Top Balancing Acting Weird

[ender]

I have two SOK 12V 100Ah batteries that I'm top balancing by wiring them in parallel and applying 14.4V with a current limit of 4A on the supply. Everything's been fine for a couple of days and today it finally hit 14.4V. This whole time the supply display has shown CC at 4A limit until it got closer to 14.4V at which point the current on the supply started dropping (makes sense) and eventually went to 0A when it hit 14.4V. It stays at 14.4V and 0A for a few minutes and then the voltage will drop to 13.9V and the supply will show 4A current again until it slowly goes back to 14.4V. It keeps doing this over and over.

Here's a short video I recorded with the supply at the bottom left and a multimeter reading the battery terminals voltage:

Can someone help me understand what's going on here?

 

[RCinFLA]

Sounds like BMS shut down for over cell voltage trip. Having two batteries in parallel would be unlikely they both trip at same point in time . Is the drop off of current in distinct steps like one trips first then some time later the second battery trips? Eventually it will stop doing that when four cells get balanced in each battery.

You might back down the absorb voltage to 14-14.2v to give balancer more time to operate.

 

[ender]

Sounds like BMS shut down for over cell voltage trip. Having two batteries in parallel would be unlikely they both trip at same point in time . Is the drop off of current in distinct steps like one trips first then some time later the second battery trips? Eventually it will stop doing that when four cells get balanced in each battery.

You might back down the absorb voltage to 14-14.2v to give balancer more time to operate.

Not sure how I'd be able to tell if one battery is tripping before the other one.

This doesn't make sense to me though. If I hit 14V and it stays there for a few minutes with 0A of current draw, why would the BMS trip? I understand if it tripped while current was being drawn from the power supply and voltage was going up but when it's already settled and there's 0A? Also, according the SOK datasheet I've attached here, their recommended charge voltage is 14.6V and BMS Charge Cut-Off Voltage is 14.6V (3.65V/cell). After seeing this, I set the power supply to 14.6V and it seems like the batteries go up to 14.6V, current drops to 0A and it stays that way without the voltage dropping down again. So it seems like 14.6V is the way to go to top balance two SOK 12V batteries in parallel. Maybe 14.4V is right at a BMS threshold of some sort so it acts weird?

 

[RCinFLA]

Balancing current is likely only 50-100 mA resistor dump per cell. When cells are out of balance, the greater the charging current, compared to balancing current, the more likely one cell will reach its overvoltage protection voltage and shut down BMS. When BMS shuts down for a cell overvoltage, the bleed current will continue after BMS shuts off. When overvoltage cell bleeds down to its BMS reset voltage the BMS will turn back on.

When you get in this situation the BMS re-engages when overvoltage cell bleeds down to about 3.5v so it does not take much charging to put it back to overvoltage trip again. This is recoverable but takes a lot of time and aggrevation. This is why you need to keep up with balancing by taking battery charge above 14.2v at least every couple of months.

Process may repeat several times until cells finally get into balance enough to allow the charger to get to its absorb voltage setting.

Reason why I asked if you saw a step drop off in two distinct steps is with two batteries in parallel it is unlikely they both will shut down at same point in time. They may both shut down but it would be one drops first resulting is a sudden drop off in amount of current, then after some additional time the second battery shuts down finally net going to zero current.

If you hit absorb voltage you likely would not go to zero current in a few minutes. It would taper off gradually over a longer time. Immediately going to zero current is a BMS shutdown.

By setting absorb to a lower voltage, 14.0-14.2v, you give a bit more time before the highest state of charge cell reaches its overvoltage trip, allowing more time to bleed balance. Lower charge current also gives more time to balance.

When you have two batteries in parallel, when BMS on a previously tripped battery reengages, the other battery will put a fast charge into the just reset battery causing it to trip again in a short time. I would not recommend trying to rebalance these batteries in parallel. It will cause more thrashing BMS resets. You can actually get into a ping-pong situation between the two batteries' BMS's. It will eventually work out but it will lengthen the time before it does.

Balance bleed on a cell typically does not start until a cell gets above 3.4v.

If you are not in a hurry just charge at about 100 mA and after several days it will fix itself.

 

[ender]

Balancing current is likely only 50-100 mA resistor dump per cell. When cells are out of balance, the greater the charging current, compared to balancing current, the more likely one cell will reach its overvoltage protection voltage and shut down BMS. When BMS shuts down for a cell overvoltage, the bleed current will continue after BMS shuts off. When overvoltage cell bleeds down to its BMS reset voltage the BMS will turn back on.

