Will excessive heat kill batt?

Nope. Look for a draw while the car is off. Anything more than a couple hundred mA is worrysome.

Good luck.

Heat WILL kill a battery over time, but not in one single hot day ... more like over a period of months. Being in the hottest friggin' State in the Union (temperature-wise, at least ... I'm sure most States in the Deep South have us whooped when it comes to sticky, nasty humid heat), the average battery out here in Arizona only lasts about three years, tops. If you get six years' life out of a battery here, either you're a snowbird that stores the car or brings it North for part of the year, or you've got a serious freak of a battery on hand. Even Optima batteries die out pretty quick from the heat out here.

As to why, exactly, I'm not 100% sure. Something about speeding up the whole electrolyte process and thereby shortening its service life, I've heard. And, down here, these people that insist on re-installing those goofy OEM foam battery blankets (not the plastic heat shields) that were meant for COLD climates only make things worse by retaining heat and baking the crap out of the battery. My theory is that in extreme temperatures - hot OR cold - batteries take a harder hit because people are always jammin' on their accessories overtime - A/C, heater, wipers, headlights, etc. - and a lot of cars, especially the late-80's models like Foxes, are equipped with puny alternators that don't put out enough juice to cover the electric bill, so to speak, so the battery takes an extra beating. Batteries, by nature, can only bounce back so many times and with so much energy before they just give up the ghost.

Up in NY, battery blankets are a good thing because it gets friggin' COLD, but out here in the desert ... WHY?!? It never drops below 32* out here but maybe once every 5-10 years on a freak chance, and even then it'll only get to, what, 30*?

Every battery I've had die on me has always done so without any warning at all - perfectly happy one day, then leaves you sitting dead in the water the next. If you get one that gives you a little bit of indication it's getting weak, count yourself either very lucky or very observant - sometimes it's just that one time it cranks a little slow that's the final hint it gives before it poops out (happened to me the one time I borrowed my sister's Honda CR-V for a parts store run).