I've been planning on doing this myself. After reading Christ's post, I'm thinking that if I kept a voltmeter connected to the battery when I was charging, I could get an idea of whether I'm getting into the danger zone by taking out too much juice.
Looking at typical car battery capacities (around 50 amp-hours), and knowing that putting 2500mAh (2.5 amp-hour) into a 5-cell battery should take around 5 amp-hours (because -- physics; boosting the 12V to the 21V that the pack needs means the current in has to be over 21/12 of the charge current).
That's actually getting into scary territory: you're using 20 amp-hours to charge your batteries (if you're charging them fully). Car batteries are designed to deliver lots of current, not to be discharged deeply. So pulling 40% of your car batteries charge out and then asking it to start your car is -- well, you can
hope the tow truck driver is good looking, but when does that happen?
So -- now that I've done the analysis, I'll definitely use a voltmeter. Or I'll run the car every second time I charge, or I'll adopt Fred's method.
Edit: Or, after commenting on
this thread and thinking about it some more, I'll use Fred's "old dead flight battery" method or I'll buy a deep-discharge lead-acid battery and stash it in the trunk.