How to DIY Charge-Balance 4 Li-ion 2000mAH

[SuzyC]

Any Li-Ion charge experts out there?

I want to DIY a 4S li-ion charge balancer and I have some questions:
When to begin/continue charge balancing in charge cycle ( at 10% of charge or  50% of charge or at 100% of charge?
    (0r actively charge balance the entire charge cycle and also while the battery pack is in use?)
 
 
 Assume I'm charging 4 series-connected cells to 4.2 V charge cut-off voltage at CC 500mA

I am going to use a dedicated 10-bit A/D MCU to accomplish this job, using opto-isolators as sw's to a NPN BJT to load a 10-ohm resistor across the battery being reduced in voltage for balancing?

[NiHaoMike]

Just buy an active balancer module. You're not going to be able to make your own for cheaper for just 4S.

[SuzyC]

I have on hand two examples of charge balancing circuits, one from a handvac  14.4V rated(4-cells) and another from an supermarket  "20-Volt" 5-cell battery pack. What surprises me is that the complexity is great, a dedicated >28pin IC is being used in both cases, 10's of discrete transistors, caps and resistors. No inductors were being used except to provide MCU power voltage..

But all the resistors are tiny!  This seems to tell me that the actual discharging to balance currents must be small, maybe in the 40-mA range..and I was  planning on using 10-ohms at 2-Watts! 

I saw the spec sheet for the super charge balancer something like a 100-pin chip made by TI, which can balance something like 17 Li cells and would be in stock sometime maybe in Feb 2023.

My first impression of charge balancing circuits is that they are over-complicated, There must be a more cheap'n'dirty way to do this without significantly sacrificing battery life or fire safety.

So, at this point, my MCU basic idea to charge balance 4 to 6 cells would be to work with lower balancing current (40mA).

But when in the charging cycle to do this..while charging? Stop charging momentarily to balance? Wait for the batteries to reach a minimum low dischage level before attempting to balance?  Repeat at 50% 75% 100% ?  Continuously?

I have given the whole idea some thought and realize what would be an allowable mismatch between cells? And I've guessed it would be around 10% or between .33V to .42V worst case, else the battery pack would need repair or replacement.

Then, if charging takes maybe up to four to five hours at 500mA CC, and the charging current is reduced by 10% shunting of charge current with overvoltage cells, then there should be sufficient time for even a 40mA difference in charging current to balance the batteries while they are being charged.

My solution would monitor each cell voltage continuously by a MCU 10-bit A/D. This could allow a cell voltage error of no more than 15 mV( because of voltage dividers needed with the top cells in the stack).  Once individual cell voltages are known, some equalizing circuit can then be developed.

[SuzyC]

Another thought to resolve. When measuring charging battery voltage, the battery voltage will be higher while charging due to the  internal resistance of each cell.

Charging past a 4.2V cutoff  level can easily damage a cell, but can the actual battery charging voltage limit be higher while charging to take account the internal resistance?

Is 4.2V a limit while charging?

[Psi]

There must be a more cheap'n'dirty way to do this without sacrificing battery or fire safety.

Maybe.
If you get 4 isolated supplies, (like 4x transformers or 4x DCDC isolated converters.) Then you can simply use a single cell LiPo charge IC on each cell because you have an isolated power supply for each. You are charging them individually so no balancing issues exist.

No need for complex system to charge in series until an imbalance exists, then switch to individual cell discharge mode using resistor, then back to charging in series. etc..

Additional comments about DCDC isolated bricks
They are cheap for lower currents, like $1-$2 in volume for 5V 200mA output.
They come in versions that have a few different input voltages, 5v, 12V, 24V usually all the same price. But they get expensive if you need more charging current than 400mA.
I guess you might be able to put two in parallel. Dunno

[SuzyC]

Thanks for the idea of using discrete power transformers, but this idea is too bulky and costly.

Charge constant current will be no lower than 500mA but upper limit maybe up to 750mA.

I need to make the charging circuits cheap and small enough to fit in the most tiny areas available in the target device being charged, (limited by the level of my technology: double-sided perfboard, or single sided PCB, no too small parts though, no SMD 0203 teeny-weeny parts.) And all parts must be easy to source, no mini-bricks, modules/custom/special/expensive and maybe out-of-stock till mid-2023 stuff.)

My design goals are a  target size < 2-in x 2-in max footprint, with an attempt to build even smaller and no power supply mini-brick power supplies or specialized charger ICs, just ordinary discrete transistors/diodes/resistors/optos,etc. and a small footprint MCU (MSSOP MCU is  possible but a SOIC 28-pin DIP would likely be my best choice MCU package size to begin my design work with.)

[Psi]

You could easy do <2x2 inch using DCDC isolated converters. They are like 10mm x 5mm x 10mm high
But you would be limited to like 200mA charge current.

[Psi]

If you're going for a typical design and have a 4 layer PCB you could probably embed the 4 discharge resistors inside the PCB layers as really long thin PCB tracks.
You'd need a 4 layer pcb to be able to dedicate 2 layers for it, and would probably need to be 4mil traces.
Could do 50% duty cycle on the fets connecting those resistors to lower avg current further if needed.
But it would save you needing to find room for 4 big resistors on the top/bot of PCB.

[SuzyC]

If a supermarket chain store can market a 20-V 2000mAH 5-cell Li-Ion battery pack with a built-in charger PCB utilizing charge balancing/temperature sensing/3-LED charge test indicator, 4-6 layer PCB?/ rugged plastic case etc for under USD $20, then maybe I can do something comparable cost wise to fit on a PCB in my devices..with a little design effort.

But what is the strategy to balance the cells?

[Psi]

You might get some ideas if you do a google image search for IMAX B6 HOBBYKING SCHEMATIC
and have a look how they did it.

That's a $30 hobbyking RC lipo/lion/nimh/pb charger that can do charging/balancing for anything from 1S to 6S

[SuzyC]

The key word here in my posting is DIY, design it myself, make it myself. That is why I am posting on an engineering site,  instead of a hobby site that struggles to find a pile of wire and pre-built modules to somehow fit together to make something work.

