parasitic draw testing

I would expect that if you were a whizz at computers you should have a basic conceptual understanding of how electricity works. It works pretty much the same way in cars. Maybe you overthinking things.

 

The Prius has a somewhat complicated system of two electric motors and one ICE motor. For the purposes of this discussion it can be simplified to:

In Park or when driving forward the ICE motor drives one of the electric motors which acts as a generator. That is stored in the big battery pack.

Braking is similar - the wheels turn one of the electric motors (acting as a generator) and the high voltage produced is stored in the big battery pack.

If the car is on the inverter takes energy from the pack and converts it to around 14V to charge the auxiliary (12V) battery and power all the 12V parts of the car.

If the car is being driven forward the pack sends high voltage to the electric motors which are mechanically bound to the ICE motor and drive the vehicle forward.

(Reverse is more complicated, look it up online if you care. Doesn't matter at all for finding parasitic draws as the car will obviously not be moving while doing that.)

 

That's great, you got a result. That is showing a 400 mA load, which is higher than it should be after 45 minutes.

Did you take a photo of (or remember what) the reading was when you first connected up the meter?

By the way, did you also turn all the interior lights off at the start?

 

If your car has the 9-speaker JBL stereo system, unplug all the plugs from the amplifier under the RH front seat (passenger seat in the US). If the meter is still connected see if the number drops. If it's not connected, just repeat what you did (with the amplifier disconnected) to get the 0.40 Amp picture.

If the number is still 0.40, then you will need to go to the engine bay fuse box and start removing each fuse, check the meter, and then reinstall the fuse one by one until you see the current drop down – hopefully to 0.03 or less. This will go quicker if you have a friend at the back watching the meter.

Make a note of the fuse you removed (or just snap a photo) that made the meter drop down and let us know which one it was.

 

Is the trunk still open? That counts as an open door and there will be a roughly 500mA draw. You reported 400mA, which might be the same. I don't know how much "door open" varies from car to car, or even door to door. I wouldn't have expected a 100 mA variance, but that is an assumption not based on data.

As long as you have access to the battery then you also have access to the pack of capacitors in the black rectangular box next to it. That has been reported as a common source for a parasitic drain. Try unplugging the cables at that box and repeat your measurements. Don't forget to plug them back in!

Note, if you want to keep the trunk open the switch can be fooled with a carabiner that is roughly the same size as the steel loop on the bottom of the hatch. To get the carabiner back out again press the release button on the hatch. Don't forget to remove the carabiner before closing the hatch. (It is all too easy to do.)

 

No good. Look at the dash while the door is open, notice the "door open" indicator light. That must be off when testing for parasitic loads.

 

20 mA is an acceptable current draw for an "off" Prius.

Why do you think there is a parasitic draw? Is this another case of "the12V battery is low" which turns out to be due to not driving the car enough? The Prius charges the 12V much slower than regular cars with an alternator, and it doesn't charge it any faster at higher RPM than idle RPM. So it can take a long time for a depleted 12V to charge. This is why a lot of Prius are attached to battery minders, because if the driving pattern has changed to "occasional short trips" it will eat 12V batteries unless the 12V is charged externally. Or make a point of taking it for a 1 hour drive every week.

 

Not having your battery clamps tied up securely is definitely a problem. Automotive Diagnostics 101. Yes it is true, and I, for one, didn't mention it because a) there was so much going on and b) you seem to have default m. o. of not believing anything anyone says.

Whether it is the problem, only time will tell.

 

Having the cable clamps on tight is good, being sure that the battery posts and clamps are clean is also good. If one of the clamps is corroded or there is a layer of gunk on the post tight may not be sufficient to get a good connection.

We recently had a problem like that on the wife's Accord. There was a starter problem and it was replaced. We were testing from a battery in a truck using jumpers to the cables in the car (which were not on the car battery), and the new starter wasn't turning over the car. But it worked fine once the cables were reattached to the car battery. There was nothing wrong with the truck battery, it had much more capacity than the car battery, but the jumper cables were making a poor connection somewhere. We think it was probably the clamp onto the cable connection at the positive battery post on the truck, since the outside of that was dirty and somewhat corroded.