I recall some old Jags had a small fan for the battery compartment.

Combination of protecting the battery and making the engine bay look neater same as the engine cover most cars come with today. Looks all ultra hi tech until you remove the cover to reveal the old smoker below.

It seemed to start when turbo-diesels started gaining ground, on some models with the jacket round the battery only on the diesels - although none of my cars have ever had it but my son's Vectra-C CDTi has one.

My present has an AGM battery under the passenger seat.

Absorbent glass mat (AGM) type batteries of the type employed in today's stop-start systems can be sensitive to overheating. Consequently the design trend is to locate the battery away from the engine compartment to avoid heat, ie. in the boot, under the seats etc. Where this is not possible some sort of barrier or shielding is the next best compromise.