I design stuff.  I design circuits. I design devices.

I don't buy modules if it is as all possible to fabricate something myself that will function well or better than an add-on module and that same time reduce wire tangle, footprint and complexity.

Reducing wiring and space is a major goal for my projects and custom  PCBs and modules of any kind are counter to my goals in reducing expense and minimizing sourcing considerations.

Thanks Psi, but I have no use for pre-built modules from China or wherever. I won't pay for something that will create a dodgy tangle of wires and boards in order to patch into my devices.

To get the best performance, integrate a circuit into my designs and get the best footprint for a charger,  my intelligent battery charger with balancing  is going to be strictly my design to be used on my different devices and fit on my PCBs,  or for now, a circuit of my own handwired perfboard prototypes.

[beanflying]

There must be a more cheap'n'dirty way to do this without sacrificing battery or fire safety.

Maybe.
If you get 4 isolated supplies, (like 4x transformers or 4x DCDC isolated converters.) Then you can simply use a single cell LiPo charge IC on each cell because you have an isolated power supply for each. You are charging them individually so no balancing issues exist.

No need for complex system to charge in series until an imbalance exists, then switch to individual cell discharge mode using resistor, then back to charging in series. etc..

Additional comments about DCDC isolated bricks
They are cheap for lower currents, like $1-$2 in volume for 5V 200mA output.
They come in versions that have a few different input voltages, 5v, 12V, 24V usually all the same price. But they get expensive if you need more charging current than 400mA.
I guess you might be able to put two in parallel. Dunno

Some time ago I had a good play with success here with exactly this  https://www.eevblog.com/forum/projects/multi-cell-lithium-charger-from-a-single-5v-supply/msg1454259/#msg1454259

@Suzy but the DC-DC bricks they are great and then DIY a proper PCB. Due to what I had on hand at the time and geography I hacked it up 

[SuzyC]

Thanks beanflying, but what you show here is exactly what I want to avoid.

It may be easy to fabricate, might approximate what I want to do,  but it is way out of hand in minimizing project space and reducing wiring complexity.
.
I want to learn how to engineer/design circuits, not learn how to stack circuit boards upon modules upon tons of expensive wiring.

DIY

[beanflying]

I think with a dedicated PCB and from scratch you put together something about 50% (or less) of the footprint. The DIP switches for variable current take up a lot of board real estate and if you want to use one charge rate are easy to omit. The DC-DC converters are required for simplicity and then buy a stash of TP4056 IC's and DIY.

The issue otherwise is you need generally a DC-DC boost converter Micro and a bunch of other associated bits and some programming time for multicell charging/balancing. I did buy a trashy Imax B6 charger a while ago from evilbay and that is another obvious option then tweak it into your project eBay auction: #263289024061 Against my good R/C chargers and testing so far they are not horrid and they are this sort of DC-DC boost and micro topology.

[SuzyC]

Thanks NiHaoMike, but this video was like wasting time watching just another blind man attempting to precise an elephant.

[Psi]

Thanks Psi, but I have no use for pre-built modules from China or wherever. I won't pay for something that will create a dodgy tangle of wires and boards in order to patch into my devices.

I was not suggesting that you buy that hobbyking charger from china. I was suggesting you look at the schematic for it to see how it works and that might help you designing your one.

For the smallest size option a dedicated 4 channel lipo charge IC is probably the best way to go.
Ideally one with built in DCDC converter. But that probably violates your rule of not using a module.

From my experience one of the best skills to have in electronics is knowing what areas of a project to design yourself and what areas to use pre-exising module.   Of course if your goal is to learn rather than get the finished product then avoiding pre-existing modules is the way to go.
Though, with one possible exception. Sometimes trying to do everything yourself leads to projects that drag on and on and never get finished.  The satisfaction of finishing the project is important.

[Psi]

The key word here in my posting is DIY, design it myself, make it myself. That is why I am posting on an engineering site,  instead of a hobby site that struggles to find a pile of wire and pre-built modules to somehow fit together to make something work.

I want to learn how to engineer/design circuits, not learn how to stack circuit boards upon modules upon tons of expensive wiring.

DIY

hm.. maybe we have a language misunderstanding.
DIY does not just mean designing things from scratch for the purpose of learning. More often DIY is throwing a solution together from whatever you can find as fast as possible. eg, throwing a few modules at a problem with some wires.

[Psi]

I did have another idea to simplify the charging design.
Though i'm not saying its a better solution, just that its simple and can fit in a 2x2 inch size easily.

You could design a single cell charger and then have 3x small ~14x9x5mm DPDT relays. One master and 2 slaves to connect your single charger across each of the 4 cells as required one at a time.
You wouldn't need any discharge electronics or load resistors. The MCU would simply cycle through the relays every minute or so, balancing is done by giving longer/shorter charge time to each cell as needed. The MCU can disable the charger before switching the relay on/off so the relay contacts should last a very long time as they are not arcing at change over.

You can get pretty small DPDT relays in the 2-5A range so that should be ok for most typical lipo charge currents.

[SuzyC]

Thanks Psi, I guess DIY means different things to different people.  For some DIY means to hack a way to accomplish something, to others it is not paying a painter to paint the porch.

Thanks for your suggestion to use relays, but this is the same problem, using more expensive, less than ideal fix-ups instead of careful design to solve this problem.

What I need to know is how many good small footprint battery-management ICs function with their accompanying small army of tiny low-cost discretes accomplish this task. Then I will be able to emulate a battery-management IC's functionality with my circutry and my MCU code.

I am well-aware that it takes some time and often many iterations to develop something that !@*#s-up into something that works well. And of course,  in that case, the optimal solution to an engineering problem often does invite use the modules that do something that is so very difficult to replicate by my own design effort.

I want to avoid using any dedicated BM IC or prebuilt module.

[tooki]

What I need to know is how many good small footprint battery-management ICs and their accompanying small army of tiny low-cost discretes accomplish this task and emulate a battery-management IC functionality with MCU code.

I suspect they are using battery management ICs.

