Using and charging a regular 13s4p battery. What happens when there is a bad cell?

ColinB

100 W
Joined
Jun 29, 2011
Messages
171
Location
British Columbia, Canada
Alternate title: How can we monitor our battery to prevent it from having an "exciting event?"

Ok, this is hurting my brain, so I appreciate your thoughts. Take the case of a typical 13s4p ebike battery. It looks like the cells are arranged in 4p first, then arranged in series. Each group of 4p cells acts as one battery, and the BMS is monitoring each of the 13 groups. All is well.

What happens when one cell in a 4p group goes bad or gets weak? How does the end user (me) determine there is a bad cell? (And avoid letting the magic smoke out?)

- I assume the cell group would have reduced capacity (by 1/4), so the entire battery capacity would be reduced.
- I don't know if the 4p group would excessively heat up during charging or discharging, causing potential problems? (Because the 3 good cells are supplying the power, or absorbing the charge, for the bad cell?)

What do you think? Thanks for the help.
 
Ok, I'm starting to look into failure modes for these types of cells. I don't know much yet.

One failure mode is voltage drop after charging. (But that was for single cells.) Not sure how a parallel group would behave.

One site said a failure mode is to drop voltage under load. (Makes sense to me.) The other cells would then feed the bad cell power to keep the voltage up. I assume this extra draw could stress the good cells. The big questions is - what would it do to the bad cell? I assume it would heat up, but by how much?

I'm just thinking out loud here - don't know if this next part is right.
Can a user notice a bad cell? Let's say the 13s4p battery is 12AH, for easy math. Each cell in the 4p group would have 3AH capacity. And lets say a cell going bad is down to 60% original capacity. That one 4p group would have 3+3+3+1.8ah, or 10.8Ah. 90% of original capacity. Let's say you normally get 80km from your battery until it's pretty low. We assume your going bad cell is happy until km 48 (60% of your 80km range) and the remaining good cells continue to prop it up until km 72.

So... in the first part of your ride, if you can monitor the voltage of each cell group, would you notice a voltage drop? I assume you must, especially by the end. And, would that bad cell (or the entire harder working cell group) heat up? Hmmm. I can see how people like to buy stuff just to experiment on them.

Colin
 
ColinB said:
What happens when one cell in a 4p group goes bad or gets weak? How does the end user (me) determine there is a bad cell? (And avoid letting the magic smoke out?)

The whole P group is affected. It will provide the same current as other P-groups, but will drop in voltage at a faster rate and hit LVC first, shutting off the battery. The effect is a lower AH capacity of this group affecting the entire battery, In an extreme case, which has happened to me, I've had a 36V12AH battery come off full charge, and shut off at 39V after 5 miles. It turned out that weak cell was only charging to 3.6V, not 4.2V because I did not have a balance BMS. Installing a balance BMS did not help. While I could get the weak group recharged, it did not have the same capacity and still cost me significant AH reserve.

The same thing has happened to me with a 4P battery when one of the welds to a cell broke, making it a 3P battery. I lost 25% of my normal AH capacity and I noticed that. In this case a balance BMS was able to start all the groups at full charge, but the 3P group still lost voltage faster.
 
Back
Top