A battery that keeps dying is one of three things: an old battery that no longer holds charge, a charging fault, or a parasitic drain from a component that will not go to sleep.
Jump-starting the same car every few days is a symptom, not a fix — and the test that identifies the real cause takes a garage about twenty minutes.
First: how old is the battery?
A typical lead-acid car battery lasts four to six years in the UK. Beyond that the plates sulphate, the capacity falls, and a battery that measures a healthy 12.6 volts at rest can still collapse the moment the starter draws 300 amps. Short journeys shorten its life dramatically, because a starter takes far more out of a battery than a ten-minute drive puts back. If yours is over five years old and letting you down in cold weather, the battery itself is the most likely answer and no amount of chasing drains will change that. Look for a date stamp or sticker on the case.
Second: is the alternator charging?
With the engine running at idle you should see 13.8 to 14.6 volts across the battery terminals. Below 13.5 and it is not charging properly; above 15 and the regulator has failed and is cooking the battery. A slipping or glazed auxiliary belt gives the same result as a failed alternator and costs a fraction to fix. The tell-tales are a battery warning light that glows dimly at idle, headlights that dip noticeably when the engine is idling, and a battery that dies again within a day or two of being fully charged on a bench.
Third: is something draining it overnight?
Once the car is locked and all the modules have gone to sleep — which takes up to 30 minutes on a modern car — the standing current draw should be somewhere between 20 and 50 milliamps. Above about 80mA will flatten a healthy battery over a long weekend. The usual culprits Bob finds are a boot, glovebox or bonnet light staying on because the switch has failed, an aftermarket alarm, dashcam or tracker hardwired to a permanent live, a failing body control module that never sleeps, a stuck relay, or a corroded connection allowing current to leak to earth. Damp is a major factor — a wet boot from a blocked drain will do it.
How a garage pins it down
A proper battery and charging test takes about twenty minutes with a conductance tester: it measures the battery''s state of health and cold-cranking performance, then the cranking voltage drop, then the charging voltage and ripple from the alternator diodes. If those all pass, the next step is a parasitic drain test — an ammeter in series with the negative lead, the car left to sleep, then fuses pulled one at a time until the current drops. That second test takes longer, often an hour or more, because you have to wait for the modules to shut down properly. Many places do the first test free; expect £50 to £120 for the drain hunt.
Real UK costs
Battery test: often free. New battery fitted: £90 to £180 for a standard lead-acid, £150 to £280 for an AGM or EFB battery on a stop-start car. Important: stop-start cars must have the matching AGM or EFB type and the new battery registered to the car with a diagnostic tool, or the charging strategy will destroy it within months — that coding is £20 to £50 and is not optional. Alternator: £280 to £650 fitted. Auxiliary belt: £90 to £190. Parasitic drain diagnosis: £50 to £120. Corroded earth strap repair: £40 to £90.
What you can do yourself tonight
Clean the battery terminals — green or white powder on the posts adds resistance and is a genuinely common cause of intermittent no-starts. Check the terminals are tight enough that you cannot twist them by hand. Make sure nothing is plugged into the 12V socket overnight, since many cars keep them live. Check the boot and glovebox lights actually go off. And if the car sits unused for days at a time, buy a smart trickle charger for £30 to £60 — it is the cheapest way to double the life of a battery on a low-mileage car.
The stop-start complication
If your car has stop-start and the function has quietly stopped working, that is often the first warning that the battery is on its way out. The battery monitoring sensor on the negative lead tracks state of charge and disables stop-start to protect the battery. People assume the stop-start system has failed; nine times out of ten the battery is simply tired. It is a useful early warning — get the battery tested before it strands you.
How long should a car battery last in the UK? — Four to six years for most cars. Short-journey driving and lots of electrical load shorten it; regular longer runs extend it.
Can a bad alternator drain a battery overnight? — Yes. Failed diodes inside the alternator can allow current to leak back through it when the engine is off, which behaves exactly like a parasitic drain.
Does a new battery need coding? — On any car with stop-start, yes. The new battery must be registered with a diagnostic tool so the charging strategy matches it. Skipping this shortens the battery''s life significantly.
Why does my battery only go flat in cold weather? — Cold thickens the oil so the starter draws more current, while the battery''s own chemical output falls. A marginal battery that copes in summer gives up at the first frost.