I would start with checking the battery terminals and cables. If there is any greenish/whitish corrosion, then take them off and clean them off. A piece of fine sandpaper would work well for that, but they do sell a tool for it at the auto parts stores as well. Make sure that the terminals are tight enough that they cannot be moved around with your just your bare hands, if they can be moved like that and cannot be tightened any more, then replace the terminals. Also, make sure that the cables are similarly tight on the other ends.
If that does not take care of it, then you have an electrical drain/short somewhere in the system. You will need a voltmeter/multi-meter. Take one battery cable off. Connect one lead of your multi-meter to the cable, and the other end to the battery terminal. You should get some sort of reading on the meter. Then start pulling fuses until a significant change happens on your multi-meter (should be a drop in voltage). I always start with circuits that have aftermarket electronics on them (like that radio, for example). When you see that change, you have identified a circuit that is drawing power and should take a closer look at it to make sure that no switches in that system are left on, that there are no bare and exposed wires, etc.
You can also check out this guide: https://www.2carpros.com/articles/car-battery-dead-overnight
But I like my way more, personally.
Tuesday, June 27th, 2017 AT 9:40 AM