Battery not fully charging, top balancing issue?

DanielJLee

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Jul 19, 2021
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Hi,

I am in the middle of my first project and I am having an issue with my cells not reaching 3.65v when charging.

When cell 2 reaches 3.65v, the BMS shuts down, but cell 7 is 3.55v.
Also after charging the cells quickly discharge to ~3.37v without anything connected, is that normal?

From my reading, this seems like a top balance issue. Is that right?
The cells are welded together, am I still able to successfully top balance them?

Here is a quick video showing the issue.

I'm having this issue with two different batteries, and two different BMS's.

Any help is greatly appreciated.

Here is the battery that I'm using, Signature Solar 25.6v 8s 100ah pack.
Here is the BMS that I'm using, JBD 8s 100a.
I'm also using the 24v, 40a EG4 charger from Signature Solar, which appears to no longer be on their website.
Attached are screenshots of the BMS settings.
 

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Yes top balance is your issue. Once your cells get balanced it should fully charge without the BMS cutting off.
 
Will I run into any issues with the terminals being welded together in series to make one pack?
 
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Yes, do not parallel a series pack. Instead use a 12v light bulb like out of an rv light or car blinker/brake light to drain the higher charged cells down to match the lowers. Just put the bulb on one cell at a time to drain it while watching the voltages on your BMS monitor.
 
Do you mean do not top balance a series pack? It's a 24v system so I'll try to find something to attach to each cell.
 
One option would be to put a small amount of discharge on the whole bank, say by about 10% or more, and then recharge again. This should give the bms bleeders a little more time to get closer. You may have to do this a few times to determine if you have a *true* issue.

From this distance, it almost appears like you are basically restarting a charge over and over trying to achieve 3.65v, which is NOT always necessary to achieve full capacity.

In other words, if you are just repeatedly doing charge cycles on an already pretty fully charged batteries without any discharge in between - in an attempt to reach 3.65v, you may be in essence trying to push the cells beyond their capacity limit already reached at a lower CV voltage.

While the light-bulb trick on a cell is faster, it is easy to go too far, or too little and you spend the whole day doing this dance.

I may be wrong, but I have the feeling that if you gave the bank a bit of a discharge between charges, giving the internal bleeders some time to do their job, and not expecting them to go to 3.65v, but to appear *balanced* in similar voltages, you'll be pleased.
 
going back to the beginning..
why on earth do you cycle your cells to 3.65 ?

there is hardly any power over that, and loading them up to 3.65 every cycle is going to shorten their life.

given above, 3.55 is already over your top limit.
as said there is no power, so voltage will rise extremely quickly above 3.5.

you could do a good top balance ( check resources section) , after set inverter/mppt/charger to bottom 2.7 and top 3.45 and be happy.

if you need more power than what you get from that range, you need more cells
 
Once charged the cell voltage will fall. Varies depending on cell size and type. Around 3.4 volts is typical.
With welded buss bars you cannot use the traditional method of top balance so there is no chance of cell damage with the bms in place.
With your pre built battery lower the charge volts to give the BMS balance time too function.
3.55 on a cell is almost full, I would be quite happy if the cells were in the range 2.55 to 3.65 volts.
Lower the charger volts to 28 volts and enjoy using the battery.

Mike
 
Do some research on Active Balance circuits. These devices actually use the charge in the high voltage cell to charge the lowest voltage cell. Over time, the cells will achieve a similar soc and slowly but surely the top balance issue will greatly diminish if not go away completely. Some cells will run off from the others no matter what you do to try to get them all acting the same.
The active balancer will slow the run away cells by increasing the bypass current, as much as 5 amps, and once the BMS interrupts charging because of max battery voltage or max cell voltage, the active balance moves the charge from the highest voltage cell to the low voltage cells until all cells are all the same voltage. This process is automatic and relieves you from having to spend literally hours manually discharging high voltage cells.
There is an issue that can be addressed through the initial corrective phase. Lithium batteries need to go through an absorption process where they are charged to some level at a constant current and then held at a max level for about 30 minutes. That last constant voltage time constitutes about 10% of the charge. If the BMS cuts the charge process early, the absorption time is aborted. To improve initial performance you can simply find the max voltage where your run away cell triggers an end of charge and set the max charge voltage just a bit below that. At least theoretically, over time you should be able to increase that max charge voltage as the batteries come into sync with a similar SOC (state of charge).

This is my personal observations on the behavior of two 100AH 4S8P LFP batteries. I was about to give up on getting these batteries to take a charge without being interrupted way too early and then I found out about Active Balance circuits. No more worries, it just works.
 
I will not advise you to do it unless you are very familiar with this.
You can balance each cells individually even if they are welded in Series.
Use benchtop power supply and apply leads across single cell set your power supply to 3.65 and whatever your power supply can deliver for current. Once battery is getting saturated it will start using less and less current. Once it drop to 0.2-0.3 A that would be 99.8% SOC. Now you move to the next and repeat until you get all cells individually balanced using benchtop power supply.
While doing this disconnect bms by following manufacturers instructions.
For fully charged lifepo4 chemistry at 3.65 when fully saturated it is normal that it will drop after 24 hours to 3.370 V +/- ( depending on chemistry). This will still be even at this V 100% SOC. Reason why I suggest you disconnect bms is because it will probably take 24 hours to complete all cells individually and voltage drop on first cell will cause if bms was left running to keep overcharging first cell you already have top balanced. And you should not ever have need to charge cells over 3.400 V . With absorption it will be at this per cell voltage 99.8 or even 100% SOC if you let absorption long enough to stop taking any current. For discharging individual cell in the series you can follow posts above by other people suggesting. Your battery pack voltage should be 54.4 V when fully charged and if not used when fully charged and absorption have switched idle. Usually over 24 hours it will stay at 53.9 +/-. And also when charging you cannot use constant current. Current is controlled by SOH and SOC of the cell. As cell is closing to the 100 SOC it will start tapering current.
 
Also in practice not balancing simply means loosening some capacity. It’s not the end of the world either. Repeated fractional C charge and discharge will then to make cells converge over time also , high C will tend to make them diverge
 
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