I am told the Calcium batteries handle the engine bay temps the best. I have been monitoring my engine bay temps with a thermo-couple for the last few months, and will see temps ranging from 55 C to 65 C or even 70 C when nearer the turbo. It varies quite a bit from place to place actually, and a lot seems to depend on the ability of the air to escape out of the engine bay, which is usually downwards and under the car. The more air you can get flowing through the engine bay, the lower the air pressure, which in turn assists airflow through the radiator and front grill.
All advice I have read indicates that batteries simply can't handle those sorts of temperatures for long periods of time. Some will last better than others, but it gets to them all eventually.
Collyn Rivers has written a bit about it, and has an excellent table in one of his books describing the expected life reduction of a battery with given heat ranges.
What sort of time frame are you talking about in between each battery?
I should clarify that I am in Nth Qld and those engine bay temps were normal hot days and taken in locations both beside and above the engine itself, plus on the battery. Temps are often down in the 40's along the firewall on cooler days.
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