AGM Battery - Very Hot and Sulfur Smell

[VVD]

Hello,

We have 3 AGM 6 cells house batteries which are 2 years old, 200Amps each. Until recently they all worked very well. They are connected parallel.
The other day, we noticed one of the batteries being less charged than others and not accepting the charge well. When touched it felt much warmer than the others and we could smell something like sulfur. We've taken this battery out of our electrical circuit but would like to understand what happened to prevent it from happening again.

We do not think our batteries were ever overcharged. Our voltage has never been above 14.10V and suspect this battery had developed some internal resistance. How can we make sure that this is the case and that our alternator or generator is indeed not overcharging?

Also, before the battery started smelling, our voltage seemed to dip unusually fast and we suspect this one battery was draining the other ones.

Any help is appreciated.

Thanks
Vincent