Why does my car battery keep going flat?
A battery that goes flat more than once in a short space of time is telling you something, even if it starts again after a jump start or a night on charge. The three usual suspects are the battery itself reaching the end of its life, the alternator that is supposed to keep it topped up while you drive, and something quietly drawing power while the car sits on the drive. Each of those needs a different fix, and guessing which one it is from the symptoms alone is unreliable — it needs a proper check on the actual car, not a list on a website.
Is it the battery, the alternator, or something else?
A failing battery usually shows up as slow starting that gets a little worse over days or weeks, then one morning there's nothing at all. A charging fault behaves differently: the car starts fine, but a battery warning light flickers on while you're driving, or the lights and radio feel weak at idle. A drain on the system — something left switched on, a faulty interior light, a module that isn't going to sleep properly — tends to show up as a battery that's fine one day and completely dead the next, especially after the car has been left for more than a day or two.
Does cold weather make this worse?
Yes, noticeably. Cold slows down the chemical reaction inside a battery, so one that's already weak can cope through September and then fail outright on a cold morning. If your battery keeps going flat and it's been cold overnight, that isn't a coincidence — the cold is finding a weakness that was already there, not creating a new one.
Can I test it myself before booking anything in?
You can do a rough check. Switch off the lights, the radio and anything electrical overnight with the engine off, then see whether the car starts cleanly the next morning. If it's flat again with nothing left running, that points more towards the battery itself or a drain than the alternator. A proper test puts a meter on the battery and on the charging system while the engine is running, which isn't something to guess at on the driveway — it's what we do on the ramp.
What does ONEFIX actually check?
Every service at ONEFIX includes a free digital health check with photographs, and the battery and charging system are part of that, so if a service is due anyway this gets looked at as a matter of course. If the car is going flat now and a service isn't due, it's worth booking it in as a look on its own rather than waiting for it to leave you stranded. We check the battery's condition, what the alternator is actually putting out, and whether anything is drawing current it shouldn't be — and you get a price in writing before anything is touched, because we don't carry out work that hasn't been approved.
Will I be charged just for someone to look at it?
Where a job needs inspecting rather than booking as a straightforward fixed-price service, we tell you what we find and put a price in writing before any work starts. You're free to say no and walk away. You only ever pay for work you've actually approved, never for the look itself.
Should I keep driving it while this is happening?
A battery that's slow to start isn't a safety fault in the way a brake or steering problem is, but a car that might not restart is still a real practical problem — stuck at work, stuck on the school run, stuck somewhere with no one around. If it's getting worse week on week rather than staying the same, it's worth booking before it leaves you stranded rather than after.
What if it isn't the battery or the alternator?
Sometimes neither part is at fault and the real cause is a drain somewhere else in the car — an interior light that doesn't switch off properly, a boot catch that thinks the boot is open, or an aftermarket alarm or dash cam wired in badly. This is exactly why a battery that keeps going flat is worth a proper look rather than a straight swap: fitting a new battery onto a car with a hidden drain just means you're back here again in a few weeks.
How do I book this in?
Book online with your registration number and a note that it's a battery or starting concern, or call the garage directly. If it isn't urgent, it's worth folding into your next service booking anyway, since the check is already included and costs nothing extra.
For related reading, see our guide on what an MOT advisory means if a battery or charging note has turned up on a recent MOT certificate.