Author Topic: Balance charging lithium cells  (Read 4765 times)

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Offline OM222OTopic starter

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Balance charging lithium cells
« on: April 24, 2019, 10:29:38 pm »
In pretty much every balance charger circuit I can find, they just use some braking resistors to discharge the cells that are charged to a higher voltage. wouldn't it be just easier to create multiple isolated supply lines and charge each cell to it's desired voltage, e.g: 4.2V and then stop charging all together? it should be more efficient too?
 

Offline Miti

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Re: Balance charging lithium cells
« Reply #1 on: April 25, 2019, 01:30:02 am »
it should be more efficient too?

...and way more complicated. Probably you're playing with RC balanced chargers. Technically, if the cells are well matched, you'll have to balance them only once, after that they should be pretty much charging/discharging in sync without much "braking". However, if the cells aren't perfectly matched, the circuit that uses shunt resistors, if properly designed, will give you a better capacity matching than the circuit that you proposed.
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Offline OM222OTopic starter

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Re: Balance charging lithium cells
« Reply #2 on: April 25, 2019, 01:46:43 am »
how come? with a voltage reference and an op amp, you can create a very exact cut off voltage  ??? you create the desired voltage, e.g: 4.2 or 4.1 volts and use one comparator per cell to deactivate the charging circuit and since they are all referenced to the same point, then the only error would be the input offset of your comparator, which you can select to be very small, even less than 1mv  :-DD also there are plenty of DC-DC isolators available in through hole packages which do boost / buck conversion as well! they are limited to low currents (500mA max I believe) but it wouldn't be much more difficult to make small converters on your PCB either  :-// you can also create small constant current / constant voltage charging circuit which adds more complexity than just charging with constant voltage and cutting off at some point, but it's presumably much better in keeping the batteries happy campers  :-/O
 

Offline digsys

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Re: Balance charging lithium cells
« Reply #3 on: April 25, 2019, 02:03:45 am »
For serious applications, we use ICs like this - https://www.analog.com/media/en/dsp-documentation/evaluation-kit-manuals/dc2064afa.pdf
There are a few manufacturers / designs out there already, and it very popular in EVs
I NEVER have it permanently connected, always as a plug in - and only when the BMS says I need to. They are very efficient, also cascade able.
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Offline OM222OTopic starter

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Re: Balance charging lithium cells
« Reply #4 on: April 25, 2019, 02:25:23 am »
very interesting ... but it seems like a very specialty item?
the method I proposed can be done with jelly bean parts however  :-//
or another way would be to have 1 constant current constant voltage charger with it's cut off point set to a voltage reference IC, and use mosfets to effectively swtich in / out different batteries. this way the offset would also be the same, so the cutoff point is exactly the same  :-DD it would just take longer to charge the batteries which is not ideal  :-//
« Last Edit: April 25, 2019, 02:27:20 am by OM222O »
 

Offline digsys

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Re: Balance charging lithium cells
« Reply #5 on: April 25, 2019, 02:57:58 am »
Quote from: OM222O
very interesting ... but it seems like a very specialty item?
the method I proposed can be done with jelly bean parts however ...
Possibly correct, but it gives the OP another method / ideas. This IC (and others) do most the work, so all you need is some Si switches
and the inductor. Some users have trialed off-the-shelf ones from Digikey etc with pretty good results. No need to design it yourself.
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Offline mikerj

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Re: Balance charging lithium cells
« Reply #6 on: April 25, 2019, 09:07:10 am »
In pretty much every balance charger circuit I can find, they just use some braking resistors to discharge the cells that are charged to a higher voltage. wouldn't it be just easier to create multiple isolated supply lines and charge each cell to it's desired voltage, e.g: 4.2V and then stop charging all together? it should be more efficient too?

How would multiple isolated charging circuits be simpler than a balance circuit (comparator, transistor and a few resistors) per cell?  It would be both more complex and considerably more expensive.  It's also likely to be less efficient if cells are well matched.
 