And I suggest you do the same, since lithium cells aren’t necessarily the most forgiving things. Unless you want to master the art of lithium charging, you’re better off leaving that to purpose-built ICs (and even that will require learning a lot about charging).

[SuzyC]

tooki..That's it exactly!

 I do want to master the so-called "Art of li-Ion" charging.  I am certain it is not that formidable a problem.

I am hoping to find some engineers with my post that know how to do this task well to give me some help.

[tooki]

tooki..That's it exactly!

 I do want to master the so-called "Art of li-Ion" charging.  I am certain it is not that formidable a problem.

Your prior posts indicated a hodgepodge of conflicting motivations:
- wanting to design from scratch
- cheap
- compact
- safe

You won’t be able to compete with commercial products on price; they have economies of scale a hobbyist cannot begin to approach.

In terms of size, using purpose-built components (like charger/battery management ICs) will be more compact than doing it discretely.

In terms of safety, a dedicated IC is going to do more. Their designers have tons of experience (and lawyers telling them to not fuck up), and have put that experience into the code and circuits inside the ICs. An added safety circuit that adds practically no cost to the IC might require a bunch of components to do yourself. (For example, op-amps to condition a signal from a thermistor to read into an ADC, but also to a comparator that shuts things off in hardware if the temperature goes too high.)

In a nutshell, I highly suggest you design something around a dedicated IC first. Trust me, doing that properly will still take much longer than you think, and will elucidate all the things you need to consider when doing it from scratch later.



FWIW, I agree with others on what DIY means, and that it doesn’t imply “made from scratch”. If anything, “DIY” implies a particular level of expertise, namely, on the low side (hence why we don’t say that professionals are DIYing when they do their work), and thus in electronics, more likely to be using premade modules, if anything!

[SuzyC]

My expectations are not hodge-podge at all. It is just that I am going to use a MCU instead of a dedicated BM IC.

I do think I am capable of learning how to accomplish this task with my own design efforts and what I will shortly accomplish is a solution  that is cheap, have a BOM that easy to source, efficient, safe, works well and has a small foot print.

Somehow,  too many think too often that painting a porch requires a PHD and have a BM whenever they are challenged.

[tooki]

And some people are delusional and think they can quickly out-engineer companies that throw ultra-experienced engineers and millions of dollars of R&D at a product.

This isn’t painting a porch, it’s building a plumbing system.



But you missed the key point anyway: conflicting.

You know that engineering adage: “Quick, good, cheap. You can have any two.”

This is similar: you can do it from scratch with an MCU. But it won’t be cheaper, and it won’t be more compact. It likely won’t be as safe, either. And it certainly won’t be as quick to engineer.

[amyk]

Another thought to resolve. When measuring charging battery voltage, the battery voltage will be higher while charging due to the  internal resistance of each cell.

Lion doesn't really do that, unlike other chemistries such as NiMH/NiCd, lead-acid, etc. that need more than nominal voltage to charge. The maximum specified voltage is the actual one. Internal resistance is extremely low and the charge must be current and voltage limited.

[SuzyC]

Amyk:  What do you consider extremely low?  I would say in 0 to 1 milli-ohms.

I have googled 18650 Li-On internal resistance and found results ranging from 25 milli-ohms(new, fully charged) to 400 milli-ohms(exhausted battery)

At >= C/2 CC charge rates, this resistance is significant in determining a precise battery EOCharge cutoff voltage that doesn't damage the cell.

Also, while a large number of charge cycles clearly shows a large loss of charge acceptance of a cell, the internal resistance remains nearly constant.

I have also learned how internal resistance can affect charge balancing.  A cell that has developed a high internal resistance will appear to be over charged in comparison to the other cells in a battery while  CC charging. Attempting to equalize voltage in a cell in this condition makes a weak cell end up with even less charge than the other cells in a multiple cell battery.

[SuzyC]

i seem to be getting little help with my original posting subject on this forum.

Replies here seem even to be unable to realize that I am able to improve my ability to design circuits and seem obsessed with the profitable encouragement for viewers of this forum to buy buy buy (modules) and not try on their own to learn how to design things themselves( Don't get me wrong, hobby ways of doing things are fine for hobby-oriented people, I am tying to learn electronics and electronic design not just make some bulky, expensive, dodgy device work by throwing money at a problem.). 

Googling, I have found an obsolete (2004) research paper about this subject (attached). There are also a few postings of some interest on StackExchange.com This paper fails to realize many of the problems and offers no realized solution.

I would also say that I can circuit design as good as the best groups of engineers by learning their technology, their methods to solve this  problem using up to date battery-charging/balancing IC's.

[beanflying]

i seem to be getting little help with my original posting subject on this forum.

Partly because a lot of us will see little advantage in reinventing the wheel when you can buy a good one for around $30.

It is 'normal' for example for low number runs in commercial products to use commercial block or modules even the big boys do it a lot with Power Supplies in all sorts of gear instead of starting from scratch. My cludge with a dedicated PCB for 4 cells will 'look' anything but dodgy and most importantly will work be simple to make and not cost much. You should never dismiss this approach regardless of project in a lot of cases it just makes good common Engineering and $ sense. Any Engineer costing a company $1k+ to do a $20-30 job will have a short company life.

I seriously suggest you read this https://dlnmh9ip6v2uc.cloudfront.net/datasheets/Prototyping/TP4056.pdf buy some of the cheap boards (cheapest source for low volume TP4056 IC's and play with them. The staged charge diagram needs to be imprinted in your head. This playing is a great way to learn the basics.

Then as you are determined to go from scratch then you need to start with a basic block layout or topology to make it more sensible to approach the relevant parts. I have included some happy snaps of the Imax mentioned earlier under the display almost certainly is some sort of STM or clone Micro driving the output channels and monitoring/controlling the voltage and current via FETS and sense resistors. Lower Left is the DC-DC boost converter as this one is designed to run off 12V but can charge 6S so likely 30VDC output. Lower Right is a larger FET and associated bits for Non Balanced charging at high currents of NiMh/NiCad etc.

[tooki]

i seem to be getting little help with my original posting subject on this forum.

