I only race outdoors occasionally, and only recently started to, but I don't charge off the battery in my car, as for me I'd rather not risk my car not starting to get me home, so I use a separate cheap car battery to do my charging.
When I bought it I did notice that car batteries vary quite a lot in capacity, the cheap ones being quite a lot lower obviously, and they do degrade over time, if your going down this route it may be worth making sure the battery in your car is a high capacity one and in good condition.
The cheap one I run is still pretty new and does significantly discharge over a days racing/charging, but so far not low enough to damage it.
If you only have a small battery it could be worth upgrading it to cope with starting your car after a days charging! A car engine takes a short but BIG load off the battery to get your engine running.
I believe the BC6 does a voltage check on the incoming 12v supply and won't charge if the supply battery is too low, I'd assume that voltage is set to protect the supply battery from over discharging and isn't set to keep enough voltage to start a car!
Last thought if you go down that route...get some jump leads just in case...at least there are many people at races with alternative batteries to get you started in an emergency!!!
