Hey there Prius heads... I've got a Priusasaurous 2001. Phantom current draw on the 12 v battery.

I recently got a gen 1, It was laid up a while and the 12v battery was dead. After attaching a few different good 12 aux batteries I now conclude there is a current loss somewhere.

I have code p3125 which I can clear using an android app. I still haven't been able to gain the sub codes.
When I clear the code the car will start but only with a really strong battery.
It drives fine charges the HV battery fine and also keeps the aux system at 14.2 v once running and will run all day if left on ready.
When I switch it off the code reappears and the 12v battery needs to be recharged or jumpered to start.
I'm using standard 12v flooded wet cell batteries and not AGM . Is it true the car won't charge them ?
I want to bring life back to this old girl...

 

Thanks for the reply, I will run those tests and check. Although I tried a new battery which I had just put in my diesel van last week It's a 100amp/hr and starts the van no problem.
The Prius wouldn't start with it today. When I'm metering at the battery and the voltage reding 12.9v but reading for the aux battery from OB2 connection is 12.3v. and drops below 12 v when I try and start. I've also tried a Optima red top I use in another vehicle.

 

Is it common for the gen 1 to display code P3125 because of a dead battery?

 

Okay I tried those tests this evening, the battery passed the headlight test. It started out at 13v and dropped to 12.6v in a the first two minutes and then only slowly dropped to 12.55v by 5 minutes.

The second test , the reading at the battery once recovered was 12.9 , the fuse box reading was nearly exactly the same (12.85-9) with no load . With the load applied the fuse box reading was .5 v lower at 12.4 v .

Also the aux supply voltage stated by the ECU is even lower at 12.1 v

I did start to doubt my connections to the battery but I did check again and I'm pretty sure they are good.


I've been trying to get techstream to read the sub code but it so far won't interface with the OB2 .

Here is the picture of the operation, I bought this car for scrap price but it has low mileage for its age ( 89,000) and a good HV battery.

Any help appreciated...
Phil.

 

I think perhaps I wasn't clear in reporting my results, The battery never dropped more than .45 volts during any of the tests. It stayed above 12.5 during all tests and an attempted start . The test did show there was a .5v difference between the battery reading and the fuse box reading with the headlights on. Does this mean something? There is nearly 1 volt difference difference between the battery voltage (at terminals) and the voltage which the ECU is is receiving (read from ob2 parameters) , but this difference was there before i began any tests. The power is going somewhere down the line.

I took this battery out after the tests and it starts a 2.5 litre high compression diesel engine so unless the Prius computers and relays are some sort of monsters on the juice it is not the batteries fault.

 

Thanks, yes that's correct it didn't drop a volt. Yes I have the rear seat removed and long leads on the meter so I can voltage whilst trying to start. The voltage of the battery (at terminal) does not drop below 12.5 when everything is fired up and i try to start. But as stated above I do believe the car is not receiving 12v at the computer. The ECU is always stating 0.6 -0.8v lower than battery voltage.

I have actually already purchased a second hand inverter , but I'm not going to change them yet until i can narrow the fault down just in case i cause more problems.

Yeah I intend to get the correct battery, the one which came with the car is the correct one from Toyota and it is still under warranty although I haven't had a chance to get to them and harass them for a replacement. I live on a small island and the weather is stormy here, otherwise i would have been up to Toyota and gotten them to run a diagnostic. They've told me they charge 70 pounds (about 100$) for the pleasure. Is that comparable prices in the US?

cheers.
Phil.

 

Thanks again for the replay and continued help,
You are right about a dodgy ground connection a being likely culprit in this sort of scenario, I've been caught out by it a few times on old wagons..... Alas in this case it's not, I had the ground disconnected earlier and it was quite clean and well secured. Damn it anyway... It would have been a nice easy fix.

I'd heard about the HV battery dying on older cars especially when left sitting and when I got the car I pulled it out and tested it on the workbench. It was testing perfect at 7.85 on every cell and 300v (the gen 1 is a 290v batt) total at the main terminal. I'm happy about this! I think it was replaced about 6 years ago.


I have found one lead now however, The current draw appears as soon as the the car is switched to on position... I pulled all the fuses one by one with the ignition on, whilst testing at the fuse box. Removing the AM2 fuse stops the loss .