Finding the cause of an overnight battery drain

[Barry]

This is a very simple and cheap test you can do on your own car. All you need is a cheap multimeter at 40-50dhs. You can probably pick one up even cheaper online or at somewhere like dragon mart. 

I'm sure some of us have experienced a problem where you parked the car up in the morning and went back to it in the morning and it wouldn't start because of a flat battery. Why did this happen? It was working fine last night?

You should first test the battery with a proper battery discharge tester. This is a bit more expensive than a multimeter and I appreciate that most people won't have or won't buy one. 

Next thing you need to check is the functionality of your alternator. You can do this with a multimeter which is one of the tools i recommend everyone should have. Set the alternator to VDC. DC volts. Measure the battery voltage across the positive and negative terminals with the engine stopped and ignition off. It should be 12 volts give or take half a volt either way. Next, start the car and measure the voltage again using the same method. Depending on the car, you should see a voltage of 13-14 volts. If you rev the engine and see a wild fluctuation the charging regulator could be fried but this is a whole other topic.

Ok, now that you're sure that your battery and alternator are fine, let's find out why your battery is draining overnight. Disconnect the negative battery terminal and connect the negative wire on your multimeter to the negative terminal on the battery. Tape it on if you have to. Next connect the positive wire on your multimeter to the cars earth lead. Again, tape it on if you have to. 

Now turn your multimeter on. Set it to the highest possible amp setting and turn it down gradually until you get a reading on the screen. Once you see a reading, this is your battery drain. Pull the fuses out one by one and watch for the reading dropping to zero. When you pull a fuse and the reading drops, you know you have found the circuit where the problem is. Check your cars fuse diagram and see what the fuse corresponds to and you know where to start looking for the problem. 

[treks]

The only thing(s) I can add to @Barry's advice here is to check the manual for your car to see what will happen if you disconnect the battery. In some cases, the ECU (and/or) other controllers could lose vital memory or programming, which means that you have just made the problem worse. 

Also, bear in mind that a minimal current drain is normal. The radio and ECU's memories will draw about 30-50 milliamps. This is normal, and will not drain the battery overnight. If the current drain approaches 75-100 milliamps there is a cause for concern, but even this will not drain a healthy battery overnight.

Current drains that cause a battery to run down overnight can be traced in the way @Barry described, but these current drains are also heavy enough to be traced by leaving the battery connected. Just figure out a way to attach the multi meter's probes to the battery terminals without disconnecting the terminals, which eliminates the risk of damaging the ECU.   

Edited February 3, 2017 by treks
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[Rahimdad]

Great information shared by @Barry and @treks