Did the bms drain the cell group?

[j bjork]

When I built the battery the cells were perfectly balanced at a little over 3,6V
It is a 13s5p with genuine, new sanyo cells and a cheap chinese bms.

I did not charge the batterys afterwards, I just connected the carger to see that everything works. I thoght it would be perfect voltage to store the battery until the bike was finished.

I think it could have been 2 mounth or so, before I was ready to use the battery. I noticed it had lost some voltage, I think it had went from about 46V to about 42V..
I thought it was a bit odd, but did not think to much about it at the time, but I charged the battery and started testing the bike.

Anyway, I started to get power cuts from the bms and took apart the battery. Every cellgroup was in perfect balance and about 3,9V I think. Exept the first group on the negative side, that had about 3,1V as I remember it.

I tried to charge, and it overcharged the other groups to 4,25 or 4,26V, and the negative group to 3,7V I think.

I then charged that group with another charger to 4,2V, it charged it with a little over 10Ah. It has been about a week since i charged, and the other groups have went from 4,26V to 4,20V and this group have went from 4,2V to 4,17V

I dont think there is something wrong with the battery, so I suspect the bms.
It seems to me that the easiest way to power the electronics in the bms would be to take the power from the first negative cell group. That could be the same on every cell count, the first negative is always precent. And probably a more suitable voltage for the electronics than the full 48 ish volt.
Is it possible that it is built this way?

 

[kcuf]

some built that way

20-50ua current quiescent consumption common adds up 24/7

sometimes bleed balance circuit short or turn on and never turn off

if so channel balance resistor will be warm to touch dissipating 50-80ma

always best to unplug bms long term storage

 

[fechter]

Most cheap BMS use the first 3 cells to power the circuit. If only the first cell is going low, it would be good to make sure the balancing shunt isn't stuck on.

Most BMS boards will have a row of resistors for the balancing shunts. If one is turned on, the resistor will get warm and it will have a voltage across it. By measuring the voltages across all the resistors, you can tell if one is on.

 

[robb]

I have the same problem. Is there a way to replace or repair the balancing shunt? I am going to replace the BMS with this bluetooth one https://www.aliexpress.com/item/13S...034.html?spm=a2g0s.9042311.0.0.49614c4degGUoP when I receive it from China. Are thye likely to have the same problem.

 

[fechter]

I have the same problem. Is there a way to replace or repair the balancing shunt? I am going to replace the BMS with this bluetooth one https://www.aliexpress.com/item/13S...034.html?spm=a2g0s.9042311.0.0.49614c4degGUoP when I receive it from China. Are thye likely to have the same problem.

It would be pretty challenging to repair a blown balancing shunt, but I have simply removed them on bad boards before. The over/under voltage cutoff points are generally done by a different chip so still work, just not the ability to drain the cell if it goes high.

Since a generic BMS board is relatively inexpensive, it works out better to just replace a bad board.

Most designs have the same weakness for potentially blowing up a channel when first connecting the board. Some are better than others. Following the proper connection sequence minimizes the chances of blowing something up.

 

[robb]

Since a generic BMS board is relatively inexpensive, it works out better to just replace a bad board.

Thanks, that is what I will do.

 

[Matador]

Since we're in the subject and I hear you guys have a lot of experience with BMS. Do you think this BMS is likely to be on that drains cells unequally (example the last 3 cells groups closer to the negative ) ? or should it be alright.

I'm currently storing it at 51.6 (avraging 3.68V per cell)

Here's the 14S BMS:
https://www.ebay.com/itm/14S-51-8V-45A-BMS-PCB-PCM-for-ebike-electric-bicycle-18650-Li-ion-LIPO-battery-/112636580964

Here's the battery I build with it:View attachment 1View attachment 2

Should I be worried, storring it until may-june ?
Matador

 

[fechter]

Since we're in the subject and I hear you guys have a lot of experience with BMS. Do you think this BMS is likely to be on that drains cells unequally (example the last 3 cells groups closer to the negative ) ? or should it be alright.

I'm currently storing it at 51.6 (avraging 3.68V per cell)
Should I be worried, storring it until may-june ?
Matador

It should be fine. The drain on the first 3 cells is about 10uA more than the others. You may want to check the balance next time you charge it. If needed, charge it fully to make it balance.

 

[Matador]

Thanks for the advice Fechter ! Will do. I did balance charge it for 12 hours the last time I fully charged it and then discharged it to 51.6V. Will rebalance everything when summer comes. I check bulk voltage every month too.

Matador