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.