Servicing5 min read
Why does my car battery keep going flat?
A battery that is flat one morning is bad luck. One that keeps going flat has a cause, and it is usually one of four things. Here is how to tell which.
By Kaine Carter
Jump-starting it and hoping is the expensive way to find out. Each time a battery goes completely flat it loses some of its capacity, so a car left with an undiagnosed drain often needs a new battery as well as the fix.
1. Short journeys
Starting the engine takes a lot from the battery, and the alternator needs a decent run to put it back. A car that only does short trips, especially in winter with the lights, heater and heated screen on, can slowly lose the battle. If it is fine after a long drive and struggles after a week of school runs, this is the likely cause.
2. An old battery
Most car batteries last somewhere around four to six years. As they age, they hold less charge and struggle most on cold mornings. A slow, laboured start is the usual sign. A battery can be tested properly in a few minutes, which is far cheaper than guessing.
3. The alternator
If the alternator is not charging, the car runs on the battery alone until it is flat. A battery warning light while driving, lights that dim, or a car that dies again soon after a jump-start all point towards the charging system rather than the battery.
4. Something draining it overnight
Every modern car draws a little power while parked, for the alarm, the keyless entry and the computers. A fault — a module that does not go to sleep, a boot light that stays on, a faulty relay — can draw far more and flatten a healthy battery in a few days.
Accessories are a common cause too. A dashcam's parking mode needs a hardwire kit with a low-voltage cut-off set correctly, so the camera switches itself off before the battery runs too low to start the car. When we fit a dashcam we wire it that way and test it before we leave. Trackers draw very little and are designed to run permanently, but any accessory wired into the wrong circuit can cause problems.
How we find the cause
Our Diagnostic Call-Out is £60. We test the battery and the charging system, measure what the car draws once it has gone to sleep, and trace any drain to the circuit causing it. Then you get a written price for the fix. Mechanical work covers Milton Keynes, Leighton Buzzard, Bedford, Buckingham and Northampton.