Offline David Hess

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Re: Balance charging lithium cells
« Reply #7 on: April 25, 2019, 10:06:10 pm »
An alternative I have though about is using an operational amplifier with a simple class-B output stage to force the voltage between two adjacent cells to the midpoint during charging.  This could also be suitable for a bank of supercapacitors.

 

Offline OM222OTopic starter

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Re: Balance charging lithium cells
« Reply #8 on: April 25, 2019, 10:57:48 pm »
An alternative I have though about is using an operational amplifier with a simple class-B output stage to force the voltage between two adjacent cells to the midpoint during charging.  This could also be suitable for a bank of supercapacitors.

that is actually a simpler approach, although still less efficient, but not by far  :-// would you be interested in developing a prototype PCB?
 

Offline David Hess

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Re: Balance charging lithium cells
« Reply #9 on: April 26, 2019, 02:38:28 am »
An alternative I have though about is using an operational amplifier with a simple class-B output stage to force the voltage between two adjacent cells to the midpoint during charging.  This could also be suitable for a bank of supercapacitors.

that is actually a simpler approach, although still less efficient, but not by far  :-// would you be interested in developing a prototype PCB?

You are welcome to the idea.  It is just one of those things I have been considering.  It is awfully simple for what it does.

It has the same efficiency as resistive loading but if only two cells are present, only one control circuit is required for the two power transistors.  Power transistor dissipation can be limited with a resistor in series with the collector or emitter.

With a micropower low input bias current operational amplifier, the circuit could be left in parallel and powered by the two cells continuously since when they are balanced, the quiescent current could be lower than the self discharge.  That brings up the problem of whether it is a good idea to enforce balancing under all conditions.

For a version which is not always attached to the battery, it would be relatively easy to add a disconnect to disable balancing and leave the output stage in a high impedance state.  Some operational amplifiers have this built in.


 

Offline OM222OTopic starter

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Re: Balance charging lithium cells
« Reply #10 on: April 26, 2019, 07:51:08 am »
why would this be as efficient as using a braking resistor? I'm not sure I understood what you said correctly, but what I understood is using an op amp and an output stage which forces the voltage to the midpoint between cells (i.e: taking charge from the battery with higher charge and forcing it to the battery with lower charge). this should be more efficient than just wasting it all as heat?
 

Offline David Hess

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Re: Balance charging lithium cells
« Reply #11 on: April 26, 2019, 01:14:57 pm »
why would this be as efficient as using a braking resistor? I'm not sure I understood what you said correctly, but what I understood is using an op amp and an output stage which forces the voltage to the midpoint between cells (i.e: taking charge from the battery with higher charge and forcing it to the battery with lower charge). this should be more efficient than just wasting it all as heat?

They are both doing the same thing.  With the operational amplifier, the transistor is being used as a variable resistance and dissipating the same power that the resistor would have to maintain balance.  Power is power no matter how it is dissipated.
 

Offline OM222OTopic starter

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Re: Balance charging lithium cells
« Reply #12 on: April 26, 2019, 03:52:47 pm »
why would this be as efficient as using a braking resistor? I'm not sure I understood what you said correctly, but what I understood is using an op amp and an output stage which forces the voltage to the midpoint between cells (i.e: taking charge from the battery with higher charge and forcing it to the battery with lower charge). this should be more efficient than just wasting it all as heat?

They are both doing the same thing.  With the operational amplifier, the transistor is being used as a variable resistance and dissipating the same power that the resistor would have to maintain balance.  Power is power no matter how it is dissipated.


I didn't mean dissipation, I meant using the op amp to transfer charge between the batteries until they are balanced, therefore very little (almost zero) dissipation. Similar to what the ICs that digsys mentioned (check the app note  / datasheet on how they balance the batteries).
 

