I've recently acquired two faulty battery packs. One is a 10ah from a elife folding bike and the other is a vruzend kit. The elife battery 10s5p was from my mates missus bike. I built him a new pack the other day.

 

When I tested the pack at the main battery terminals it measured 14.7v when I opened it up all cell groups were 4.1 expect for the last group which was 0v.

 

My mate told me he left it on charge for a week or so like he used to with a trickle charger on his motor bike battery.

 

Does it seem likely that a single cell failing would completely drain the whole group down to 0v or is it more likely a bms fault.

 

I'm not sure how to test the bms other than to put some new cells in the pack then charger it and just test it over a few days to see if the bms is draining that group.

 

The vrezend kit battery has the same issue but at the opposite end of the pack

What ever the cause the dead cell groups are scrap, the issue likely lies with the BMS draining them.

What ever the cause the dead cell groups are scrap, the issue likely lies with the BMS draining them.

Yeah the 0v cells are in the scrap bin, I'll have to try and test both the bms

If a cell gets an an internal short and it's connected to others, they're all shorted.

 

Sometimes, the BMS bleed resistors stay on. The bleed current is only about 100mA, but that will drain a fully charged cell group to 0v in about 4 days. If you want to keep the BMS, you must test for this specific fault.

Well I've finally found the time to look at the bms issues. I had 5 lg cells with simerlar specs and capacity to the existing cells in the elife battery pack. I've connected them up and so far the pack hasn't drained down at all. It's been sat at 41.6v all day with no drop as yet. I did notice that if I check the voltage on the charge port though it's slightly lower at 41.1v. Is this normal?

 

I also found another issue that possibly caused the cell group to fail on the vrezend pack. The charger that was charging that pack was over charging slightly as the charger output voltage was 43.1 instead of 42v. I know that's not massive if you split the extra 1.1v over the full pack it it carnt have helped.

I did notice that if I check the voltage on the charge port though it's slightly lower at 41.1v. Is this normal?

I can't say it's normal, but it's not necessarily a problem either. The BMS has a mosfet between the charge port and the cell-pack on the negative wire, so your connection is not direct. Maybe there's a protection diode too. You'd have to get your meter out to find out where the drop is happening. I wouldn't wory about it if the pack is charging to the correvt voltage and staying charged.