Why is the UPS fan turning on continuously?

I have a Luminous Cruze 2 KVA pure sine wave UPS with 2 12V Exide batteries. Facing the following issues with this setup in the last 1-2 months:

1. The UPS fan is turning on more frequently, like every 10 mins
2. Battery water level is some compartments are dropping more frequently than other compartments
3. Battery acid smell has increased in the recent past, it used to be non existent before
4. Electricity usage seems to have increased significantly in the recent past

I called in a technician from the Sales shop from where I purchased this UPS 2 years ago, he checked the battery load and checked the voltages and said it was fine and asked about battery backup, I said the backup is fine - it is fine, then he suggested to check with Luminous support and suggested it could be due to the UPS over-charging.

Due to the increased Acid smell, I have kept the UPS under bypass mode since last 2 days, but the it still seems to be charging - I can tell this as the battery is still somewhat hot and the UPS fan is running once in a while - not as regular as before, but it is regular.

I checked with a Luminous technician, he said with the symptoms mentioned it must be the issue with battery and suggested to get 2 spare batteries and test voltage cut-off at 27-29V.

I checked with the person who checked the battery first, he said he doesn't have any old batteries in good condition to check this.

I'm not sure what is the next step here. I'm not able to use the UPS now, and even when it's kept it in bypass mode, it is making fan noise once in a while.

How can I diagnose the issue further? Please recommend next steps.

Thanks in advance.