Replies here seem even to be unable to realize that I am able to improve my ability to design circuits and seem obsessed with the profitable encouragement for viewers of this forum to buy buy buy (modules) and not try on their own to learn how to design things themselves( Don't get me wrong, hobby ways of doing things are fine for hobby-oriented people, I am tying to learn electronics and electronic design not just make some bulky, expensive, dodgy device work by throwing money at a problem.). 

The problem is that you don’t seem to accept that making things yourself — which everyone here understands the joy and educational value of — is rarely compatible with “cheaper” and “smaller”.

If you said simply: “I want to make this myself from scratch to understand how it works. I know I could buy it cheaper, but I want the experience”, nobody would disagree, and you’d get plenty of help. We all do projects like that.

But your arrogance of thinking you can out-engineer teams of people who have spent years designing dedicated chips and/or products around them? Nah, not a snowball’s chance in hell you’ll manage that. Even if you design a circuit that does everything a dedicated chip does, and does it equally well, it will not be as small and cheap as the dedicated chip. And your flippant responses to people giving you reality checks aren’t doing anything to curry favor.

I would also say that I can circuit design as good as the best groups of engineers by learning their technology, their methods to solve this  problem using up to date battery-charging/balancing IC's.

So now suddenly you are willing to use dedicated ICs? That’s a very sensible approach. Later you can build your own truly from scratch.

P.S. I’m speaking from personal experience here — I learned a ton about charging systems by building one NiMH and one LiIon charger, each around a dedicated IC. And I actually would like to build my own from scratch at some point. But the learning from starting with the ICs was invaluable.

[salihkanber]

Chinese packs even dont have balancing circuits and they work well for some years

[SuzyC]

 beanflying "Partly because a lot of us will see little advantage in reinventing the wheel when you can buy a good one for around $30."
Again, beanflying, I am not interested in buying modules or even dedicated BM IC's if I can avoid it if I can design my own circuits.

I am interested in furthering my experience in designing circuits, to learn how to design circuits.

I am not a newbie. I have been designing and successfully building my own CPU-based power supplies and Li-Ion chargers for  projects for over 20-years.

I am therefore not trying to "start from scratch."

Buying a module is not engineering, it is a hobbyist's desperate attempt to achieve a dodgy result by buying clumsy modules that are bulky and not integrated into a design of their own.

My advantage is clear, to design my own circuits to not have to buy a module or IC that I can myself design to emulate and therefore then allow me to refine my design and then build something that best fits in the small spaces available in my project.

Tooki: So now suddenly you are willing to use dedicated ICs? That’s a very sensible approach. Later you can build your own truly from scratch. 

I never said I was willing to use dedicated ICs. I don't understand how you misread what I posted to think this is true.

My goal is to try to achieve the smallest footprint without resorting to buying a dedicated chip and certainly not a pre-built module. I may not be able to closely achieve that goal but I will make that effort.

My solution may not even be as cheap, but it will be my design. It will by my design and undergo testing, so I am sure to say it is able to very closely work as well as the pre-built module or dedicated IC. 

If I design a circuit to accomplish my goals, I will be able to completely control the circuit operation, integration into a PCB and size to fit my needs for performance, parts sourcing, cost and safety.

Finally, I can say (after quite a lot of Googling)  that charge-balancing is not really complicated or something "taboo" to play with. 

Charge balancing needn't be complicated nor unsafe to attempt.

I had hoped my topic posted would  have been about sharing experience with charge-balancing instead of spending post after post attempting to convince others that I am not interested in modules.

[beanflying]

Given your last post then we are clearly wasting or having our time wasted then  Everything you have typed so far just doesn't gel with the following.

I am not a newbie. I have been designing and successfully building my own CPU-based power supplies and Li-Ion chargers for  projects for over 20-years.

I am therefore not trying to "start from scratch."

Any Li-Ion charge experts out there?

I want to DIY a 4S li-ion charge balancer and I have some questions:
When to begin/continue charge balancing in charge cycle ( at 10% of charge or  50% of charge or at 100% of charge?
    (0r actively charge balance the entire charge cycle and also while the battery pack is in use?)
 
 
 Assume I'm charging 4 series-connected cells to 4.2 V charge cut-off voltage at CC 500mA

I am going to use a dedicated 10-bit A/D MCU to accomplish this job, using opto-isolators as sw's to a NPN BJT to load a 10-ohm resistor across the battery being reduced in voltage for balancing?

[amyk]

At >= C/2 CC charge rates, this resistance is significant in determining a precise battery EOCharge cutoff voltage that doesn't damage the cell.

Note that still putting full charge current into the cell as it approaches the upper voltage limit will wear them out much faster than a more gentle tapering off.

[SuzyC]

Thanks Amyk for focusing on the actual topic of this post.

Always in charging LI-Ion, current must throtle down once the 4.2V (or 4.15 or 4.1V) desired cutoff voltage limit is reached.

Proper charging always allows the constant-current to decrease during the voltage float phase of charging, once the target cutoff  voltage is reached while charging.

The only way to force current into a Li-Ion battery over an established level in charging would be to increase the allowable charging voltage.  Exceeding the voltage cutoff limit is damaging and not permitted in any charging scheme I use.

This is why voltage balancing seems enticing, because some cells could reach the cutoff voltage before others and equalizing prevents these cells from being damaged by over-voltage while allowing any lower-voltage cells to continue charging.

What I've learned so far:

Charge-balancing is accomplished in two ways. The first is to switch on a shunt (resistor or constant-voltage shunt regulator) across a cell that is too high in voltage (relative to other cells in the battery) to slow its charging.

The second way (and I am not sure if it is used in modern charging circuits) is to momentarily shunt a capacitor across a higher-voltage cell and then transfer this charged capacitor to shunt a lower-voltage cell to discharge this charge-transfer capacitor.

The second idea seems enticing because it does not waste charging current, but could be less safe and very complicated to achieve in a charging circuit without a very large number of extra parts. It would also mean the charging circuit would have a larger footprint and be more expensive in part costs.

