I have a Planet Bike Alias. Good light, suits my needs. Unfortunately the battery is just about done. I would like to put together a basic battery pack with NiMH d cells. The PB battery is 6V 4.5ah. Would 4 d's at 1.2v each equal 4.8v and not be enough to run the 15watt MR11. Would I need a 6 battery bundle instead, which would equal 7.2v, no? Also does anyone have experience with AccuPower batteries and chargers, and Thomas Distr?

Yes 6D would be good for that light. 6C would work too and be lighter weight. Check out mouser.com for battery holders of either type (plus AA too). Thomas Distributing is a good site; I've ordered from them many times. Accupower also has a good reputation.

Thomas Distributing and Accupower are high quality choices.

You mention the PB battery to be 6V, which would suggest that you use FIVE NiMH cells, unless you're planning on overvolting the thing. The PB battery is 4.5 Ah; if you get NiMH batteries from someplaces online, you should be able to find C cells that are roughly that capacity. If you go for D cells, 10 Ah is fairly typical. However, for some reason the C and D Energizer cells you find at supermarkets are only 2.5 Ah, no better than their best AA cells.