By the way, do NOT deep-cycle a lead-acid car battery. I think that is an appropriate thing to do with some other rechargable battery technologies, but not lead-acid batteries. According the deep cycle battery faq on solar-electric.com,
Lead-Acid batteries do NOT have a memory, and the rumor that they should be fully discharged to avoid this "memory" is totally false and will lead to early battery failure.
So at the moment I'm trying to resurrect that battery, by running it part way down and then recharging it slowly.