Offline Siwastaja

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Re: Balance charging lithium cells
« Reply #13 on: April 26, 2019, 05:13:10 pm »
What you describe is called "active" or "redistributive" balancing. While widely discussed on academical (and hobbyist) level, very rarely used in the real world. Simply because: li-ion cells have almost negligible self-discharge rates, and the differences between self-discharges between the cells (which determines the balancing need) are even lower.

Think about this: a typical cell self-discharges around 2%/year. Let's say it's neighbor self-discharges by 2.1%/year; the difference to balance out is 0.1%/year. At, say, 10Ah capacity, this is 10mAh per year. Assuming an example pack of 10 series cells at 10Ah, this would be around 0.37Wh wasted in heat - in a year! Approximating at $0.15/kWh, the cost for this electricity would be around $0.00005, per year.

Now, passive balancing BMS cost per cell is about $0.50, and well designed redistributive is about $2.00 - assuming both types are cost optimized for mass market. (This is based on my own designs of both types, so I have a general idea of what it costs. Sadly, the differences are actually even bigger, since redistributive balancers are expensive niches.)

Saving $0.00005 per year, the payback in a 10s pack (10 * $1.50) would happen in 300000 years. Of course, assuming that everything else, such as quiescent current, stays the same, which is a ridiculous assumption.

In larger packs, the numbers are less ridiculous. Yet, it's highly improbable that an active system ever makes sense, even in large systems. They are used in very specialized conditions. Another chemistry in the future might require the re-evaluation of redistributive balancing; li-ion is just too good.

Of course, if this pack is in any electric vehicle, the extra consumption of carrying a few dozen grams of extra electronics with you makes the bottom line negative.

Factor in any cost for developing the more complex system, and it falls apart completely.
« Last Edit: April 26, 2019, 05:16:50 pm by Siwastaja »
 
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Offline OM222OTopic starter

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Re: Balance charging lithium cells
« Reply #14 on: April 26, 2019, 06:13:57 pm »
I haven't done any actual testing with batteries, but when it comes to capacitors (especially super capacitors) they need to be balanced every single time as they have different capacities and discharge and discharge at different rates and become unbalanced very quickly. the cost of that would add up very quickly in the waste heat (especially given the main goal is charging at insanely high currents. e.g: 5 or 6 amps!). So I would assume batteries would be the same and discharge at different rates (regardless of self discharge). especially the recycled ones that are used in most DIY power wall type applications where the batteries might not even be the same model (e.g: mixing samsung cells with LG ones). you don't want to be throwing out tons of your input power simply because one cell has been charged to its limit.
I agree in a single pack that wouldn't make much sense but if you scale it to a power wall type project, waste energy scales just as much so efficiency is important.
« Last Edit: April 26, 2019, 06:15:43 pm by OM222O »
 

Offline digsys

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Re: Balance charging lithium cells
« Reply #15 on: April 27, 2019, 12:02:23 am »
Totally concur with Siwa.. on his comments, that's why ours is a plug-in maintenance option, NOT embedded in the pack. It is only ever used IF the BMS says
it needs it. Race batteries do get stressed a heck of a lot more than any other use scenario. To complete an event, we may go well past "ideal" operation.
Also, NEVER leave ANY active device (capable) on a battery pack. It can be very dangerous, should it fail, and transport options are FAR more restricted than
a pack of raw lithiums. We've seen the after effects of "shunt" failures.
Super caps are another ball game. I have been working on a different implementation of super-cap usage the last couple years. Hope to have results in a few
months. It will do away with the very wasteful balancing problem.
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Offline OM222OTopic starter

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Re: Balance charging lithium cells
« Reply #16 on: April 27, 2019, 12:13:13 am »
so you mean once the batteries are balanced initially there is no need for balancing them ever again?
then why do balance chargers even exist as opposed to a CC-CV charger with different cut off voltages for different packs?
 

