Hi,
I was driving my patrol today and a smell like rotten egg was coming through the vents. I have a dual battery system. The normal starting battery has white on top of it. (It's also not starting well after a few hours of not driving it.)

I am going to take the battery out to clean it and check the fluid levels. If one cell has dried out does that mean the battery has had it? Or could I fix it my topping it up?

Also the rotten egg smell would be from the battery acid wouldn't it?

Thanks