SLA batteries and heat

I am still using sealed lead-acid (SLA) batteries, because they are cheap and give good service for several years in my experience. The power needed in my glider is modest so even on a long flight the battery does not get fully discharged, that lengthens its life. I take the battery home when I am done flying, charge it, and keep it in moderate temperatures.

Last summer I kept a spare battery in the glider trailer, in case I forget to bring a battery to the airport when I go there to fly. At the end of the season I tested the charge capacity of that spare battery along with my other ones and some owned by our club. I found that the battery that was stored in the trailer was pretty much useless, even though it was good before that summer, and not that old. It held much less charge than some much older batteries that I have. It was never used during the summer, and its voltage held up, but measured capacity (amp-hours) was reduced to a fraction of 1 AH by the fall.

Also, some club batteries that were perhaps left in the club gliders for weeks on end lost most of their charge capacity, despite being less than 2 years old. The latter may be due to being left in an uncharged state for a while, I don't know, since the club does not keep battery charging records. But it could also be due to the hot summer temperatures in the glider cockpits, whether tied out or in the trailer? Not Arizona-hot, but even in Vermont on sunny summer days the temperature inside a glider on the ground can reach well above 100 degrees F.

So, is it useless to store a spare SLA battery in the trailer?