In last weeks, we are planning to rebuild our headlamps for longer working time. My decision is to buy 4x 1.2V rechargable D (R20) cells and put them to cover with switch. I have found batteries with 9Ah, which should make my 5V Xenon super strong bulb light for a looong time. I was tired of all the calculating, so I made little tool – Battery charge calculator. You can check it out here. I hope it can be useful to somebody.