Prompted by a member called Robert who was asking, sharing in case it helps anyone else.
Robert had an issue with his starter battery and wanted to replace it but of course is space constrained in an older vehicle. His 72Ah battery was the right size, but how many Amp Hours do you need?
A standard 1.6 litre air cooled engine requires a starter motor such as the Power Lite one from JK. That one is a 1.4 kilowatt starter. Converting kilowatts to amps you need to change 1.4KW to 1,400 watts and then divide it by the voltage, in our case 12 volts.
1,400 / 12 = Around 120 amps.
For two litre engines, you will need a little more. For a customised engine, who knows?!
If you look at The battery charge quick reference guide you know that you do not wish to flatten the battery completely as that will break it. Ideally avoid going more than 30% depleted.
If you know that you never use more than a minute on the starter motor to get the engine into life, that is 1/60th of an hour. Running that 120 amp starter motor for an hour would be 120 amp hours, so 1/60th of that is 2 amp hours.
As long as you have no current leaks and are not sitting in your vehicle draining the battery with a stereo, a fridge, lighting or other circuits on the starter motor, as you can see, a minute to start the engine on a 1.6 litre air cooled engine will drain 2 amp hours out of your battery. Even the smallest and cheapest car batteries will cope with that, but for peace of mind, don’t buy the cheapest battery in the shop!