What Causes Runaway Cell Voltage?

What Causes Runaway Cell Voltage? 2022-09-06

Quite often, a cell will hit a High (or low) voltage unexpectedly and/or faster than the other cells. (On the DIY Solar Forum, this is usually called a ‘Runaway Cell’. ) The runaway cell can eventually cause the BMS to unexpectedly shut down ether charge, discharge, or both. This paper gives a brief description of what the likely causes are and what to do about them.

I would like to thank the many forum members that helped by providing valuable review comments on the early draft of the paper. A special shout-out goes to @HRTKD for his extra effort and review.

Additional comments on how to improve are always welcomed.

Note: This paper is discussing LiFePO4 batteries used in typical DIY solar systems. These batteries almost always use BMSs with top-acting balance circuits. Furthermore, these systems typically have relatively low Discharge and Charge rates as compared to the size of the batteries. Systems with other characteristics or from other industries are beyond the scope of this paper.
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Confirmed in live installation all to be true.
It is important to do individual cell bleeding at high knee while charging - current is generally low there, and a bulb bleeder will work quite quickly. But may need to be done several times to get them all to true balance.
Would be useful to mention stripped screws (obvious from not being able to torque it properly) for cells that don't have welded threads on them and how to fix it. Also maybe a note about cells that aren't fully balanced yet and have an active balancer on them a thermal camera will show it "hot" even though that isn't necessarily indicating a problem.
Thanks for the feedback! I just added this to the list in the document:

* Stripped threads on cell terminals that can not be properly tightened. (Do a search on the forum to find resources and discussions about what to do for stripped threads. One recently popular solution is to force thread a ¼" stainless steel stud into the 6mm hole. It will self tap into the hole)