20Ah LiFePO4 Battery

Commonly used in golf buggies and mobility scooters, I have been using this for a while and its light weight (2.5kg) makes getting around in a C5 really easy.

http://www.ebay.co.uk/itm/Ultra-Max-12v-20Ah-LiFePO4-18-Hole-Lithium-Golf-Trolley-Battery-/120901844487

I can usually get about 7 miles out of it when the motor is used a lot. The downside is the price, this particular battery cost £190, though for that extra cost you get more than 3 times as many charges out of it compared with a lead acid car battery (1000+ vs 300) plus these batteries provide a constant voltage.



You can get larger capacity LiFePo4 batteries, this site in particular allows you to specify various combinations of voltage/capacity:

http://www.pingbattery.com/servlet/StoreFront