All you do is unscrew the caps and make sure the fluid level is up to the top. If not, fill ONLY with distilled water. Otherwise tap water has "metals" in it that are not good for your battery. As that is how a battery "dies" is when the metals off the plates falls off and fills the bottom of the battery, in essesence "shorting" the battery.
As to heat and batteries. When I was in Saudia Arabia, one of the iussues we had was keeping the vehicles running due to the batteries dying after 6 months, sometimes less due to the EXTREME HEAT. It would be 140f on a thermometer in the sun, so under the hood it would be WELL over that. Vehicle running or not, thisis bad for the battery and we had an extreme problem with that. We were replacing literally thousands of batteries.
And batteries aren't cheap, especially the Hummer batteries. Which we had hundreds of hummers to take care of.
Chez