6 reasons why your car battery power can be drained and how to fix them

Does your car fail to start in the morning? Does your car battery lose power sooner than expected even when the car’s alternator system seems to work properly?

If so, then may be a sign of your car’s battery draining fast.

In this post, I will share the 6 common causes of your car battery draining flat even with the ignition off and how to fix them.

1. Aging battery with a less ability to hold charge

Aging batteries tend to lose their ability to hold charge. They tend to drain faster than newer batteries.

Aging car battery
Aging car battery

You most likely have an aging battery if:

  • the car’s alternator works well, the battery leads are firmly fixed to a well serviced battery yet it continues losing power fairly fast even when fully charged
  • the battery age is close or past the specified lifespan by the manufacturer

Fix: Replace the aging battery with another of the type recommended by the car manufacturer.

2. Poor quality battery by the manufacturer

Newly purchased batteries can fail prematurely if they are of poor quality. Before you can conclude that it is a bad battery, check that:

  • your battery leads are connected firmly to the battery
  • your car’s alternator is working properly


  • If it is a bad battery, you need to return it to the shop. Always buy batteries with a warranty
  • Check that the battery being purchased has genuine positive reliability reviews and honors its warranty.

3. Electrical fault in the car’s wiring

Does your car have a parasitic load that is consuming power even when the ignition if turned off? Perhaps a trunk light that is accidentally left ON, a faulty cigarette lighter, faulty car alarm?

To check if you have a parasitic load, with the ignition switched off:

  • Disconnect the battery lead from the positive battery terminal.
  • Switch a multi-meter to measure DC amperes and connect one lead of the multi-meter to the wire disconnected from the positive battery terminal then connect the remaining multi-meter lead to the positive battery terminal.

You should expect battery drain of about 25 mA. Current drain in excess of 100 mA is a sign that an appliance is still connected and draining battery power or faulty wiring.


You can isolate the problem by removing the fuses from the fuse panel one at a time until the ammeter reading drops to the normal level of about 25 mA or take your car to the mechanic.

4. Poorly serviced batteries

Is your car battery a Flooded Lead Acid or Sealed Lead Acid battery? If it is a flooded lead acid battery, make sure that it is topped up with distilled water up to the recommended level.

For both Flooded Lead Acid or Sealed Lead Acid battery types, confirm that the cable clamps are firmly holding the battery terminals.

Inspect for and remove any greenish yellow residue that may have formed on the battery terminals. This residue or loosely fixed Battery terminals ca affect a battery’s ability to charge fully and thereby draining flat.


  • Confirm that the battery cable clamps are firmly fixed to the battery terminals.
  • Remove any greenish yellow residue that may have formed on the battery terminals.

5. Battery self discharge over an extended period (if not trickle charged)

Batteries left unattended for a a week or more can self- discharge significantly draining the battery over that period. When the battery voltage is below 11 volts, the car fails to start.

An attempt to start may result in a clicking sound or no sound at all.


Should you plan to leave your car unused for an extended period, consider:

  • Connecting a Solar trickle charger to keep your car battery charge topped up
  • Having a friend or someone periodically starting the car and revving it about once in a week to keep your car battery charged.

6. Bad alternator

A bad or failing alternator will not be able to fully charge a car battery resulting in the battery draining faster than usual. A properly functioning alternator with has an output voltage of between 13.3 to 15V with the car’s engine running.

You can also confirm that an alternator is working properly by:

  • With the car engine running, checking that the car lights shine brightly. Dim lights are a sign that the alternator is not producing enough power and therefore the lights are drawing power from the battery.
  • The car light’s brightness does not increase when the engine is revved.


If you suspect a faulty alternator, you can confirm with and have it replaced by a qualified mechanic.

In summary,  if your car battery is draining faster than usual, this may be caused by an aging battery, new but bad battery, faulty wiring, bad alternator or battery self-discharge.

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