Troubleshooting Smart-UPS - APC: UPS Cooling fan activates with out good reason

[Kiriakos GR]

Smart-UPS by APC models them designed for medium-high loads applications they are really intelligent due the amount of their internal sensors.
Several electrical parameters (information’s) they are collected and logged or displayed at the product faceplate due LED lights or by an LCD screen at the newer models.   

Two days ago my own Smart-UPS SUA 1000XL this started to activate internal cooling fan every few minutes with out apparent (obvious) reason.
In my latest battery configuration I am using a pair of 36Ah (12+12V) deep cycle batteries.
Due their large dimensions they are placed externally of the SUA 1000XL and therefore internal UPS battery compartment this is empty.

Use of external battery packs this is a tricky business, mostly because battery pack temperature this is not monitored as it does an internal battery pack.
Overheat of internal battery pack it would be partially detected due UPS motherboard sensors and this it could become a reason to alerts you.

Now these symptoms of activation of internal cooling fan every few minutes with out apparent (obvious) reason, they did alert me.
My first thought was to check battery temperature and both seemed hot at 50 Celsius.
Then I took my AC/DC clamp meter and I did measure charging current, this was at 3A when max capacity of internal charger this is delivery of 9~10A.
Next step was of me monitoring charging current for an hour of time and measurement this did not changed at all.

First conclusion this is that SUA 1000XL was activating it internal cooling fan so to cool down it own battery charger circuit.

Second conclusion this is that something forcing continues operation of internal charger circuit at higher than expected as regular current for battery top-up maintenance.

Individual battery inspection shown that single battery was overheated from the too.
They seemed both warm, because they were touching its other, and the healthy one this worked as heatsink for the overheated one.

Addition of  50ml distilled water inside of  2V cell-element  this did not helped, and what happened was specific (damaged) battery to faster deteriorate ( in just few hours) and voltage output this dropped down to 0.5V

Theoretically speaking, if these Smart-UPS by APC models they were designed to also monitor charging current,   then remotely monitoring of UPS due software, this it would be able to deliver more specific information’s of battery cells status.
Monitoring of battery voltage alone this is not enough as warning prior having an total collapse of singe or multiple battery power packs (of two 12V batteries connected in pair).