Car Battery Keeps Dying — Causes and How to Fix It
A car battery that repeatedly goes flat despite being charged or replaced is a symptom of an underlying electrical fault. Replacing the battery will provide temporary relief but will not solve the problem — the new battery will also go flat. Finding and fixing the root cause is essential.
Why Does a Car Battery Keep Discharging?
The Battery Cannot Hold a Charge (Sulphation)
Lead-acid batteries degrade over time. A battery that has been repeatedly deeply discharged loses capacity through a process called sulphation — sulphate crystals form on the plates and reduce the active surface area. A battery test using a conductance tester gives a precise measure of remaining capacity.
If the battery tests as failed — below 60% capacity — replacement is the only fix.
Parasitic Drain (Current Draw When Ignition Off)
The most common cause of a battery that goes flat overnight or within a few days. A healthy vehicle should draw no more than 25–50mA after all modules have gone to sleep (typically 10–15 minutes after locking the car). A faulty component drawing 200–500mA will drain most batteries within 24–48 hours.
Common sources of parasitic drain:
- Stuck relay (windows, sunroof, central locking)
- Infotainment system not fully shutting down
- Interior light not switching off (boot, glove box)
- Aftermarket equipment (dash cam, amplifier, tracker) wired incorrectly
- Faulty alarm module
We test parasitic drain using an ammeter in series with the negative battery cable, then systematically remove fuses to locate the circuit responsible.
Faulty Alternator (Not Charging)
If the alternator is not charging the battery while the engine runs, the battery slowly depletes while driving. Symptoms include the battery warning light, dimming headlights at low speeds and the battery going flat within a day or two of a full charge.
A healthy alternator outputs 13.8–14.4V at idle. We test this during every electrical fault diagnosis.
Corroded or Loose Battery Terminals and Earth Cables
Poor electrical connections reduce charging efficiency and can cause the battery to discharge overnight if a relay keeps cycling due to low voltage. We inspect and clean all main connections.
Short Journeys
Frequent very short journeys (under 10 minutes) do not allow the alternator to fully recharge the battery, especially in winter when headlights, heating and heated seats draw maximum current. Connecting a battery conditioner (CTEK or similar) once per week maintains battery health in this usage pattern.
DIY Checks You Can Do
- Check all interior lights switch off when you lock the car
- Look for any aftermarket accessories left switched on
- Check the boot light and glove box light switch off when closed
- Charge the battery fully with a battery conditioner (not just a trickle charge), then test
How We Diagnose a Repeat Flat Battery
- Battery health test (conductance)
- Alternator charging output test
- Parasitic draw test — current measurement with engine off, modules asleep
- Systematic fuse removal to identify the drain circuit
- Full earth circuit inspection
Book Battery Diagnosis in Reading
Call 07305 523333. We carry battery test equipment and can carry out a comprehensive electrical check at your home. We cover Reading and all Berkshire towns.
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Frequently asked questions
How do I know if it's the battery or the alternator causing the problem? +
A battery that starts the car fine but goes flat after sitting overnight usually points to a parasitic drain or a battery that cannot hold charge. An alternator fault usually causes the battery warning light to illuminate while driving and the battery to go flat after a few minutes of running. We test both.
Can a new battery also fail quickly? +
Yes — if the charging system (alternator) is faulty or a parasitic drain is present, a new battery will discharge just as quickly as the old one. Always diagnose the root cause before replacing the battery.
What is a parasitic drain? +
A parasitic drain is an electrical component that continues to draw current when the ignition is off. Normal standby current draw is around 25–50mA. A faulty component — a stuck relay, a malfunctioning infotainment system, an interior light that won't switch off — can draw hundreds of mA, draining the battery overnight or over a few days.