Phil - don't confuse capacity (A-hr, energy) with "CCA" (cold cranking amps, the ability to turn the engine, power). You can have all the A-hr in the world and still not get an engine to start because you need several hundred amps in an instant which (from a battery design perspective) is not actually that much energy but is a lot of power. Fitting a larger petrol tank to your car makes it go further, but not faster!
