Dont use a battery tester! A proper multimetre or voltmetre set to v dc and ensure you let the voltage settle before you take a reading!
check voltage with car not running (12-13v), start car wait a few seconds check voltage with engine running (13.5-14.5v). turn on all lights, blowers and heated rear window, check voltage (11-12.5v). report back what you find if your unsure.
