I've fitted this to my S1 -

Odyssey Extreme Racing 35 Battery (PC925)

- not cheap and you have to be careful not to 'cook' it by over trickle charging (see other posts) but has a good reputation otherwise.

James

Aero8 , Series 1 - Boston Green
Mercedes EQC 400 AMG
Smart Brabus Convertible
Honda Monkey Z125