Just to say, try and look after them, make sure they are charged and filled correctly with deionised water (i.e. maintain them), as I picked up a new battery for my 200sx (S13) today, and it cost £79, eek. Man they have really gone up in price.
The 'spare' I had was dead as a dodo as well, even after being left to charge all night on low. Don't ever fast charge a battery, quickest way to screw the plates up, unless of course it is specifically designed for it.
Matthew