Start with the simple things. Auto parts stores can test your alternator for bad diodes, which if failing can cause this. Next, disconnect battery neg and hook up amp meter, then remove one fuse at a time to find which one changes the reading, then figure out what that fuse powers and check for something like a glove box door loose and not turning off the light. I have even seen a relay sticking on, or one being replaced by the wrong type such that the pins were wrong and the coil was always on. You can find this by feeling the top of each relay with everything turned off. If one is warm or hot you have found the problem.