A car battery that dies overnight, or goes flat after a few days parked, is telling you something. Either the battery can no longer hold charge, the car is not recharging it properly, or something is drawing current while the car is switched off. A battery drain test separates those three causes so you are not left jump-starting the same car every morning or paying for a new battery that goes flat just like the old one.
Jumpstart Near Me carries out battery testing at the vehicle from our Oldbury base, for 12V cars and vans. If the car will not start when we arrive, we get it going first, then work through the checks below.
What a battery drain test involves
The first step is the battery itself. A load and conductance test shows how much charge it holds and how healthy it is. An old battery that has lost capacity can look like a drain problem, because it simply runs out sooner than it used to. If the battery has failed, there is little point hunting for a drain until that is dealt with.
Next comes the charging side. With the engine running, we measure the charging output at the terminals, at idle and with loads switched on. A weak alternator leaves the battery partly charged after every journey, so it gives up after a night or two standing.
Finally, with the car locked and its electronics allowed to go to sleep, current draw is measured with the car switched off. A small draw is normal on any modern car: the alarm, the clock and the keyless receiver all take a little. A parasitic battery drain is a draw that is higher than it should be, or one that never stops because a module stays awake.
Common causes of car battery drain
Some causes are simple: a boot, glovebox or vanity light that stays on, a phone charger or dash camera wired to a permanent feed, or an aftermarket alarm or tracker. Others are less obvious, such as a door or boot switch that tells the car it is still open, or a control module that fails to shut down after the car is locked.
Keyless entry can play a part too. On some models a key left close to the car keeps systems awake for longer than expected. Short journeys, long periods parked and cold weather all make an existing drain show up sooner, because the battery starts with less in reserve.
When the fault is in the wiring
A drain test tells you whether there is a drain and roughly where it is not. If testing points to a wiring fault, a sticking relay or a module that will not sleep, that repair is a job for an auto electrician. You will know the battery and charging results first, so you can explain the problem clearly and avoid replacing parts that are working.
If the battery is the cause, we can supply and fit a matched replacement at the vehicle, with registration or coding where the car requires it.
Arranging a drain test
When you call, tell us the make, model and registration, how long the car stands before it goes flat, and anything fitted recently, such as a new battery, a tracker, a dash camera or other accessories. Mention whether the car has stop-start and whether any warning lights have appeared.
Our service runs 24 hours a day. Call 07886 626640 or send your location on WhatsApp and we will quote before setting off.
Replacement questions
What is a parasitic battery drain?
It is current drawn from the battery while the car is switched off and locked. Every modern car draws a little; a parasitic drain is a draw that is higher than normal or never stops.
Why does my car battery die overnight?
The usual causes are a battery that no longer holds charge, an alternator that is not recharging it properly, or something drawing current while the car is parked. Testing all three shows which it is.
Is it the battery or a drain?
A battery test comes first. If the battery has lost capacity it can go flat quickly with a normal draw, so replacing it may solve the problem. If the battery tests healthy, the charging system and current draw are checked next.
Can you fix the drain as well as find it?
Simple causes, such as a light left on or a failed battery, can often be sorted at the vehicle. Wiring faults and modules that will not sleep are referred to an auto electrician, with the test results to explain the problem.
Choosing the next step
Compare the full mobile battery replacement service with battery testing before deciding what your car needs. Read why a car battery keeps going flat, our help page on a battery that keeps going flat, and when you need an auto electrician.