Low 12-Volt Battery when I start my prius

I’ll have a go. I have an old ammeter with an upper limit of 150mA so that should work. Can I check the connectors go between the negative terminal of the battery and anywhere on the car body or engine?

I tried them on the negative terminal and the engine block, and it showed zero charge...

 

You need to ensure the only way current can flow is through the meter. What I did:

1. Disconnect negative cable clamp at the body, connect a long (insulated) wire to it and isolated it from contact (stuff the connection zone) into an insulated rubber glove).
2. Using the bolt that held the cable to the body, clamp a second wire.
3. Run both wires out across the hatch threshold (ends isolated from each other) and gently close hatch.
4. Wait about 1/2 hour, with fobs(s) well away from the car.
5. Connect up meter to the leads, for amperage reading. Start with amperage scale first, then (if manually selecting range) the highest milliamperage scale, then gradually step down. Just to avoid possible overloading the meter, if there was excessive amperage for some reason.
6. Disconnect meter, ensure leads are isolated, open hatch, disconnect wires and restore battery cable connection to body.

 

Then......you need to be VERY careful not to do anything that will "wake up" the car after the meter is connected.
If it does come to life, the meter likely will go up in smoke.

 

Most meters will have a fuse on the low current range but not the 10 A range. You could add your own fuse in line with the test setup to be safe. Or use a clamp meter, but it will be hard to find one that can accurately measure a few mA DC.

 

Off the top of my head I can think of two constant drains: the smart key system and the coolant heat storage system.

 

Many of the ECUs have a +B supply connection, to keep their memories alive. Clock. Safety Connect if you have it. Occasional evaporative emission self-tests, in some models anyway. ...

 

I don't think the primes (or any 4th gens?) have a coolant heat storage system.

 

Nor do the Gen III's. They use an Exhaust Heat Recirculation System.

 

Also, it wouldn't make sense for the Prime specifically. I often go weeks without using the engine.

 

The only issue I can see - if you just walk around the car - it seems to sometimes make a noise.

I unlocked my car from inside the house yesterday (to take the photo of the battery). Opened the door, opened the bonnet release, closed the door. And as I opened the bonnet, I could hear at least one relay or something give a sigh. No idea what it was. Or why. But I didn't have my key with me - was 7 metres away.

 

Every time you open the driver's door when the car has been parked, the brake booster pump runs. It uses a lot of power. I think it's on a 40A fuse or maybe even larger. Just walking by the car turns on the interior lights, but I'm not sure if it does anything else.

 

If someone tried to find out about current flow, it could be quite confusing. Not like in the old days when you disconnect the clock, attach a meter to see if there is any flow.

 

These Toyota hybrids sure make a lot of weird funny sounds. I think the Plugin models make even more weird sounds than the regular Prii do. The sounds are not funny though when the 12v is low and left wondering why.

I've heard the soft humming noise on the passenger side of the car at least once if not 2 or 3 times. Since it happens so sporadically I don't pay too much attention and just try to make a mental log of where it was and what it sounded like.

( I don't see the video attachment with the sound ) only the jpg attach in the OP, If it was happening enough for me to get a recording of the sound I'd be looking more closely to try to figure out what that sound is.

 

I remember a discussion on PriusChat soon after Gen 4 came out. So I went and sat in mine - can't remember how many, but I think about ½ doz I could hear inside. Then I walked up to it, unlocked, but didn't get in - and logged about 3 or 4 I could hear from outside.

Just as well it doesn't have Electro Pneumatic Suspension - it would have great fun putting on a display just to show off.

 

The last 12 postings since #42 have put me off using my ammeter!

It’s a basic one with only one setting (max 150mA) so no way to protect it from blowing if something unexpected came on.

I have too little knowledge of this little gem of a car, and these last posts are a warning that there could be any number of devices that might wake up when I’m playing with this in series!

Thank you all for this interesting discussion though!