The warning signs

Slow, laboured cranking; headlights that dim at idle; a battery or charging warning light; electrical gremlins; and needing a jump-start more than once. Age matters too — many batteries weaken after around three years, especially in hot summers.

Battery or alternator?

A battery that keeps going flat isn’t always the battery’s fault — the alternator may not be charging it, or something may be draining it while parked. A quick test of the battery and charging system tells you which, so you don’t buy a battery you didn’t need.