What can drain a car battery when it is off?

Does your car battery drain overnight? Do you have to jumpstart your car every now and then?

The problem of car batteries draining flat affects many car owners and the cause might be straightforward or complex requiring a specialist mechanic to trouble shoot.

Here is a list of the top 4 causes of battery drains and have to solve them

1. Parasitic drain on the battery

The recommended place to start when investigating battery drain is to look out for parasitic loads. It may be because of a faulty device, or device that is automatically switched on by the car’s system or a device that is unintentionally left switched on.

Sometimes the offending devices are not turned off by the car’s ignition and are hardwired into the car’s electrical system.

Common devices from experience that are the source of car battery drain are:

  • faulty stereo, stereo amplifier or one that is accidentally left on
  • faulty security system
  • after market items hardwired into the car’s electrical system with a high battery drain
  • headlights accidentally left on
  • interior lights – dome or trunk left on
  • hazard lights left on
  • car’s ac set to auto and automatically coming on? bad fan, relay

Sometimes the cause of the problem is not obvious especially when it was installed by the previous owner. Trouble-shooting in this case may require patience or you can simply consult a speciliast mechanic.

2. Bad or Aging battery

A bad or aging battery looses its capacity to hold charge. When the battery falls below 12 V, the car will fail to start. A large voltage drop e.g. 12.8V to less than 12V when a fully charged battery is disconnected from the car’s electrical system and left standing overnight is a probable sign of a bad of aging battery

3. Battery self-discharge

Even brand new quality batteries lose some of their charge over time. To prevent battery drain from self-discharge

  • Connect a trickle charger to the battery if planning to park for a long time
  • Have the car started about once a week and the engine run at least 1,500 rpm to keep the battery charge topped up

4. Bad Alternator

A bad alternator can fail to fully charge the battery resulting in the battery appearing to drain so fast. You can check for a good alternator by checking for a voltage at alternator of between 13.8V – 14.5V with battery still connected to alternator while the engine is running at least 1,500 rpm.

Lower voltages is a likely sign of a bad alternator.

The four common causes of car battery drain are also summarized in the table below:

No.Possible cause of battery drainHow you can testSolution
1Parasitic drain from an appliance left on (car stereo, amplifier, interior lights), faulty wiring or malfunctioning device (car headlights, car A/C, blower fan)Switch off the car’s ignition.   Connect an multi-meter set to read amps between the  positive wire of the battery and the positive battery terminal.   Current more than 50mA is a sign of a parasitic loadSwitch off the appliance left on; Replace the faulty wiring
2Bad or Aging batteryDisconnect fully charged battery from the car’s electrical system and leave standing overnight or for a day.   Large voltage drop e.g. 12.8V to less than 12V with no load is a sign of a bad or aging batteryReplace the battery
3Bad alternator not charging the car battery fullyWith battery connected to alternator, switch on engine and run at least 1,500 rpm.   Voltage at alternator should be about 13.8V-14.5V.   Lower voltages are a sign of a bad alternatorReplace the alternator
4Battery self-dischargeCar parked for about 2 weeks fails to start   Car is able to start after overnight parking or when parked for about 3 daysConnect a trickle charger to the battery if planning to park for a long time   Have the car started about once a week and the engine run at least 1,500 rpm to keep the battery charge topped up

Related Topics

How you can check if an alternator is draining the battery
Why power inverters may be bad for your car