Offline digsys

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Re: Balance charging lithium cells
« Reply #17 on: April 27, 2019, 12:29:29 am »
There are many answers to THAT question. I have to run very soon, so I'll do a couple -
We receive "hand matched" cells from manufacturers, who are our sponsors. ALL production processes have tolerances, and there is NO WAY they can afford
to batch match every single cell (we're talking about millions of cells). They have no idea what pack sizes are being made anyway. So they ship them to
equipment / vehicle manufactures. They ALSO do not have the time to "hand match" every cell ! so a simple / cheap balancer is installed.
In our case, we STILL spend the time to run 3x FULL EXTRA balance cycles on our already "specially" balanced batch, and always find 1-2 out.
BUT, once they are matched well, AND (big AND), the cells are run within limits, they rarely ever need it again. At the very least not under 1-2 yrs, and then
it may be minor. Outside of "proper" operation conditions, which people often PUSH, all bets are off. On top of that, people are often "re purposing" cells etc.
In short, it is a cheap / easy way to have them start life as best as possible. more later
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Offline Siwastaja

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Re: Balance charging lithium cells
« Reply #18 on: April 27, 2019, 06:18:11 am »
I haven't done any actual testing with batteries, but when it comes to capacitors (especially super capacitors) they need to be balanced every single time as they have different capacities and discharge and discharge at different rates and become unbalanced very quickly.

Supercaps and li-ion cells are completely different beasts. Don't assume any similarity.
 

Offline Siwastaja

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Re: Balance charging lithium cells
« Reply #19 on: April 28, 2019, 12:26:30 pm »
so you mean once the batteries are balanced initially there is no need for balancing them ever again?
No one said that. This being said, sometimes this is the case.

I'd guesstimate that most of the commercial multi-cell li-ion packs do not use either balancing or even cell-level monitoring. At least considerable number; including well known brands, without any problems.

Quote
then why do balance chargers even exist

Sometimes the leakage differences are large enough to shorten the product lifetime. My previous example number set isn't such a case; it would do fine without balancing. But assume 10 times worse leakage difference; this is still possible in the real world. Now, if the battery unbalances at about 1% per year, it starts to be a real problem at about 5 years - so balancing helps. But even then, the "payback for redistributive balancing" would still be 30000 years!

So, lithium ion cells tend to be almost good enough to require no in-system balancing whatsoever. This creates the awkward "it depends" situation, necessitating real system level understanding.

In reality, sometimes balancing is needed, sometimes not. When balancing is used, it does so little work that it's important to optimize for:
* Reliability
* Low cost

instead of "efficiency", or a fancy feature set. I have seen so many utterly failed li-ion balancer products that started from a complex list of feature requirements, even though in reality, they are irrelevant.

So when the balancer is needed, it needs to reliably sit there, and do it's minuscule yet important job when needed, and only when needed - and correctly.
 

Offline KL27x

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Re: Balance charging lithium cells
« Reply #20 on: April 28, 2019, 09:30:54 pm »
Quote
So, lithium ion cells tend to be almost good enough to require no in-system balancing whatsoever. This creates the awkward "it depends" situation, necessitating real system level understanding.

In reality, sometimes balancing is needed, sometimes not.

In my opinion, the balancing is rarely necessary for increasing cell life, regarding longevity or number of cycles. But it can very slightly increase the cell capacity right after a full charge. When I say slightly, I mean very slightly, and this applies mostly if the battery is used immediately after charging it. So balancing would be important for something like an EV, where you have massive weight/space in the batteries. Or a cell phone, which it going to be on 24/7 and sucking battery right after you unplug it. (Not that a single cell battery ever needs to be balanced, of course).

But in many other uses, it is not going to do a whole lot of improving. Say for something you recharge then put away. And it may be more effective just to buy batteries with a larger capacity to start with and/or adding more and/or larger batteries.

