Look at the pattern. Starting trouble after the car has been parked, with no warning light while driving, suggests the battery. A battery warning light while driving, or a newly fitted battery going flat again, suggests the charging system. Test both before buying parts, because the RAC says the warning light doesn't always mean the battery itself is faulty.
Before you spend money
Replacing the wrong part is the expensive mistake. The RAC notes that the battery warning light can be triggered by a faulty alternator, a bad electrical connection, damaged cabling or a worn-out battery, so the light alone doesn't tell you which part to replace.
Questions to answer
- How old is the battery? Check the date or label on the case, or your service records.
- When does the problem happen? At start-up after parking points to the battery. While driving points to the charging system.
- Has a new battery gone flat? Then the charging system, or something draining the battery while parked, should be checked before you fit another.
- Has the battery been flattened repeatedly? The RAC says a battery needing jump starts every time you use the car is likely to need replacing, or the car has a more serious electrical problem.
Get it tested
A battery and charging test tells you which part has failed. If the battery is the problem, Jump Start Car fits a replacement on site from £150, including the battery. Call 0333 567 1785 to confirm the price for your car. For alternator faults you'll need a garage.
For quick roadside clues, see dead battery or alternator?
Frequently asked questions
Can a battery fault look like an alternator fault?
Yes. Both can cause flat batteries and warning lights, which is why testing beats guessing.
Should I replace both at once?
Not unless testing shows both have failed.
Does a new battery need setting up?
The RAC notes that many modern cars need battery registration or electronic resets after a new battery is fitted. Check your handbook or ask the fitter.