Adding second battery to car.

[brrapp]

I've got an old Ford pickup, it has a 2.5 diesel engine which can be a bit slow to start if it's been sat for a week or two as I don't use the pickup regularly. I'm going to add a second battery tray and wire in a second battery to give it a bit of extra cranking power. I just happen to have a brand new spare battery sitting (my son wrote his car off a few days after I'd fitted a new battery). The only problem may be that the main battery is a big one 940Amps and the spare one I have is just 570Amps Can anyone foresee any problems linking these two different sized batteries in series?

[brrapp]

battered said:

Yes, you will f* everything. Wire them in parallel. Black to black, red to red. Alternatively, if you don't understand why series wiring is a very stupid and dangerous thing to do, leave it well alone.

Sorry, I meant in parallel, just got my words mixed. It was just the different cranking Amps I was concerned about. I know it wouldn't make a difference in the short term (would be just like getting a jump start from a smaller car), but don't know if it will mess up the charging system.