Pylontech US2000 Cell over 3.6V

Batteries
1

Good day Everyone

For interest’s sake, I have a few questions:

  1. What would cause one cell of the 15 cells in a Pylontech US2000B to have a higher V value than the rest of the cells to the extent that it gives an over voltage alarm?
  2. Does it have something to do with the BMS or with the cell itself?
  3. Can a single cell be replaced or do you need to replace the entire battery?
  4. Would a firmware update help in any way?

Thank you in advance.

Kind Regards,

Corné

2

A cell being pushed over 3.6V could mean one of many things.

  1. It could mean there is a damaged cell in the pack. This could be the same as the cell that is experiencing the high voltage (the reduced capacity causes it to fill up faster), or it could be a different cell that is low.
  2. It could mean the battery is not balanced and needs some time to balance.

First, it is important to know that having one cell “jump out” is perfectly normal. When the battery is full, one cell will always tend to sit a little higher than the rest.

The ability to reach 52.5V (which is thelow end of the charge voltage range Pylontech wants) relies heavily on the cells being almost perfectly balanced. All 15 cells need to reach 3.5V more or less simultaneously, and if there is even a small imbalance, one cell (the one with the lowest capacity, even if it is just marginally less than the rest) will jump out to above 3.55V, and Pylontech considers that a “high voltage warning” condition.

Something to look for, is to check at what SOC the warnings arise. If the high voltage warning shows up while the battery is below 90%, or a low voltage warning shows up with the battery over 50% SOC (or both, that is also possible!), then you are probably dealing with a bad cell.

If a high voltage warning shows up somewhere above 90% SOC, then it probably just needs time to balance again. You usually do this by just keeping the battery at a fully charged voltage for several days to give it time to balance.

(Note, for other readers, that my opinion on this has changed. In the past I advised slowly cycling between 80% and 100%. I have been convinced that just keeping it full and letting the passive balancers bleed of excess energy is a faster way to balance).

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