battery charging

I have an inverter that plugs into cig lighter. I keep drill and flashlight battery chargers plugged in all the time. Never had any problems.
My lighter is on a switched circuit, it dies after 10 minutes, hence the battery. and the actual voltage of my 12V is 13.2 so.....we'll see. I had an inverter in my old van and I left it on for the weekend to charge my Makita drill, killed battery dead. Battery probably wasnt all that great to begin with.