Battery shuts off at 50% Do I have a bad cell?

[navygreen33]

I just built a 52V 14s7p pack. Took it out for a spin, everything is working great. I'm almost home with 50% battery and the thing cuts off. I turn it back on, it's showing ~48.5V. I go a couple feet, it shuts off again.

After recharging, I've taken a reading of each series and it looks like something is going on with the first couple of series. What do you think I should do? Will the BMS eventually balance these out? If I have a bad cell in one of these series, how can I figure out where it is?

Here are the readings I'm getting:
3.78 3.73 3.64 3.7 4.07 4.2 4.2 4.1 4.2 4.2 4.2 4.1 4.2

 

[amberwolf]

All those groups that aren't about 4.2v are all low, so all of them have a problem.

They may all have cells with high internal resistance / low capacity, or

they may all have bad parallel interconnects between the cells, so some cells aren't contributing to the pack,

or they may all have bad BMS channels that are stuck on (continuously draining those channels).

Or some combination of the above.

To test for interconnect poblems: measure the voltage across each cell in a parallel group, by touching *only* the cell itself, and not any of the interconnect material, you can tell if the interconnects are a problem, because you'll get different voltages. The one(s) with different voltage than you see at the interconnects are the ones with the problem.

To test for bad BMS: leave the pack on the charger constantly whenever you are present to monitor it, until those groups reach the same full charge as the rest of the pack, then disconnect the charger and let it sit, for at least hours, and if necessary, days. If the voltage of any of those groups drops, relative to other groups, then either the BMS balance shunts are stuck on, or the cells are self-discharging.

To see which of those two it is, you'd have to disconnect the BMS balance connector from the cells (which according to this thread:
https://endless-sphere.com/forums/viewtopic.php?f=14&t=90581
has a risk of destroying the BMS)
and then let it sit again. If the cells stop dropping in voltage, the BMS is draining them. If they don't, the cells are bad and self discharging (at least one in each group).

 

[goatman]

how did you build it, spot weld or solder?

 

[navygreen33]

how did you build it, spot weld or solder?

Solder

 

[goatman]

I had a similar problem on one, cold solder joint and sometimes on the negative side in the center theres a tab inside the battery that lets go and then that battery is dead.

did you pre solder every cell then check its voltage before soldering the pack up?

did you use solid copper wire or stranded wire? solid copper will cause bad/cold joints

 

[flat tire]

How the hell does someone build a battery, and provide cell group readings, but not know the battery needs to be balanced? WTF!!