I recently discovered some corrosion on the belly of my Cessna 180 that was later traced back to the battery vent tube a few feet forward on the bottom of the fuselage. Apparently for a reason I still have not determined, the fairly new battery I installed decided to offload some of its electrolyte that discharged through the vent and resulted in the bit of corrosion I found. The reason I tell this story is because my A&P suggested that maybe at some point it was overcharging and boiled off, so it got me to thinking about batteries mounted on the firewall in an environment MUCH hotter than underneath my baggage area. There were some posts here not long ago with people offering temperatures they have recorded in this very area that were hot enough to be a real problem (I would think) for a battery with plastic housing. Are batteries designed to withstand boiling temperatures as can be expected in this area? I get the desirability of mounting batteries there-favorable CG, a lot less heavy cable required, easier access, etc. But what about temperature?