Why does the battery keep going flat

Hi @derekmead49264

Use a DMM's (digital multimeter) Voltmeter function to measure the battery voltage.

With the engine not switched on, the battery voltage should be in the range 12.0V - 12.6V DC.

With the engine switched on and running¹ (idling - handbrake on, transmission in Park (AT) or Neutral (MT) the battery voltage should be in the range 13.8V-14.5V DC.

If the battery voltage is <12.0V DC with the engine off, it could be a faulty battery not holding its charge

If the battery voltage is <13.8V DC with the engine running, it is not being charged correctly. This could be due to a faulty alternator or voltage regulator.

If the battery voltage is >14.5V DC with the engine running, the battery is being overcharged and it could be being damaged by the overcharging. This could be due to a faulty alternator or voltage regulator.

If the battery voltage is between 12.0 12.6V DC with engine off and is between 13.8V -14.5V DC when engine is idling, then there could be a parasitic quiescent current drain on the battery when the engine is off that is discharging the battery when it shouldn't be.

The normal quiescent current from a battery (engine off) is in the range of 50 - 80mA (milliAmps).

With the engine off, disconnect and remove the -ve battery cable from the -ve battery post on the battery and connect the DMM (Ammeter function) in series connection between the -ve battery cable and the -ve battery post on the battery and check what the quiescent current is, i.e. -ve battery post → Ammeter → -ve battery cable.

If it is >80mA you need to remove all the fuses, one by one and check what the quiescent current is with a fuse removed. It may be quicker for two people to do this i.e. one to remove the fuses, one by one (so as to not confuse which fuse belongs where) and one to watch the meter to observe the current measurement. Perhaps start with the passenger compartment fuse boxes as these usually haves fuses that are lower rated and more specific to a feature rather then the engine bay fuse box where higher rated fuses are located and some of which supply more than one lower rated fuse.

If it is still >80mA, replace the fuse and remove the next one etc, until you see that the quiescent current falls back down to the normal value. Once it does, check what circuit the fuse supplies power to i.e. fuse number description in fuse box lid or the owner manual.

Once you know what circuit is causing the excess quiescent current to flow, when it shouldn't be, you're halfway to finding out what's wrong.

¹ Be safety aware when measuring the battery voltage as the engine is running! Watch where you place your body, hands etc and watch for loose clothing etc so that it's not caught in any moving parts of the engine