Originally Posted by 2manybikes
Do you have some kind of voltage regulator in the circuit to protect the bulb ballast? Do you know what voltage your battery puts out when it is fully charged?
I have read about a couple of systems built with too high voltage and the ballast was damaged. It may not happen right away, it may take time.
That happened to me. I was using a 14.4v NiMH directly into the HID ballast. The ballast is listed as 14.8v maximum voltage. The NiMH exceeds that for the first hour after a full charge, putting out about 16.9 at first. My first HID ballast lasted about 5 hours. Trailtech replaced it for free.
I put a voltage regulator in line, mainly because I happened to have a really good inverter-style digital voltage regulator sitting around that I'd built for something else. It is super-efficient, only drawing about 1ma for its own use. If I had to use an analog regulator like a 7812, I would have just cut one or two cells out of the pack and run it unregulated at a lower voltage.