From time to time the auxiliary battery in my 2012 Prius C is drained so much that it becomes hard to start the car. When this happens the voltage drops to about 10.5V. The battery is about 4 months old. With all doors closed and lights off, the parasitic current jumps between 103 and 130 mA. This is measured with a multimeter connected in series between the battery negative post and the battery nevative connector. Not sure what the normal value is, but think it should be much lower, probably around 20 mA. My car doesn't have a SKS. The key needs to be inserted to drive the car. There are two fuse boxes under the hood. To find the culprit, do I need to unplug every fuse and replace it with multimeter leads? Or is there some simple way to do it?
Just leave the multimeter connected to the battery and pull the fuses out one at a time, the draw shown on the multimeter will drop when you have pulled the fuse on the circuit that is drawing the excess current. That said, be aware that most modern cars will often have circuits that draw power for awhile, like ten to twenty minutes, after the key is turned off. If this is an intermittent problem it can be very hard to track down. Do a Google search for "parasitic draw on a 2012 Prius". With luck you might find that someone has already solved this problem.
You can use any goat; there's nothing special about finding it. The key is, you have to bring two goats, and cast lots. The one that wins (from a secular goat's perspective), you then lay both hands on, blame for the parasitic drain, and release into the wilderness.
Maybe the battery isn’t charging fully? It’s also a small battery. 130 ma would take 4 days to draw 12.48 amp hours from the battery. If the amp hour rating is 50, the battery should be 75%. Maybe that’s my too simple understanding. You may want to think about what happens when you disconnect the battery then put your multimeter in the circuit. What about the computer has to be reset, even off? I don’t know all what happens.
From Arts Garage in Berkeley, I found though a link on Earthling Automotive - Electric Car Repair in San Francisco site Does your Prius need a jump start every time? Check these things. This link is discussing how to find a parasite draw in a Honda, should be good starting point for howto locate the draw in any car, I'd think, anyways. Honda battery draining overnight? Here's how to solve the mystery!
There is also the evaporative emissions test, which runs several hours after shutdown. The car would be expected to draw a few hundred mA for a few seconds, maybe a minute before returning to background drain level. Sometimes you can hear the pump and valves ticking a bit when that happens.
