10s4p x2 in series power cut out too soon

[Hans_J._plourde]

Hey,
I got 2(10s4p) LG MH1 connected in series and fully charged I get 84v
The controller is generic Chinese 48 to 72v and 26 to 33A
My issue is that between 65v and70v the voltage drops to zero

Either the bms or the controller cut out the power
Thé LG MH1 cell data sheet says that the normal discharge is at 2.5v per cell however the cut off occurs above 3.3v

3v would be acceptable
Any suggestions?

 

[glennb]

Voltage sag under load? Can you give it a tiny bit of throttle when the battery’s at 65-70 without it cutting out?

 

[E-HP]

Hey,
I got 2(10s4p) LG MH1 connected in series and fully charged I get 84v
The controller is generic Chinese 48 to 72v and 26 to 33A
My issue is that between 65v and70v the voltage drops to zero

Either the bms or the controller cut out the power
Thé LG MH1 cell data sheet says that the normal discharge is at 2.5v per cell however the cut off occurs above 3.3v

3v would be acceptable
Any suggestions?

Looking at the discharge curves, seems that at 33A, the 20S4P pack may be sagging about 10V.

 

[Hans_J._plourde]

Voltage sag under load? Can you give it a tiny bit of throttle when the battery’s at 65-70 without it cutting out?

It’s 2 new battery packs made from refurbished LG mh1 And I have charged it 5 times so far

And when power cut off/out I’m riding it and the voltage display shoes countdown voltage as it fades out
I think that one cell is weak or unbalanced, that would explain the early cut off.

Yet before I cut open 1 or both I hope I could get some troubleshooting advise

 

[glennb]

Without knowing the BMS and controller LVC there’s nothing to suggest weak cells.

3.3V is relatively high for a BMS or controller LVC, but still an appropriate one for many/most cells, and particularly in a “refurbished” battery.

You might also want to calculate how much energy remains in MH1 cells below 3.3V before trying to extract it.

Have you tried to isolate which device causes the cutoff?

Could you use a coulomb counter to see how much energy is going in or out of the battery?

Your battery could indeed have problems. Holding an 84V charge tells you little about its health without knowing balances. If the cells were not meticulously tested, sorted, labelled and balanced before the pack was built, you’re pretty well guaranteed to encounter BMS cell level LVC before the pack drains to 50V (20s X 2.5V).

You’re working blind without knowing more about the controller LVC, the BMS LVCs and balancing abilities, and how much energy the battery is storing.

If you can unplug the balance leads to use them for probing with a multimeter, you might choose to. However, you might want to first establish that there is a high likelihood of weak cells.

 

[docw009]

Measure the ten group voltages in both batteries. One of the groups is sitting a few tenths of a volt below the rest, When the battery is down to 65-70 volts, the average group voltages are running 3,2-3.5V per cell. All you need is one group running a little lower than that and a little extra load triggers enough voltage sag to shuts off one of the two batteries. In my opinion, it is not unusual for a well used pack, even made with MH1's, to shut off around 33V.

The fix is to see if the battery can be balanced so all ten groups in both batteries can be equalized. You either have a balance BMS that can do this, or you do it manually, charging the low group(s) til they are at the same level. If the cells are healthy, they will stay in balance, If not, they will drift apart with time.

FYI. when one of the series packs shuts off, its BMS transistors will have to withstand the entire battery voltage of 70V. They are probably rated for 80V so it shouldn't be a problem. If you had two 13S packs, it would be a problem.

 

[Hans_J._plourde]

Thanks for taking the time to reply
The BMS is à Chinese generic without info about LVC and same for the controller
I feel that I must take both packs and find the weak cell(s) and replace it (them).
If there are no guarantees that the problem is indeed faulty cells I cannot come up with any other suggestions
I’ll keep you posted if it interest you
Thank you again for your insights
Cheers

 

[Hans_J._plourde]

Looking at the discharge curves, seems that at 33A, the 20S4P pack may be sagging about 10V.

Hey E-HP, it's been a while but I wanted to come back to your comment because I believe that you are unto something here and I wish you could elaborate a bit and perhaps suggest a way to rectify that. Because if the controller shuts off around 65v it always and only shuts off while accelerating ...