NASA has made available to the public a multi-cell charging scheme that uses pulse transformers to individually charge cells. This idea is not economically practical in most instances in non-space use, but could be very important on any space vehicle or other outer space device, where all power supplies need to not waste any energy.

[tooki]

beanflying "Partly because a lot of us will see little advantage in reinventing the wheel when you can buy a good one for around $30."
Again, beanflying, I am not interested in buying modules or even dedicated BM IC's if I can avoid it.

I am interested in furthering my experience in designing circuits, to learn how to design circuits.

I am not a newbie. I have been designing and successfully building my own CPU-based power supplies and Li-Ion chargers for  projects for over 20-years.

I am therefore not trying to "start from scratch."

Nobody said you'd be starting from scratch. I said you intend to build it from scratch, which means "not relying on premade things", i.e. in this case, avoiding modules and ICs. I used "build from scratch" to avoid using "DIY", which has connotations that might not be applicable here.

Like, haven't you ever heard anyone say "I'm going to bake a cake from scratch"? That means starting with flour, sugar, eggs, milk, baking powder, and vanilla, as opposed to starting with a cake mix that only needs eggs and milk. It doesn't mean that it's their first time baking!!

Tooki: So now suddenly you are willing to use dedicated ICs? That’s a very sensible approach. Later you can build your own truly from scratch. 

I never said I was willing to use dedicated ICs. I don't understand how you misread what I posted to think this is true.

Easy, because your lousy punctuation and run-on sentences make it easy to misunderstand what you mean.

You wrote:

I would also say that I can circuit design as good as the best groups of engineers by learning their technology, their methods to solve this  problem using up to date battery-charging/balancing IC's.

That sentence construction (Do X by doing Y) means that the Y is qualifying how you will do X. Using italics to mark X, and bold for Y, your sentence parses as either:

I would also say that I can circuit design … by … using up to date battery-charging/balancing IC's.
or
I would also say that I can … solve this problem using up to date battery-charging/balancing IC's.



Either way, you end with the qualifier of using modern ICs.

My goal is to try to achieve the smallest footprint without resorting to buying a dedicated chip and certainly not a pre-built module. I may not be able to closely achieve that goal but I will make that effort.

My solution may not even be as cheap, but it will be my design. It will by my design and undergo testing, so I am sure to say it is able to very closely work as well as the pre-built module or dedicated IC. 

If I design a circuit to accomplish my goals, I will be able to completely control the circuit operation and size to fit my needs for performance, parts sourcing, cost and safety.

You say that now. But above, you stated repeatedly how your goals were to:
a) match or undercut the price of a low-cost product from a supermarket
and
b) minimize space

Well the first one is practically impossible anyway, so let's ignore it. But if space savings matter, which they seem to be since you bring it up every time, then you'd want to go with a dedicated IC.

As I said in a prior reply: if your actual #1 goal is to avoid using anything premade, even ICs, then you should not have kept insisting on low cost and compact.

Finally, I can say (after quite a lot of Googling)  that charge-balancing is not really complicated of something "taboo" to play with. 

Charge balancing needn't be complicated nor unsafe to attempt.

Nobody said balancing was taboo. Without knowing your skill level (which you didn't share until your final tirade), the etiquette here is to err on the side of safety, which is why you will see people express warnings about lithium charging.

I had hoped my topic posted would  have been about sharing experience with charge-balancing instead of spending post after post attempting to convince others that I am not interested in modules.

The first person to mention a module to you, to which you immediately freaked out, said to look at a module to see how they do it. They did not say to buy the module! 


The reason you're getting so much pushback, SuzyC, is your awful attitude. You've been on the defensive since the first reply was posted, a reply you didn't even read properly. You attack everyone, and then oscillate between acting like a newbie and claiming you're super-experienced, ultimately spending more time whining than anything else. Your sloppy use of terminology also makes me very skeptical of your claimed expertise.

Always in charging LI-Ion, current must throtle down once the 4.2V (or 4.15 or 4.1V) desired cutoff voltage limit is reached.

Proper charging always allows the constant-current to decrease during the voltage float phase of charging, once the target cutoff  voltage is reached while charging.

You are always in either constant-voltage (CV) OR constant-current (CC) mode — you can't be in both at the same time. (Float charging is something else, btw.)

When the CC phase has finished by reaching the termination voltage, the charger switches to the CV phase, during which current (not "constant current") drops until reaching the termination current.

This is why voltage balancing seems enticing, because some cells could reach the cutoff voltage before others and equalizing prevents these cells from being damaged by over-voltage while allowing any lower-voltage cells to continue charging.

I think a better word than "enticing" is "necessary". But yes, that's exactly why you need balancing.

Charge-balancing is accomplished in two ways. The first is to switch on a shunt resistor across a cell that is too high in voltage (relative to other cells in the battery) to slow its charging.

The second way (and I am not sure if it is used in modern charging circuits) is to momentarily shunt a capacitor across th higher-voltage cell and then transfer this charged capacitor to shunt a lower-voltage cell to discharge the transfer-capacitor.

The second idea seems enticing because it does not waste charging current, but could be less safe and very complicated to achieve i a charging circuit without a very large number of extra parts.

Another way it's done is with inductors as energy store. See e.g. how the Analog Devices LT8584 does it. But it definitely adds lots of parts (even more so when rolling your own control circuit.)

There are also some systems that use an additional, dedicated cell as the energy store for balancing.

[SuzyC]

Thanks Tooki for your help in and pointing out some of the confusing phrases and word choices I have used.
I don't always have time to both try to explain whats on my mind and then carefully edit my postings.

However, when I scan over the many topics posted on this forum, my main complaint is seeing too often almost unintelligable, very vague postings. I choose to make clear my ideas and spell correctly and pay attention to grammar, and that in itself, seems unusual.

If I was that perfectly careful, I would be writing a book! If I was writing a book, I wouldn't have time to work on my current  projects.

I do have a strong tendency towards run-on sentences.
Thanks for reminding me.
I will be more careful to try to parse my phrases into separate sentences.