When you get in this situation the BMS re-engages when overvoltage cell bleeds down to about 3.5v so it does not take much charging to put it back to overvoltage trip again. This is recoverable but takes a lot of time and aggrevation. This is why you need to keep up with balancing by taking battery charge above 14.2v at least every couple of months.

Process may repeat several times until cells finally get into balance enough to allow the charger to get to its absorb voltage setting.

Reason why I asked if you saw a step drop off in two distinct steps is with two batteries in parallel it is unlikely they both will shut down at same point in time. They may both shut down but it would be one drops first resulting is a sudden drop off in amount of current, then after some additional time the second battery shuts down finally net going to zero current.

If you hit absorb voltage you likely would not go to zero current in a few minutes. It would taper off gradually over a longer time. Immediately going to zero current is a BMS shutdown.

By setting absorb to a lower voltage, 14.0-14.2v, you give a bit more time before the highest state of charge cell reaches its overvoltage trip, allowing more time to bleed balance. Lower charge current also gives more time to balance.

When you have two batteries in parallel, when BMS on a previously tripped battery reengages, the other battery will put a fast charge into the just reset battery causing it to trip again in a short time. I would not recommend trying to rebalance these batteries in parallel. It will cause more thrashing BMS resets. You can actually get into a ping-pong situation between the two batteries' BMS's. It will eventually work out but it will lengthen the time before it does.

Balance bleed on a cell typically does not start until a cell gets above 3.4v.

If you are not in a hurry just charge at about 100 mA and after several days it will fix itself.

Had to read your post a few times to internalize what you were saying but now I get it. Thank you for the thorough response.

At this point the only thing that confuses me is the fact that when I set charge voltage to 14.6V it eventually drew 0A and stayed there without this issue occurring. Mind you, the current slowly drops, it doesn’t just drop to 0A in one shot. So it doesn’t seem likes it’s the BMS tripping.

 

[RCinFLA]

Had to read your post a few times to internalize what you were saying but now I get it. Thank you for the thorough response.

At this point the only thing that confuses me is the fact that when I set charge voltage to 14.6V it eventually drew 0A and stayed there without this issue occurring. Mind you, the current slowly drops, it doesn’t just drop to 0A in one shot. So it doesn’t seem likes it’s the BMS tripping.

Then it is balanced well enough to make it to charger absorb voltage without one of the four series connected cells exceeding 3.7v overvoltage trip point.

For LFP cells, charging voltage rise on cell is very small until it reaches near full charge at 3.4v. Then its cell voltage increases rapidly. This causes the highest state of charge cell to jump in voltage quickly, near full charge, tripping its overvoltage limit. With all four series connected cells balanced they will all hit the finish line at the same time during charging.

Same applied to full discharge point. Weakest state of charge cell will drop quickly below about 3.0 vdc so it will trip undervoltage limit first. Result of imbalance is you will not get full capacity out of battery. If cells are not matched in AH capacity (not the same as balanced) you will only get the capacity of weakest cell.

 

[ender]

Then it is balanced well enough to make it to charger absorb voltage without one of the four series connected cells exceeding 3.7v overvoltage trip point.

For LFP cells, charging voltage rise on cell is very small until it reaches near full charge at 3.4v. Then its cell voltage increases rapidly. This causes the highest state of charge cell to jump in voltage quickly, near full charge, tripping its overvoltage limit. With all four series connected cells balanced they will all hit the finish line at the same time during charging.

Same applied to full discharge point. Weakest state of charge cell will drop quickly below about 3.0 vdc so it will trip undervoltage limit first. Result of imbalance is you will not get full capacity out of battery. If cells are not matched in AH capacity (not the same as balanced) you will only get the capacity of weakest cell.

All of this makes sense and it's what I've understood based on my readings. Still not sure why 14.4V caused this "issue" but 14.6V did not. Unless of course by the time I tried 14.6V the cells had had enough time to balance enough at 14.4V that when I went to 14.6V, the BMS did not have to do anything.

Maybe my confusion lies in the fact that Will states in his video to top balance using 14.4V but the SOK datasheet says recommends Charge Voltage of 14.6V and that Balancing Voltage is <14.4V. I'm not sure I understand what they mean by Balancing Voltage "<14.4V". Does this mean that if you set your power supply to 14.6V, that the BMS won't balance? If you can clarify this spec and why they use "less than 14.4V" maybe that will clear up my confusion.

Thank you for your time and help with this!