Heat soak starter coil, or weak battery?

[GoldOxide_imp]

Heat kills batteries far more than cold does.

I agree with Houtman.

As the battery ages, the redox reaction eventually weakens to a point where there is more water than desired in the cells. Water, with oxygen, will corrode the cell plates and heat will accelerate the reaction, while cold will slow the reaction. As the cell plates corrode and deteriorate, maximum cranking current will decrease (as well as overall capacity). If a compromised cell freezes, the water will expand causing potential physical damage to the cell.

AGM and newer technology batteries are more stable in this regards. However, AGM are still lead-acid based redox reactions.

The best one can do is to ensure the battery remains healthy by avoiding under and over charge situations during the designed lifespan of the battery.