A. First, we need to understand that the primary job of a starter battery is to flow a large current (amperage) for a short time, in order to start a vehicle. In order to do that efficiently, the battery must have low internal resistance. In very simplified terms, the larger the battery is in capacity, the lower the resistance of that battery is, and the better it can crank a vehicle under high loads. Lead-acid battery makers have therefore used AH (capacity) ratings as shorthand to indicate sustained cranking ability, rather than a real time usable capacity. The lead-acid capacity rating itself is based on a complete discharge. But because lead-acid batteries loose capacity at higher rates the more it is discharged, and because their internal resistance rises as they are discharged, the actual capacity for a starting battery which can be used is the top 30% reliably. Discharge any lead-acid battery in excess of 50% can damage the lead-acid battery if done repeatedly and it will encroa