As I am posting replies, I am also, in the process, getting a better understanding on my mindset and ways I present my understanding of a problem and the way to achieve my goal.
My postings are a process, an evolution of the ideas that I am playing with.

If I knew exactly what I was doing, I wouldn't bother to post a topic here.

I am actively trying to better understand  the problem I am dealing with and then hope I can even change my ideas when I can better understand how to accomplish my goals.

-------

While in constant current charging, the "constant current" remains constant at an ever-decreasing value once the farget float voltage is reached. (Both at the same time?) As I understand it, perhaps I should say that constant current mode evolves into a mode of decreasing current limiting to prevent overvoltage. In any case, it is simply a choice of words.
It is necessary to determine the  point that the constant-current decreases to reach the end of charge current level that terminate charging.

[David Hess]

I have always though the way to go for a simple circuit is a voltage follower with class-B output stage to drive each junction between cells.  The collectors of the output transistors are driven from adjacent cells and have a series resistor to limit current.  The reference voltages come from a string of precision resistors, which can be high value if the input bias current is low.

[SuzyC]

Thans David!  I am trying to conceptualize what you've posted and I am not quite disambiguating the circuitry required.

I know its probably too much to ask, but a simple schematic is worth a thousand words. 

With your idea, the cost of implementation, circuit complexity and footprint, as well as cost pop-up first in my mind.

I see a lotta difficulty determining how to control the charging real-time total voltage applied to the batteries.

[David Hess]

A voltage follower with a class-B output is shown below.  Essentially it just forces the output to the input voltage applied to the non-inverting input, with the transistors boosting the output current.  Only one transistor conducts at a time.

A further refinement would be to disconnect the output of the operational amplifier from the bases of the transistors to disable balancing, with an analog multiplexer or maybe a FET.  Without this the circuit would balance the cells during both charge and discharge, but this would not necessarily be a problem in all applications.  Some operational amplifiers include a shutdown function which disconnects the output which could be used for this and would be ideal.

The operational amplifier and output transistors are only running on the voltage supplied from adjacent cells, so a micropower type of operational amplifier should be used unless shutdown is used.  For the same reason the input resistor divider should use high value resistors, which also means that the operational amplifier should be a low input bias current type.

Place small power resistors in series with the collectors to limit the current.  Voltage across each transistor is about 2 volts depending on the voltage drop across the power resistors, so figuring 2 watts rating from power surface mount parts, a balancing of current of 1 amp is feasible, but of course it could be higher or lower depending on the application, and under normal conditions the battery state will limit the current lower except under high current charge or discharge.

[SuzyC]

Thanks David for posting a reply and a simple schematic. I must say this idea is so very different from anything I would have imagined and I can certainly see how this approach would be avoided because:
The cost of the additional parts, esp. regarding the  use of bipolar power supplies and adding an opamp per battery.
This circuit might require these charge-equalizers to be located on the battery.
It needs a way to precise current and voltage in each balancing module and for the whole string of cells.


The complexity is excessive.  Because of the use of bipolar supplies, this design looks like in might be found in a mains/battery powered oscilloscope or other expensive instrument.

[SuzyC]

As my study of charge-balancing continues I am wondering if charge balancing is a good idea to use at all.

Salihkamber posted that "Chinese packs even dont have balancing circuits and they work well for some years "


If any cell set has a failing battery, charge balancing can only exacerbate the problem.
A cell that has developed a high internal resistance will appear to be over charged in comparison to the other cells in a battery while  CC charging. Attempting to equalize voltage in a cell in this condition makes a weak cell end up with even less charge than the other cells in a multiple cell battery.

If any cell starts to be significantly different in charge voltage or charge acceptance or series resistance, this would indicate that cell is gone south and there is an impending  need to replace the whole battery pack. Balancing doesn't help.

Battery packs have very identical cells, they should remain that way during the service life of the battery when properly charged.

Is this guess correct?

[David Hess]

The cost of the additional parts, esp. regarding the  use of bipolar power supplies and adding an opamp per battery.

The operational amplifiers are supplied from adjacent batteries.  It could be built into a battery pack.

This circuit might require these charge-equalizers to be located on the battery.

Nothing prevents the balancing modules from being located remotely if all of the junctions between batteries are available.

It needs a way to precise current and voltage in each balancing module and for the whole string of cells.

Each balancing module uses a pair of precision resistors, which are relatively cheap, to establish a reference voltage between two cells based on the existing cell voltages.

The complexity is excessive.

It uses common inexpensive parts and requires no state control unless shutdown is desired and even that could be tied to a charge indicator.  It is more complex than a dedicated ASIC, and requires two power transistors in place of one.

Because of the use of bipolar supplies, this design looks like in might be found in a mains/battery powered oscilloscope or other expensive instrument.

I have seen it used in older dual cell battery packs for consumer stuff, but it would be a good candidate for balancing cells used in battery backup of continuously powered equipment.

[David Hess]

Chinese packs even dont have balancing circuits and they work well for some years

How much longer would they work with active balancing?

I ended up taking apart a Milwaukee M12 battery and they do not use active balancing, although the do monitor the voltage of every cell.

LiFePO4 cells should require less active balancing because they can tolerate more overcharge.

[SuzyC]

Thanks again David.

You say you took apart a Milwaukee pack, but you didn't reveal if the matching of cells and their voltages remained the same after long use.

[Psi]

On the first page I suggested you look at that IMAX B6 HOBBYKING SCHEMATIC  because that is built from discrete components and a MCU for overall control, which is exactly what you are talking about building and will give you lots of ideas.
But for some reason I got pushback against the suggestion. Or maybe it just seemed that way from your post after.

However I suspect most of the issues in this thread are related to miscommunication and terminology issues.

I think you got a lot of hacky/module solutions recommended to you because you said...

There must be a more cheap'n'dirty way to do this without significantly sacrificing battery life or fire safety.

[SuzyC]

Psi thanks and I have perused the schematic and have found it to be an unintelligible hobbyist creation. Almost all of the interconnect labeling is completely blurred, making the schematic totally useless.

