A brand-new battery that keeps going flat is almost never the battery’s fault. A battery stores power; something else is either not putting enough in or taking too much out. Work through these causes in order, starting with the cheapest to check.
1. Loose or corroded terminals
A clamp that is loose or crusted with corrosion can stop a healthy battery from delivering or receiving current. The car then cranks slowly or clicks, and the battery never fully charges. Check that both clamps are tight and the posts are clean. Our guide shows how to clean battery corrosion.
2. The charging system is not charging
If the alternator or its belt is failing, the battery runs down however new it is. Measure the voltage with the engine running. A healthy charging system typically shows about 13.7V to 14.7V, though smart alternators can vary. A reading near the resting figure (about 12.6V for a full battery) while the engine runs points at the charging system. Use the voltage checker or the dead battery or alternator checker.
3. Something is draining it when the car is off
A light, module or accessory that stays awake draws power overnight. Common culprits are a boot or glovebox light that stays on, an aftermarket stereo, alarm or dashcam wired to a permanent feed, and a faulty relay. A garage can measure the drain with a meter and pull fuses one at a time to find the circuit.
4. Short trips and long parking
A battery needs time on the road to recover what starting used. AAA notes that short trips that do not fully recharge the battery wear it down, and long periods parked do the same. If this describes your driving, a periodic longer drive or a maintainer helps. See storing a car: maintainers.
5. The wrong type for the car
A start-stop car needs AGM or EFB. A standard flooded battery in that car can fail early. If you are not sure what yours needs, read AGM vs EFB vs flooded. Some cars also need the new battery registered so the charging system treats it correctly; see battery registration and coding.
6. The battery was not new, or not fully charged
A battery that sat on a shelf for a long time can start life partly sulphated. Check the manufacture date on the label when you buy, and if you suspect a faulty unit, take it back with the receipt. The warranty calculator shows what a replacement or credit should look like.
A quick order of work
- Tighten and clean the terminals.
- Charge the battery fully, then test it. See how to test a car battery.
- Measure voltage with the engine running.
- If the charging voltage is fine, ask a garage to test for a drain.
- Only then suspect the battery, and use your warranty.
If the battery does turn out to be faulty or the wrong one, the replacement cost calculator shows what a correct one should cost.