Doesn't seem a likely source of engine drag to me. If you have a 30 amp alternator running flat out and it's putting out full output at
16v(before regulation), that's just over a half horsepower. Even if you've got a horribly inefficient alternator running into a dead short with bad bearings, it's unlikely to absorb more than 1 HP. If you start screwing with running down regular car batteries, you're going to have a lot of grief. There's a minimum voltage needed to just keep today's ECU and injection systems running, factory specs on my VW say 8v and that's 8v under load, not 8v with a VOM with the engine off. My experience with car batteries running down has been mostly bad. After one or two instances of leaving lights on all day, most will short out and won't charge anymore, the plates are thin and warp from the heat of charging.Stan