Rapid voltage drop on electric scooter - BMS issue?

[Dishwashertaco]

Hi,

I bought a second-hand Mercane Widewheel 2019 1000w scooter a few months ago, and have since had a few problems with it. I recently changed the hall sensors in the rear engine, and was hoping this would fix everything. Rear engine now works fine, but another issue has appeared.

Yesterday i went for a 10km ride with my scooter - while riding on flat terrain with over half of the supposed battery capacity remaining (13s6p), suddenly the voltage dropped rapidly and the scooter powered off. Later testing (with a multimeter connected to the charge port) shows that turning on the scooter causes the voltage to drop rapidly until it stops at 3,9v. The voltage recovers slowly as soon as the ignition is turned off. ( https://youtu.be/MzVENA5MNBk )

Connecting the charger for a few seconds fixes the issue. I'm guessing this is a BMS related problem, and connecting the charger resets the BMS(?). As this has happened before (when simultaniously having a broken hall sensor - which made the confusion even bigger), its not a one-time incident. I'm thinking either I have a malfunctioning BMS, or the BMS is doing what its supposed to, and there is a problem with the battery pack.

I tore down the battery earlier, and have already tested the voltage of the 13 parallel packs (at the balance lead connected to the bms). Testing showed that all the packs were within 0,02v of eachother - which is supposed to be acceptable to my knowledge(?).

Are there any other tests i can do to narrow down wether my bms is broken, or there are other health issues with my pack?

Pictures of battery pack and bms: https://imgur.com/a/NrNDbgl

I have already ordered a 13s 40a "smart/bluetooth BMS" of aliexpress, as to my knowledge, the BMS on my pack doesn't balance charge my cells, and i thought either how, it would be useful to have the option to monitor my battery cells.

Thanks in advance for any input!

 

[Balmorhea]

You may have a weak cell group, or an out of balance pack. In that case, the BMS is functioning correctly to cut power when a single cell group drops below the minimum voltage.

You can easily check cell voltages by unplugging the ribbon cable and probing the plug contacts with a multimeter. If they’re not all very close in voltage, that’s most likely your problem.

 

[amberwolf]

Yesterday i went for a 10km ride with my scooter - while riding on flat terrain with over half of the supposed battery capacity remaining (13s6p), suddenly the voltage dropped rapidly and the scooter powered off. Later testing (with a multimeter connected to the charge port) shows that turning on the scooter causes the voltage to drop rapidly until it stops at 3,9v. The voltage recovers slowly as soon as the ignition is turned off.

This means that at least one cell has dropped below LVC under load, and then the BMS shut off to protect the cells from overdischarge.

So one or more cells is not the same capabilities as all the others, and has less capacity and higher internal resistance.

Since you say your BMS can't balance the cells, then you'd have to either do as you are doing (replace the BMS with one that can) or manually do so with a single cell charger if it's one or a few low groups, or drain off the high ones (like the BMS would) if it's one or a few high groups. Once they are all equal, then recharge with the main charger thru the BMS. IF they are all within a few hundredths of a volt at 4.2v full charge, then they're probalby close enough. Otherwise, rebalance again, and recharge, utnil they are.

Keep in mind that since the cells started out balanced within a couple hundredths of a volt per your test, but then you ran into this problem, if they're now out of balance or they are out of balance under load (when they might still read fine without a load) then they are not well-matched to each other, and will always get out of balance under the load they're seeing on your system.

To fix that, you'd need to either replace the pack with well-matched new cells, or test each of the cells so that you can build matched groups of them with the same group capacity and internal resistance, so that the groups stay balanced and supply the same capabilities.


This type of problem is common, even with brand new packs, whenever they are not made of well-matched cells, and not many packs appear to be made that way. (and you can't know which are and whihc aren't until you test it under loads to see if voltage drops are identical, and capacity of each group is identical (they all drop in voltage equally thru the whole discharge curve, and then rise in voltage equally thru the whole recharge curve).



The problem can also be caused by poor parallel interconnects in a group, so that not all the cells are actually connected. Thus that group would have lower capacity and higher Ri, so it sags more in voltage under load and empties quicker durign a ride.