Unequal cell voltage on charge of Lithium 13S3P battery.

[Waynemarlow]

Trying to trouble shoot a 13S3P pack of Samsung INR18650-35E for a friend and have run out of ideas.

The problem is is the parallel pack of 3 batteries closest to the Negative ( 1st pack ) is charging at a different rate to all the rest of the pack. At the 4.2V switch off of the charger the first pack is 4.2 and the rest is 3.82V. The BMS then seems to switch in and drain all the cells down to the 3.8V level over night.

The spot welds and interconnects all look to be correct and with good welds.

I suspect also that 1 group of 3 in the centre of the pack has a poor connection on 1 cell as it too slightly over charges by 0.2V. Under load this group will drop significantly lower than the rest of the pack at lower voltages and trip the BMS early.

Any clues as why the first pack is charging differently to the rest of the pack ?

Thanks

Wayne.

 

[amberwolf]

The problem is is the parallel pack of 3 batteries closest to the Negative ( 1st pack ) is charging at a different rate to all the rest of the pack. At the 4.2V switch off of the charger the first pack is 4.2 and the rest is 3.82V.

If by "first pack" you mean this particular one, then if it's charging faster than the others it means it's capacity is much lower, or it's resistance is a lot higher, if they all started out at the same empty voltage down near 3.3-3.5v.

The BMS then seems to switch in and drain all the cells down to the 3.8V level over night.

Well, that part is weird, unless it is one of the "active balancer" types, which seem to generally attempt to put all cells to the same level as the lowest cell (which will kill an entire pack if the lowest cell is shorted!). A typical BMS should only drain cells down to whatever it's HVC is set to, if they go above that.

I suspect also that 1 group of 3 in the centre of the pack has a poor connection on 1 cell as it too slightly over charges by 0.2V. Under load this group will drop significantly lower than the rest of the pack at lower voltages and trip the BMS early.

That is probably a cell with too high internal resistance, whihc would do both of the things you describe.


It sounds like the cells in the pack are either just not very good, or are worn out--either way, they probably all should be replaced.

You could replace just the ones that are a problem *now*, but if the rest of them are the same age, they're close to having the same types of problems.

 

[Waynemarlow]

amberwolf thank you for the reply.

This is a newly built pack of new Samsung 35E cells from a know supplier, we always test for voltage levels before building a pack, but to have to check on resistance of each individual cell would be something we have not done before.

Could this possibly be poor spot welds, I can see the interconnecting strip welds but not the actual welds to the battery themselves.

Is there a technique that I can test each block of 3's resistance whilst built so that I can identify which sets of 3 are causing the problems ? If we find a set that's not performing as the others we could start to unpick the cells.

Thanks

Wayne.

 

[amberwolf]

Voltage sag under load *is* a direct test for internal resistance. If you get different amounts of sag on different groups while at the same state of charge and a high enough load, then the internal resistance of the groups is different, which means the cells are not matched very well, if at all. If the difference between them is high, then there's a problem with one or more of them (the worse the sag, the higher the resistance).


While under the high load, you can also measure across the cell itself to the tab material welded to that end of that cell, and there should be essentially zero volts. If there's a voltage there, then there is high resistance at that connection.

If it's a broken weld, pushing down on the tab against the cell will change the voltage drop, decreasing it.

 

[Waynemarlow]

Thanks

Yes almost certainly its poor welds, just poking around on the negative end of the one set that's charging at a faster rate and I've been able to pries the spot welds off with almost minimum force, shame as all the interconnects which are on top are welded well and to get to the battery connection to check I'm going to have to strip them off first.

Almost at the point that economically it maybe better to scrap the pack.

Thanks for your time guys, appreciated