Slow cranking: battery or alternator?

Slow cranking (engine struggling to turn or not firing first time) can be due to the battery or the alternator. Here is how to tell them apart and what to do.

Common symptoms

Slow start (engine cranking slowly before firing), lights dimming at idle, battery warning light on. After starting, if the engine runs normally but the battery light stays on or lights dim when driving: the alternator is not charging or not charging enough.

Weak battery

Battery nearing end of life, discharge (lights left on, long standstill), cold reducing capacity. The starter lacks current: engine cranks slowly or does not start. Recharging or jump start gets you going; if the battery is over 4–5 years old or often goes flat, replace it.

Failing alternator

The alternator recharges the battery when driving. If it stops charging (regulator, brushes, diode), the battery runs down and the battery light comes on. When stopped, lights and electronics drain the battery; after a while, not enough current to restart. Have charging voltage (alternator) and battery condition checked.

What to do

Check battery voltage (multimeter) and charging voltage (engine running). If the alternator is not charging: avoid long trips and have it repaired soon. My Auto Check-up helps you describe symptoms for an initial estimate before visiting the garage.