Regarding APC Matrix 5000 UPS thermal detection...

[BillP]

On occasion, i have had MATRIX APC 5000 units where the smartcell overheated in thermal runaway and was not either detected or otherwise taken out of circuit by the UPS.

Is this normal?  Do these units not have thermal interruption or detection for the smartcells?

[Terry_Kennedy_apc]

Those Matrix units are ancient but pretty reliable - I still have 3 of them running at various locations.

Every APC model I've looked at (this is mostly "classic" models at this point, I haven't looked at any of the newer units) that supported expansion batteries or modules has all of the modules on a parallel bus. Even if the UPS had a relay to disconnect the bus from the UPS, the remaining battery packs would try to equalize the voltage in the pack with the problem. Normally what happens is that one of the cells shorts or develops low resistance, which drops the output voltage on the bus. Since the other packs are still running at nominal voltage, this results in a higher-than-normal voltage across the cells in the pack with the problem, which then leads to another cell shorting. Repeat until noticed. "Cell" here refers to one of the [usually] 6 cells within a single battery, and a pack has a number of batteries (in the case of the Matrix, it is 4 12V batteries in series). Nothing to do with "Smartcell" (the name for the whole battery pack on a Matrix). 

Different APC models have varying amount of intelligence in the add-on batteries. The older Smart-UPS add-ons didn't have any sort of intelligence, and you had to tell the UPS how many add-ons were installed. The Matrix Smartcell-XR has some minimal intelligence, but it is limited to monitoring the voltage and current, turning on the "replace battery" LED and letting the Matrix know about it so it can beep. 

[BillP · Accepted solution]

Good to know…

It seems to me that an enterprising electronics rebuilder might put a thermal breaker into the system for the smart cells.

That would take a hell of a relay though.

I'll look into it.