Originally Posted by ItsJustMe

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.