Battery falling out of balance

Helpmeplz

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Jan 25, 2023
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When I ride my bike there is always one or two lifepo4 cells that drops down to 2.6v or some other very low voltage and the rest are all more or less the same good voltage like 3.26v. But when I charge ithe battery all the cells go back to being within 1% of each other.

My BMS ballence leads all seem fine.

Anyone have any ideas
 
Helpmeplz said:
When I ride my bike there is always one or two lifepo4 cells that drops down to 2.6v or some other very low voltage and the rest are all more or less the same good voltage like 3.26v. But when I charge ithe battery all the cells go back to being within 1% of each other.

My BMS ballence leads all seem fine.
If the cells are dropping in voltage under load, there are several possible problems, but the most common is failing cells, just like for the other scenario (at rest), below:

The cells (or groups) that read low *at rest* when all the others are not are lower in capacity than the ohters, if at full charge the cells are balanced (within a hundredth of a volt or so, when at 3.4v or higher). So however many Ah you've gotten out of the pack at that point is how many Ah the entire pack is good for, because of those lower-capacity cells.

Since 3.2ish volts is where LFP cells tend to sit when they are not right nearly empty or completely full, the fact that all the others are right about the same voltage at that time doesn't mean that all the others aren't of varying capacities as well.

It could be from age, cell defects, mismatched cells, etc., but with one exception, the only option to fix this kind of problem is to replace any cells that do not have the capacity (or other capabilities) you require the pack to have.

The exception is that a pack made of multiple paralleled cells in groups that are then seriesed together (the most common ebike pack design) can have broken interconnects in the groups that have low capacity. This disconnects some of the cells from the pack, so they don't contribute anything to it except weight and volume. Sometimes this is a visible problem, but if it's broken spotwelds, the strip can pass over a cell and *look* connected, while not actually being electrically connected to the cell anymore. There are other interconnect failures that can do the same thing.

If the pack is older (a few to several years), it might be time to replace the pack itself, unless there is someone that can replace the cells for you (or you want to learn the processes and buy all the equipment to do so., which itself will almost certainly cost as much as a new pack would, for the typical ebike battery).
 
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