What could cause battery temperature to rise by almost 10 degrees?

[LarryK]

SMT1000C being monitored by EcoStruxure IT Expert sent alerts this morning that the battery temperature was rising above alert thresholds.

Sure enough, over the course of several hours, the temperature rose from around 80F to around 90F - and gradually (thank goodness) came back down to around 84F.

Another SMT1000C literally 9 inches away did not go through this cycle.  Temperature was roughly 81F throughout.

What I'd like to know is:  What could cause this spike to occur?

Thanks!