In addition to failing/faulty battery there are a few reasons why a battery might go flat: Weather – A car battery is 35% weaker on a cold icy frosty day which is why it's then you have a higher chance of being unable to start up your engine.   Human Error – We have all left the headlight or interior lights on, or not closing the boot properly can all possibly flatten a vehicle's battery overnight.   Driving Habits – If your vehicle mainly goes on a lot of small short trips then the battery may not catch up enough charge to recover the energy lost from starting up the engine initially. Lead-acid batteries also lose power gradually over time when not in use, so batteries in vehicles which are rarely driven will eventually end up going flat.   Old Battery – Old batteries do not hold as much charge as a brand new one. Lead-acid batteries last approximately 42 months therefore if a battery is older, be mindful that it is on borrowed time.