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Car battery keeps dying: causes and how it's tested

By Altay Hasanli, auto electrician

Short answer

If the battery goes flat overnight or after a few days, it's usually one of three things: a worn battery, an alternator that isn't charging enough, or something drawing current while the car is off (a parasitic drain). A drain is found by measuring it; replacing the battery on a guess often doesn't help.

The most common causes

  1. 1

    Parasitic current drain

    After the car is switched off, a module, relay, infotainment unit, alarm or aftermarket device doesn't go to sleep and drains the battery. In most cars the normal draw once the modules sleep is around 20–50 mA.

    Fix: Wiring and electrical repair

  2. 2

    Worn-out battery

    The voltage can look fine while the capacity has dropped. Especially with batteries older than 3–5 years, only a load test shows their real condition.

    Fix: Battery test, replacement and registration

  3. 3

    Alternator undercharging

    A failing diode bridge, voltage regulator or belt means the battery never fully charges while driving. Only ever making short trips can do the same.

    Fix: Alternator and starter repair

  4. 4

    A module that won't sleep, or a software fault

    In modern cars, if one module on the CAN bus doesn't go to sleep, the whole network stays awake. Sometimes a software update or correct coding fixes it.

    Fix: Computer diagnostics

What you can check yourself

  1. When you lock the car, make sure the cabin, boot and glovebox lights go off.
  2. Think of any aftermarket equipment (alarm, dashcam, head unit): it's the most common source of a drain.
  3. With the engine running, most cars should show 13.8–14.7 V at the battery; if it's lower, have the alternator checked.
  4. Check the manufacturing date on the battery.

When to stop driving

  • If the battery is hot, swollen or smells of rotten eggs, stop charging it and have it checked right away.
  • If the battery warning light comes on while driving, the alternator isn't charging: the car may soon stall.

How we find the cause

  1. 1We load-test the battery.
  2. 2We wait for every module to go to sleep and measure the drain.
  3. 3We check the circuits one by one to find what's drawing current.
  4. 4We check the alternator's charging voltage and ripple (diode bridge) with an oscilloscope.

This guide is general information: one symptom can have several causes, and only a diagnostic shows the exact one.

Services and prices

Frequently asked questions

What's a normal parasitic draw?

In most cars, around 20–50 mA once the modules are asleep. Much more than that can flatten the battery within days.

Will a new battery fix it?

Not if the cause is a drain or the alternator: a new battery will go flat too. Finding the cause first is cheaper.

How long does finding a drain take?

It depends on the drain: a constant one is usually found the same day, an intermittent one may need longer monitoring. We'll give you an estimate after the first check.

Other problems

Describe the symptom and we'll tell you what to do

Call, or send photos and videos of the problem on WhatsApp: we'll tell you upfront whether the car needs to come in.

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