BIAC, The circuit seems not to make any good design sense to me and my lasting impression is (if this circuit really works at all) it is still obsolete and extremely over-complicated using far too-many components because it is fabricated using cheap and dirty thirty to 40-yr-old technology.

Altogether quite crude in design. I don't see a MCU in the schematic attached, and if it was shown there or on another page, and yet without the commented source code, the whole use of the schematic remains working with a bag of worms.

I don't pretend to be a MSEE just matriculated from MIT, but the level of my expertise shows this schematic is far too  primitive to give me any help.

[tooki]

As my study of charge-balancing continues I am wondering if charge balancing is a good idea to use at all.

Proper technical forum etiquette is to do your basic homework before asking for help.

If any cell set has a failing battery, charge balancing can only exacerbate the problem.
A cell that has developed a high internal resistance will appear to be over charged in comparison to the other cells in a battery while  CC charging. Attempting to equalize voltage in a cell in this condition makes a weak cell end up with even less charge than the other cells in a multiple cell battery.

If any cell starts to be significantly different in charge voltage or charge acceptance or series resistance, this would indicate that cell is gone south and there is an impending  need to replace the whole battery pack. Balancing doesn't help.

The whole point of balancing is that you don't overcharge a weaker cell, because doing so causes a vicious cycle of accelerated aging. You use balancing to protect the weak cell from overcharging, so that it isn't overstressed.

The cells in a pack are never identical, so even if they start out with identical capacities, they won't stay identical forever. So you have to protect the weaker cells, lest you constantly overcharge them, causing damage to them, further reducing their capacity.

Battery packs have very identical cells, they should remain that way during the service life of the battery when properly charged.

Is this guess correct?

Nope. "Identical" cells, even from the same production lot, will not actually be identical, and will not age identically.

The fact that practically all properly-made commercial series battery packs use balancers should make it obvious that it needs to be there; they'd save the expense if it were useless (never mind harmful).


This entire reply of yours really makes me question your claims of supposed expertise with lithium charging. Either you don't actually have that experience, or you've been abusing your cells over the years.

Psi thanks and I have perused the schematic and have found it to be an unintelligible hobbyist creation.

Yeah, I mean, IMAX is merely one of the most respected names in battery charging, particularly in the RC (radio control) world, which pushes cells hard and thus needs to handle them carefully. But what would they know… 

Almost all of the interconnect labeling is completely blurred, making the schematic totally useless.

Regarding schematic sharpness: You have to download the PDF of the schematic, not just a preview JPEG, silly goose! So that's entirely, 100% on you.

(I googled "IMAX B6 HOBBYKING SCHEMATIC" and literally the first hit is on elektrotanya, which has a nice clear PDF for download.)

This page in Russian has a more detailed schematic, including MCU: https://ru-radio-electr.livejournal.com/1295897.html

BIAC, The circuit seems not to make any good design sense to me and my lasting impression is (if this circuit really works at all) it is still obsolete and extremely over-complicated using far too-many components because it is fabricated using cheap and dirty thirty to 40-yr-old technology.

…which is literally precisely what your design intent is for your charger, since you expressly want to eschew modern integrated technology.

Altogether quite crude in design. I don't see a MCU in the schematic attached, and if it was shown there or on another page, and yet without the commented source code, the whole use of the schematic remains working with a bag of worms.

The only part of that reply that has any merit: I couldn't find a schematic of the MCU section, either.

I don't pretend to be a MSEE just matriculated from MIT, but the level of my expertise shows this schematic is far too  primitive to give me any help.

 

You certainly pretend to be an expert, when clearly you're not! (I'm not an expert, either, but at least I know where I'm at!) You certainly aren't knowledgeable enough to be making proclamations about "good design sense" on circuits you don't even understand...

[SuzyC]

Tooki, thanks for the valuable criticism of my criticism of the schematic.

Funny, before this day, I always thought IMAX was a kinda big screen place to watch moving pictures.

[Psi]

Part of being an engineer is knowing how to pull useful info out of a sea of crap info.

One note, be aware that there's many companies making clones of the IMAX chargers due to their popularity.
That Russian link appears to be the official one, but always treat anything you find on the internet as potentially fake.

[SuzyC]

Psi..should I wear gloves and a mask?

[BradC]

The IMAX charger is a pretty simple and reliable unit used by many RC modelers with batteries that have a nasty habit of bursting into flames if you mis-treat them. There are several well reverse engineered schematics and an open source firmware for it. Perhaps you can examine that to get a handle on how it handles the balancing algorithm in the MCU.

https://github.com/stawel/cheali-charger

I rebuilt a ryobi 18V lithium on the weekend, and I deliberately unbalanced the cells. The Ryobi board seems to start actively balancing them once one cell reaches about 4V. So it holds them all back until all cells seem to have reached 4V then it continues to keep them balanced up until charge termination. I thought it looked like a pretty robust algorithm.

Edit: that was a Ryobi 18V not 10V. Fat fingered

[David Hess]

You say you took apart a Milwaukee pack, but you didn't reveal if the matching of cells and their voltages remained the same after long use.

The pack was relatively new and I thought I had damaged it with excessive load, (1) but more likely I just overheated it.  When I charged it later, it worked fine.

(1) I used a new 1.5AH pack in their M12 circular saw.  It worked, but not well because it was too underpowered.  The larger capacity packs worked much better.

[SuzyC]

Thanks David Hess and BradC for sharing their experience.
What I want to know if the cells in your battery packs became unbalanced or not.

I have come to the (yet unsteady) conclusion that charge-balancing:
1) Is absolutely necessary if two or more battery packs are used in parallel or in series.
2) Of no advantage if a battery  pack is used in service alone, or doesn't have voltage taps on the pack.

Balancing a pack during charging does nothing to prevent some battery cells in a pack from becoming out of balance in use, especially when the battery pack is being used near the discharge cutoff voltage. This is because the cell/cells that were fixed-up by charge balancing would likely reach their fully discharged voltage faster than cells in a pack that were functioning better and didn't need balancing in charging.

