General > General Technical Chat
LiFePO4 balancing.
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shapirus:
I don't know. It sounds to me like you are trying to reinvent the wheel for an unclear purpose.

Here is my personal example. Three LiFePO4 batteries. All three use Heltec active capacitor-based balancers which balance the cell voltages continuously regardless of the SoC level, unless disabled via the RUN jumper.


1. 4s, 600 Ah.





2. 4s, 18 Ah.




3. 8s, 18 Ah.

No direct access for multimeter, so in this case cell voltages are taken from the BMS reporting over a serial port.


--- Code: ---Command: cellVoltages - Cell Voltages Information
--------------------------------------------------------------------------------
Parameter       Value           Unit
cell_01_voltage 3.409           V   
cell_02_voltage 3.411           V   
cell_03_voltage 3.411           V   
cell_04_voltage 3.411           V   
cell_05_voltage 3.411           V   
cell_06_voltage 3.411           V   
cell_07_voltage 3.411           V   
cell_08_voltage 3.409           V   

--- End code ---


The only situation where this wouldn't be sufficient that I can think of is very high-capacity batteries made of poorly matched cells where the balancing current achievable by these balancers won't be high enough to catch up. But they can be parallelled, if necessary.
Siwastaja:

--- Quote from: shapirus on March 09, 2023, 01:17:00 pm ---Heltec active capacitor-based balancers

--- End quote ---

Fun to see how the world goes on, and it seems Chinese are now producing cheap redistributive balancers and people are apparently buying them. Didn't know that. And I'm sure they work better than the overly expensive and complicated products I have seen years ago.

... but then again:

--- Quote ---The quiescent current is about 12 mA

--- End quote ---

So an energy saving device the only purpose of which is to possibly maybe save 1mA is wasting 12mA by just doing the saving.

If it works, then it could be usable with semi-faulty packs which have high-leakage cells. Although such cells could be a fire risk anyway.
shapirus:

--- Quote from: Siwastaja on March 09, 2023, 03:18:42 pm ---
--- Quote ---The quiescent current is about 12 mA

--- End quote ---

--- End quote ---
...a significant part of which is the indicator LED. Can be removed if necessary, and then again, there's the on/off jumper which can be shorted, for example, only when the charger is connected.




--- Quote from: Siwastaja on March 09, 2023, 03:18:42 pm ---So an energy saving device the only purpose of which is to possibly maybe save 1mA is wasting 12mA by just doing the saving.

--- End quote ---
It's not an energy-saving device, at least, it's not its primary purpose. Having a balancer is essential to prevent an uneven degradation of the cells and, in case there is no per-cell voltage monitoring, to prevent cell damage.

When there is no balancer, even with well-matched cells, disbalance, even if tiny in the beginning, increases over time, cycle after cycle. When the battery is used in buffer mode (e.g., a typical UPS), battery without a balancer will slowly drift out of balance even if no switching to battery occurs, because the self-dicharge rate of the cells is inevitably unequal while the charger will keep the total voltage of the pack constant.

Different types of balancers exist. Some require the user to make sure that the cell voltage at which they start balancing is actually reached at the target charging voltage of the entire pack (so as not to end up with one cell being at 3.65V and others at 3.35V or so).

For LiFePO4 packs, especially in buffer mode (float charging) and/or with final charge voltage well below the typical 3.60V/cell (say 3.35-3.45V/cell) these capacitive active balancers are what I find to work best for me. It's as plug-and-play as it gets, no worries about the acceptable voltage range or anything else. I think they also go into a soft power-off state when the cell voltages go below a certain threshold to prevent overdischarge (such as when the battery is kept unattended for a long time), but I'm not 100% sure about this.

For LiPo packs used in the cyclic mode, those typical for the RC hobby use, I'm satisfied with the built-in resistor-based balancer in my Turnigy Accucell 6.
Siwastaja:

--- Quote from: shapirus on March 09, 2023, 03:54:45 pm ---It's not an energy-saving device
--- End quote ---

Compared to a classic dissipative balancer, it is. Both perform the job of keeping cells balanced just as well.

There is, of course, a corollary to saving energy: larger balancing currents also become possible because power dissipation is not a problem. Although I highly doubt the need for such high balancing current. My experiments on aged, damaged, second-life 240Ah LFP pack showed that 40mA balancing current is sufficient, with a proper algorithm. Other products of the time used dumber algorithm and worked well with 200mA-500mA. I find it extremely unlikely that 5A balancing current is truly needed.

Voltage ranges, user experience, plug/play or not, is totally independent of if the underlying type is dissipative or redistributive.

OP is considering designing their own BMS, for whatever reason. "Why do that when you can buy an off-the-shelf product" is useless advice, although one can and should look at off-the-shelf products for motivation.
Siwastaja:
If a redistributive balancer with high current capability continuously balances the pack (at any voltage), there will be two sides to this coin:

1) if the cell capacities differ a lot, usable pack capacity will increase. Classic top or bottom balanced pack gets the capacity of smallest cell. Continously balanced pack gets the average capacity of the cells (assuming 100% efficiency for redistribution)

2) if the cell capacities do not differ a lot, but ESRs do differ, usable pack capacity will decrease, as the balancer misidentifies small voltage differences as the cells being at different SoC even if they aren't, and redistributes charge incorrectly, having to then later correct itself. Energy is wasted in this back-and-forth redistribution.

This uncertainty is why I'm not a big fan of continuous redistribution. It looks nice when the voltages are seemingly equal, but equal voltage at every moment is not exactly the right metric to look at.

The big case for this continuous strategy would be packs where cell capacities are unstable and large differences emerge. With current li-ion not having such a problem, this future chemistry with said problem should be competitive in some other way.
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