diyBMS Incident/Failure How to make sense of it?

I have been running a diyBMS installation on a 24v pack of 280aH batteries in a travel trailer for the past few months and have been very pleased with the results. It has functioned virtually without incident until recently.

The trailer has been inactive this winter. I have watched the batteries on an almost daily basis by monitoring the pack voltage on the small LCD monitor. The voltage has been dropping slowly.

This past week it reached the 24.0v level so I decided it was time to warm up the battery pack and recharge it. In the process, I discovered that when I put a voltmeter on the individual cells they read between .21v (1 cell) and 2.5v with an actual pack voltage of 12.8v . . . a far cry from the 24v the LCD was reading. I’m in the process of evaluating the cells to see if there is anything salvageable.

I can make NO SENSE of what I am experiencing! I’ve loved the diyBMS but without an understanding of what went wrong, I don’t know that I can trust it moving forward.

Thoughts . . . ?

Pack voltage is the sum of what the modules are sending to the controller. I would start with checking each module is measuring the correct voltage, and the modules are well connected to the terminals.