This is because my experience is that the bad/weak cells start to self-discharge faster, but it doesn't appear to be constant/linear. The bad cells seems to quickly SD from 4.20 down to say 4.00-4.08. So even if you go through the hoops to get all cells in a series to 4.20, the bad cells lose that last bit of full charge relatively fast, anyway. So in balancing them, you are not necessarily fixing something that is constantly drifting farther out of whack until it is causing some major problem. Those individual cell voltage discrepancies will reappear on the very next charge. You will have to re-balance them almost just as much on every subsequent charge cycle until you replace the weak links, and you are doing that only to reclaim a relatively small increase in the total charge.

This is somewhat limited in sample size. I'm probably over-extrapolating and oversimplifying and jumping to some conclusions. People are still designing products that occasionally catch on fire. But I wonder how much of that is simply due to manufacturing faults in the cells, themselves.

edit: Hmm, I guess the way to describe it is that assuming there IS a creep over time issue that is actually corrected by the occasional cell balancing, this issue might be riding on top of a larger discrepancy that is not getting "fixed" in the same way.
« Last Edit: April 28, 2019, 10:27:28 pm by KL27x »
 

Offline OM222OTopic starter

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Re: Balance charging lithium cells
« Reply #21 on: April 28, 2019, 11:05:47 pm »
if what you are saying is correct, then one should be much more concerned with the health of their pack, rather than it's balance. I.e: measuring individual cell voltages and comparing how much they discharge and warn the user if one or a few of them are degrading much faster than the others  ??? this will prevent some major failure and the batteries are cheap enough to replace anyways!
 

Offline KL27x

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Re: Balance charging lithium cells
« Reply #22 on: April 29, 2019, 12:02:33 am »
I'd put it that

IF one is more concerned about pack life than capacity per charge, it MIGHT be in your best interest to only balance the cells every X charge cycles rather than on every single charge cycle. But I don't know for sure if that is the case, even if this is correct under at least some specific set of circumstances. :)

This also assumes to some extent that the battery might be maintained by cell replacement rather than fully replaced. Like if your parameters say the device must run for X time for the battery to still be considered good, the one that is balanced every time might meet that for a higher number of cycles but in that time suffer slightly more decay/wear on the good cells... but no one cares, because at that point the entire battery will be replaced, anyway. The one that is not resistor-bled-overcycled as often might drop below that X limit sooner due to that weak cell not being corrected on every cycle.... but it might be in better shape after some maintenance including replacing individual cells.

 

« Last Edit: April 29, 2019, 12:14:58 am by KL27x »
 

Offline jbb

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Re: Balance charging lithium cells
« Reply #23 on: April 29, 2019, 04:07:05 am »
It may not happen very often, but if you get a small self discharge imbalance in a series pack then you can lose huge amounts of capacity. I’ve had this happen to a couple of infrequently used packs; you take a few hundred mAh out and one cell cuts off with undervoltage. Then you change it and another cell cuts off with overvoltage.

For your own use, you could probably ignore it and see what happens.

For mass production, you’d run the risk of customer dissatisfaction and needing to replace packs.

In any case, LiIon and LiPo packs should probably include under and over voltage protection for every series cell. Because fires are bad.
 

Offline Siwastaja

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Re: Balance charging lithium cells
« Reply #24 on: April 29, 2019, 03:24:33 pm »
In any case, LiIon and LiPo packs should probably include under and over voltage protection for every series cell. Because fires are bad.

On the Internet forums, everybody "knows" this is always required, otherwise a fire ensues.

In reality, BOSCH products do not go up in flames, despite not having any series cell monitoring whatsoever.

Most fire incidents are due to cell manufacturing defects, typically in single-cell applications.

But, your distinction between monitoring and balancing is a correct and an important one. Cell voltage monitoring may be important for both safety and functionality. Balancing is a secondary improvement which may or may not be used with cell voltage monitoring, to prevent pack capacity reduction caused by imbalance (where OVP and UVP events are caused by different cells). Balancing rarely "protects" the pack.
« Last Edit: April 29, 2019, 03:30:22 pm by Siwastaja »
 


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