Xiaoxiang's "Charge Balance" / Overkill Solar's "Balance only when Charging"

[Inq720]

I think these two "wordings" are for the same feature. I had my built battery sitting for two months at about 40% discharge (reading here that storing at 100% SOC was bad on cells). No external loads were on the battery during the two months. The Bluetooth was on as I checked in on the battery every so often. I went to charge the battery and the BMS kicked out with 2 cells reaching 3.65 volts. The two other cells (1 and 4) were at 3.364 and 3.372 volts.

Overkill says,

When enabled, the balancing routine will only be performed when charging.
When disabled, the BMS will enter Static Balance mode, where it will balance when both charging and
discharging.

I am now manually charging the two low cells because the BMS can't seem to handle the large discrepancy. Is there any information on how much delta voltage an Overkill Solar BMS can manage? Also, when in this "Static Balance" mode it sounds like it will try to keep them balanced all the time. Why would Overkill default to this being off? Why would using this feature be a bad thing?

Thanks.

 

[Diysolar123]

well, after carefully watching a smart daly with an amp meter on the balance leads, they do NOT balance anything.
Now granted I have a limit on accurate current reading so I cannot read anything below 10ma, but if the balance current (whether its active or passively draining) is less than 10ma, its pointless.
I think the only thing that happens is that if you let the cells charge, the BMS will disconnect when any cell of your pack hits the max voltage (3.65 or whatever you have configured it as)...then the cells will, quite naturally, all settle back to 3.3X'ish. Thats not balancing, its how lifepo4 cells react if you are not float charging them.

I expect that passive balancing when not charging is turned off because, well, think what that is doing...
It is "draining" off power in your cells even when there is no load.

 

[Inq720]

My volt meter has limited accuracy as well and considering the app has 3 significant digits, I'll go with that. Just for shits and grins, I turned off "Charge Balance" and for the first time, I finally see something in the circles to the right of the battery voltages. I'm guessing that indicates Cell 1 is (B)alancing Cell 2. Picking the lowest cell (#4) would have made more sense to me.

I understand that static balancing is "using" power. My logic goes something like this... The power is being used from one cell to charge another. We're only talking mA rates and in a previous test, I measured a chemistry efficiency of 97%. So... I may be losing 3% by doing that transfer, but we're talking fractional amp-hours. In the long run of having the cells better balanced and getting to full capacity would seem to outweigh the small bit of power being used. Are you suggesting that there are other losses that might be more significant?

 

[Gazoo]

Passive balancers bleed off the high cells using resistors and the balancing current is very small. As I recall it's only 70 or 90ma's with my 8S Overkill.. This is one of the reasons parallel top balancing is a must when using non capacity matched cells.

An active balancer transfers energy from high to low cells and uses much higher currents.

I have played around turning off balance while charging and other settings and it doesn't help because the balancing current is so small. I would suggest you either parallel top balance your cells, or charge them all individually to 3.65 volts. I would do the latter. There is no need to disassemble your pack. Turn balancing off and make sure there is no load connected to the pack.

 

[Inq720]

Passive balancers bleed off the high cells using resistors and the balancing current is very small. As I recall it's only 70 or 90ma's with my 8S Overkill.. This is one of the reasons parallel top balancing is a must when using non capacity matched cells.

An active balancer transfers energy from high to low cells and uses much higher currents.

I have played around turning off balance while charging and other settings and it doesn't help because the balancing current is so small. I would suggest you either parallel top balance your cells, or charge them all individually to 3.65 volts. I would do the latter. There is no need to disassemble your pack. Turn balancing off and make sure there is no load connected to the pack.

Thank you...

Now that explains the difference between passive and active (waste highest cell versus transferring it to the lowest).
I had just finished re-top-balancing as you suggested... each cell up with it still assembled.

Thanks.

 

[Bri from IA]

FWIW, I use a JBD BMS (Xiaoxiang App) with the balance function turned off. It only balances in the low milliamps, 20mA max. And I also have a JK active balancer in parrell. It doesn't provide any BMS protective fuctions, but does provide 2amps, i.e. 2000mA of balancing current from the highest voltage cell to the lowest cell. That's 100x and makes a huge difference.

You should know that charging with higher currrents, i.e. fast-charging, can make the cell-imbalance issue worse. If you charge with lower currents, it's easier to see a voltage spread develop and also easier for my active balancer to keep up.

 

[Corbury]

This was super helpful - i’ve seen this exact problem! I guess I need to 1) reduce my charge current; 2) get a JK active balancer; 3) re-top balance the pack (with the cells assembled) - i will search for a guide on this. Thanks!

 

[meetyg]

After experimenting alot with the JBD BMS balancing modes, I use it with "charge balance" off.
Recently, Andy from Off-Grid-Garage discovered the same behavior as I did, he also recommended to turn it off.
Basically this is how the two modes work:
1. If you have both "balancing" and "charge balancing" enabled, the BMS will only start balancing when all three conditions apply:
A. your cells are at a higher voltage than balancing voltage set
B. the voltage deviation is higher than voltage deviation set
C. the BMS senses charging is going on (this may depend on your specific model, but in general if you are charging at lower than 400ma, the BMS can't sense that).

2. If only "balancing" is enabled, but "charging balancing" is disabled, it's the same conditions as above, except for condition C.

Having that the BMS balancing current is so small, and that it can't sense very low charging rates, it fairly not practical to use "charge balancing".

I found that when I set the "balancing voltage" to a good number (say 3.451v per cell), it will keep on trying to balance even when not charging, when cells are at rest. Given enough time, they will balance (at least for a good quality pack that was properly top balanced, and depending on the capacity and imbalance of course).

But if your cells don't rest alot (meaning you cycle them often) you will probably need an active balancer anyways.

Here are Andy's experiments and conclusions about this:

 

[Porch]

Based on what I am seeing in my batteries, the small mount of discharge current the Overkill uses to balance the batteries is basically doing nothing. I am adding a Heltec to all my packs.

 

[OzRhys]

I can't find the original thread but I used this for my first charge. I built my battery and charged it. No top balancing, the first charge had one cell with a .2 difference. CATL 120a batteries for those who care. They all were resting for about 1 year at 3.2v.




I charged it up and then turned off the balancing only when charging option on my JBD BMS and it slowly took the difference out over night. I left the cells with no load for another day and then put a load on.



Over all I think if you buy cells from a good supplier who matches cells you don't need to balance before installing and the JBD can effectively balance the cells if you tell it to balance when it's not charging.

Disclaimer: I have no idea what I'm doing and I'm following others advice from experience.

 

[Brett V]

Cells showing as balanced at 3.3 volts is basically meaningless because you’re in the flat area of the cells SOC. I use the same method as Andy with the balance while charging set to off. Top the charge off at 3.50 - 3.55 and let the balancer do its thing as the cells settle back down, cutting off balancing at 3.40 volts.

 

[paircycle]

Ever thought about the fact that the cells are perfectly fine? The cause could be the voltage drop at the busbars, terminals or connection cables. At high currents, this causes cell drift. In addition, when charging currents fluctuated greatly due to cloudy weather in the upper SOC range, the voltages on some cells increased greatly.
Especially the long connection between cell 8 and 9 (16s) has regularly led to cell overvoltage or undervoltage.
Could solve the problem with me by better connections. The same resistance between all cell connections.
Now even at high currents everything well top balanced.