A brand new battery can be had for £30 believe it or not, just shop around a wee bit. Get one at 44Ah minimum, the ecu will detect an excessive voltage drop during starting if you get a smaller one, and you will get immobiliser problems / engine start lockouts.
For those who dont know this, the Ah rating means battery capacity (supply current versus time):
100Ah = 100A for 1 Hour to 1A for 100 Hours.
44Ah = 22A for 2 Hours, 44A for 1 hours etc etc
A golf starter which isnt the planetary reduction type like on new cars, will draw 300 to 400 amps during a cold start, the colder weather, the thicker the oil etc so a 44Ah battery even fully charged can only supply 400A for 6 minutes then its dead. A 100Ah one can do the same for 15 minutes.
If however you use a smaller one - say 30Ah, the voltage will dip when the current drawn exceeds 75% of its capacity, and can drop to 8v or less, which confuses the ecu - remember its still got to get sufficient power as well as all the other systems on the car, while the cranking engine is sucking it up like a vacuum cleaner.
