Here's one other factor some people aren't aware of. A starter works at what is essentially a "dead short" condition. Its electrical resistance is less than one ohm, a tiny fraction of an ohm, to be more specific. As the armature speeds up to its normal running speed, a situation called "counter EMF" develops, which tends to prevent the current from increasing without limit. If a starter slows down, either from insufficient battery voltage or a tight engine, the current draw starts to increase geometrically. That can kill a battery or result in internal damage to the starter.
NEVER, EVER KEEP GRINDING ON A STARTER IF IT SLOWS DOWN TO A CRAWL! You're likely to ruin both the battery and the starter.
Jerry
