Prius 2001. New owner. 12v battery is draining even with another good battery jumpered on. Why?

As soon I turn on the ignition it pulls the aux battery way down below 12v. I've used different batteries which I know are good but it pulls them down rapid...

 

Thanks for replying,

I did that yes. connected a big 120 AH leisure battery directly, charged and steady 12.9 v. It drew down to below 12v within 30 seconds of switching on the ignition. The car is issuing a P3125 code. The car won't start , it just switches every thing on and off when i try.

 

Hi there folks, Thanks for all the replies/ suggestions. That gives me a few more things to check over. I'll give a bit more back round for clarity.
I bought the car a week ago from a lady doctor who owned it in England, She had moved back home to Portugal and the car had been layed up for a couple of months and the 12v battery was dead, She said the car had been driving fine when it was left there, although she had to replace the 12 v twice in three years under warranty from toyota (which gives me the suspicion something was wrong). Her friend managed to jump start it and drive it (turtle mode) to local mechanic who kept it for weeks and done nothing but tried and failed to charge it's totally dead 12v battery. SO anyway i took a chance and bought the car for scrap price, had it shipped from the mechanic in England to Ireland and then then towed it on and off another ferry to the island where I live.

The car only has 89 000 miles so i thought i'd take a chance on it, nearly every car I've every had is twenty years old or more so I'm only used to old machines with plenty things wrong with them.

So far I got rid of the dead toyota 12v, (which is still under warranty) bolted on two heavy cables to the top of it's own terminals so i could connect a large 12v battery with big terminals (i have a few of these 120AH deep cycle batteries use for an off grid setup.) I understand that you should use the correct size AGM for the car and I will, if i get the car running but for now surely any good 12v with capacity should okay for testing right?

I checked the 100 Amp fuse under the hood in the fusebox and it seems intact .
After reading on here that a flat HV problem can cause all sorts of problems i moved on to that.
I took the big battery out and stuck it on the workbench. It measures 300v at the output terminals and 7.85 on each cell. That seems okay ? Gona put it back in today.

One piece of vital information is that, as the lady was having 12v trouble for a few years i think the car has been jumped quite a few times (there was a jump pack in the boot). It could be quite possible it had been connected reverse polarity by someone.... who knows.......

I'm gona check the connections to and from the inverter and remove the abs relay as suggested.
Also I can buy a salvaged inverter for 100 pounds so I think i'll get one anyway.

CHEERS PHIL.

 

What I consider a big battery:
} hurts your back, legs and arms to lift
} Big massive sparks fly out when you do something wrong
} Cost a big heap of money

 

yes, I suppose you're right.... I've heard of people welding with mobile phone batteries....Rmay635703 reckons i should try and produce smoke. Exciting these Priuses....

 

There did appear to be a pump stuck on for the brakes. I could here it running when i tried to start the the car, I struggled for a while trying to remove the main ABS solenoid from the drivers side fuse box, opened the clips but the thing wouldn't budge. I ended up loosening the whole fuse box to get a look underneath and check for melted wires, but all seemed okay there. Anyway after leaving the car switched on and jumpers connect connect to another running vehicle so the battery voltage stayed about 12.8 the noise stopped and the current drain on the 12v stopped. I cleared the codes with an android app and then i got the car to start. I let it run and could see it was charging the 12 v steady at 14.2 it was also charging the HV battery which SOC was okay at 52% . I took it for a drive and it charged the HV battery fine . Then when i switched the car off again and tried to restart it started draining the 12v down and wouldn't restart again. red triangle and P3125 back again. The app I'm using won't give me any sub codes just parameters. got tech stream on the laptop but no cable yet. I'm just glad to hear the thing running at all to be honest.

If it is a short, do you think it is likely it could stop shorting when the car is running? I got the car going today and once running the 12 v battery was sitting nice and steady at 14.2. could it be dc -dc 12v charging system is putting enough out to keep the voltage steady? I still didn't get any sub codes......

 

I would love to believe that but I tried another 110 Amp/hr battery in today(fully charged and good condition) and it can't start the car without being jumpered to a running vehicle.

The car seems to drive fine once going, then when I switch it off the voltage of the battery drops to 13v as you would expect, but when I immediately try and switch the car back on, the voltage drops to below 12 in seconds. Sure sounds Ike a dud battery but it ain't.

I'm metering at the battery with a multimeter ,but the 12v supply voltage stated by the ECU is always 0.5 - 0.7v lower.
I tried pulling fuses , the only one that made any difference was AM2

 

Thanks for the reply, and I appreciate your insight.
What are the risks involved in jumping off a running vehicle.
Can it cause particular problems with a Prius?
Right now it's the only way I can get the car to run. It will stay in ready mode all day with no problem.
I managed to put a new code on by pulling out fuses. This code now won't clear.

• DTC: C0278
• Description: Hydraulic modulator solenoid valve relay - circuit open