UPS Fan not spinning when charging

[Parthu1708]

I have a pretty old (~6 years) BX1100C-IN for which I've recently had the batteries swapped as the old ones got very weak. The UPS now manages to power the connected load (just a router & a cordless phone) for sufficiently long periods.

But since the swap, I've noticed a peculiar behaviour. Whenever the power goes out & the battery comes online, the fan starts spinning (as it should & always did). But as soon as the mains power comes back on, the fan immediately stops spinning.

Shouldn't it continue to spin while the batteries are charging up again?

Also, the fan randomly comes on at odd times when there is no apparent power loss event or fluctuation (my new SUA1500I-IND sitting in a different room attests to that).

Should I be concerned about the battery overheating as it charges with no fan cooling it?

[Teken]

Depending upon the UPS design some of the fans are temperature controlled to come on at a predefined temperature threshold which is not related to a power quality issue on the line.

None of the lower end consumer APC UPS come with a PWM fan however. Some higher end models the fan is voltage controlled to speed up / down based on how warm the unit is.

Again, this is NOT a true PWM fan where it can ramp up and down anywhere in the RPM range! 

As you noted when the unit is in battery only mode the fan will come on.

One would expect the internal fan would also come on during the charging cycle as the battery will eventually become warmer in the process. 🤦‍♂️

Every APC UPS does include a crude thermistor to determine the ambient temperature of the battery compartment. 

Given you live in India I would simply ensure the unit respects the installation requirements of spacing, no obstruction, and is operating in a clean climate controlled environment.