Again, this is because:
a) All cells in a new battery pack are very closely matched when new and should remain that way during their service life if the pack is charged and discharged in use without over-stressing the batteries.

b)When the occasion arises that one or more cells in a  pack need balancing, (without any accidental unbalancing events) then something is pathologically different with one or more cells(might have lower charge acceptance/higher internal resistance) and this means the battery pack is damaged and needs to be replaced.

If the in-use battery pack only becomes lightly unbalanced  it might still be acceptable to use the pack with compromised performance expectations.

Are my guesses correct?

[SuzyC]

 I tried to download the hobby schematic using the Russian site: https://ru-radio-electr.livejournal.com/1295897.html
Couldn't make heads or tails of whats what on this site and also I have tried other sources of this schematic in PDF form by my browser doesn't quite load pages correctly on these webpages and I was unable to get the download, but many thanks for trying to help.

What I did see in the blurry-labeled schematic I did download was a hodge-podge of circuits that seem to be extraneous integrated in to the design. The battery charging source seems to be omitted or in error because the cells seem to be stacked in reverse polarity without the schematic showing a negative CC power supply for charging.

Tooki/Psi: I did notice some interesting circuits on the blurred schematic that I was able to download,  I am quite familiar with every circuit on this page and I have long known how to design these blocks,

(And I can proudly and arrogantly say "Not to any advantage here in learning about or making my own circuit design!"),

Again, nothing but thanks again for being so nice to try to help.

BradC:  I see firmware code available for download, but having not the schematic to apply the Cheali code to, it doesn't  quite be of some help.
Many thanks, and your comment that the balancing act doesn't begin until 4V is very valuable help.

[BradC]

I tried to download the hobby schematic using the Russian site: https://ru-radio-electr.livejournal.com/1295897.html
Couldn't make heads or tails of whats what on this site and also I have tried other sources of this schematic in PDF form by my browser doesn't quite load pages correctly on these webpages and I was unable to get the download, but many thanks for trying to help.

Have you seen this one? (attached)

To answer your previous question, as the pack ages the batteries become unbalanced in use. The BMS shuts off the output when the lowest cell hits the threshold.

[SuzyC]

BradC, many thanks again for responding!

I assume, when you mention "The BMS shuts of the output"  you are referring to a BMS circuit built into the battery that prevents over-discharge of the Royobi by actively monitoring all the cell voltages during use.

Does the BMS inside the battery pack also balance the battery while the battery is in use or while the battery is idle?

Or is it the case that  there is a BMS/charge-balancer built into the 18V Royobi battery that only balances while charging?

[BradC]

BradC, many thanks again for responding!

I assume, when you mention "The BMS shuts of the output"  you are referring to a BMS circuit built into the battery that prevents over-discharge of the Royobi by actively monitoring all the cell voltages during use.

Yes

Does the BMS inside the battery pack also balance the battery while the battery is in use or while the battery is idle?

No

Or is it the case that  there is a BMS/charge-balancer built into the 18V Royobi battery that only balances while charging?

Yes

[Psi]

Does the BMS inside the battery pack also balance the battery while the battery is in use or while the battery is idle?

No

I've never seen this either.

But it wouldn't surprise me if it is done in some specialized cases.
If you had a few cells at only 85% usable capacity while all others at 100% you would definitely gain free total usable pack energy by using the total pack voltage to maintain a slight charge on those cells using an isolated DCDC converter while the pack was being discharged. It wouldn't take much charge rate to compensate for the 15% and ensure all cells reached 0 at the same time. Instead of a few cells reaching 0 while all others have 15% left. etc..

[BradC]

If you had a few cells at only 85% usable capacity while all others at 100% you would definitely gain free total usable pack energy by using the total pack voltage to maintain a slight charge on those cells using an isolated DCDC converter while the pack was being discharged. It wouldn't take much charge rate to compensate for the 15% and ensure all cells reached 0 at the same time. Instead of a few cells reaching 0 while all others have 15% left. etc..

I've recently seen a "flying capacitor" balancer board for continuous balancing but I don't recall where.

[Psi]

If you had a few cells at only 85% usable capacity while all others at 100% you would definitely gain free total usable pack energy by using the total pack voltage to maintain a slight charge on those cells using an isolated DCDC converter while the pack was being discharged. It wouldn't take much charge rate to compensate for the 15% and ensure all cells reached 0 at the same time. Instead of a few cells reaching 0 while all others have 15% left. etc..

I've recently seen a "flying capacitor" balancer board for continuous balancing but I don't recall where.

I'm guessing by the name that that system switches both legs of a cap from one input source to another output source to transfer charge between cells.

[SuzyC]

Psi:  You got that right.  I found a mention of this technique while googling that attempts to use a 4066 analog gate for this task, but IMNHO this idea is not to be tried without adequate ventilation and a fire extinguisher close by.

BIAC, its not efficient and ends up robbing Peter to pay Paul.

This idea shows some merit in preventing some cells in a pack from being reversed-charged and damaged (and gain extend the use of  nearly over-discharged cells to achieve some more time of the use of a battery pack.) This might be important where even a short time of extended operation of a battery in a device would could be well-appreciated, worthwhile or mission critical.

[David Hess]

Balancing a pack during charging does nothing to prevent some battery cells in a pack from becoming out of balance in use, especially when the battery pack is being used near the discharge cutoff voltage. This is because the cell/cells that were fixed-up by charge balancing would likely reach their fully discharged voltage faster than cells in a pack that were functioning better and didn't need balancing in charging.

That is always going to be the case if different cells have a different capacity.  Either charging will be terminated early to prevent overcharging some cells, or discharge will be terminated early to prevent over-discharging some cells.  Charge balancing is about maintaining the maximum available capacity of a battery pack by preventing cells from becoming further out of balance.

It should be better to redistribute charge between cells during discharge, and this is feasible, however it comes at a high cost for only a minor improvement in capacity.

In the case of the Milwaukee M12 battery system where the cells are only monitored, charge balancing can be accomplished through controlled overcharging because LiFePO4 cells are used.  Lead-acid and NiCd batteries can also be balanced through controlled overcharging.