Lots of comments about battery issues on the 2011 final thread.
We made a discharge tester using components from RS and a "hours elapsed" timer off ebay (£5 I think). I will post details, RS stock numbers, schematic and pictures so others can replicate.
The device discharges the batteries at 18A through power resistors until a 10.6V threshold is reached then switches off, leaving zero current draw on the batteries and the discharge time on the timer.
This will enable you to identify failing batteries, pair them up & choose best batteries for particular race strategies. The circuit is simple, 1 relay, 1 transistor, one push button, a zener diode & a big slab of metal to heatsink the resistors.....(OK there's a diode in there too....)
Others have used headlights as load resistors in the past, only problem with these is that the resistance is unknown & (massively) variable.