Why is my battery draining?

Hi @rangsit

Check that the battery is being correctly charged.

With the car parked safely, transmission on Park (or Neutral) and the handbrake applied, start the engine and then use a DMM (digital multimeter - DC Voltmeter function) to measure the voltage across the battery terminals.

The voltage should be between 13.8V - 14.5V. If it is <13.8V the battery is not being charged correctly when the engine is running. This could be due to a faulty voltage regulator or alternator.

If the charging voltage is OK, with the engine turned off, disconnect the -ve (negative) battery cable from the battery and then connect a DMM (digital multimeter - Ammeter function) between the battery cable and the -ve battery terminal i.e. the Ammeter is in series connection between the cable and the battery terminal and check what current is being shown on the meter.

With most cars the quiescent current flow is 50mA-80mA.

If the meter registers more than this, start methodically removing all the fuses, one by one and check what the current is with a fuse removed. Replace the fuse before removing the next one and so on until the meter shows that the current has fallen back to within the normal range.

Once you know the fuse (when removed) that causes the current to drop back,, check the fuse box layout diagram in the fuse box lid or owner manual to find out what the fuse is used for to supply power to in the car.