Welp I finally made an account single owner of an 07 prius with 387,000 miles four hybrids batteries and a lot of other work done (by me and anyone I could get to help). This chat has been extremely useful to me over the years but I got one I haven't seen posted or fixed. So when you shut the car down (ready to off) ALL of the 12 volt systems go down every single one. Sometimes after a bit the car will come back into acc on it's own without anything else going on (suspect intermittent relay). If you put 12 across the jump terminals car comes up fine drives well no error codes" but if your shut it down your jumping it. I even took it to a dealership thought it was the main and aux battery. I did that. Issues still the same. considering putting a kill switch in line with a 12V right now I am soo tired of jumping this thing. Please help.
It still sounds like your aux battery is either bad or being drained. Go to your local parts store and have them check your 12 volt battery if it is good sounds like you have a parasitic drain on your battery. Check youtube on how to check that. If you use a disconnect switch on your 12 volt battery the car will constantly going through a relearn of the ECM. Do you have a voltmeter to read battery voltage? Check all connections and grounds.
I agree, the Smart key system stays powered even when the car is off there could be a short. Though the intermittent turning on to the accessory mode, I think is a clue. That would only happen if the power button was pressed without the break pedal being depressed right? Why would putting 12v across the jumper terminals work consistently if the problem was the power button? It’s got to be the 12volt or that computer back there that runs it. iPhone ?
12V battery is good! I will look in to the sks thing tomorrow the key has been dead for a couple years (needs inserted) also did note that I was able to jump it the other day without the key inserted which would be odd for me.
I'd say the problem might be a 12v not holding charge. But you are saying it's good, so need to do some more tests
Using dc voltmeter what is the 12 volt reading on the front jump point wit the car OFF. With the car still OFF turn on the headlight high beams and what does it measure then? Sounds like very clearly you have a bad 12 volt battery like 1.2 million posts on this site. They all say the same 2 things: One is : " I had it tested and it tests good" Two is: "There must be some kind of draw on the battery" Do my test. Its the only one you need to do. As a comparison my car has a very good 12 volt battery with sitting voltage with car off is 12.53. With headlights on and car OFF it measures 12.1 That's the best you can get in this car with the tiny battery.
Bad 12V battery is the most likely problem. But if the 12V battery really is good, then it’s very likely problem at the 12V battery connections. There are couple of possible bad connections near 12V battery. Terminals tight to battery? Negative cable ring terminal to body bolt lose? Or connection corroded there? Negative cable to ring terminal crimp lose or corroded?
Thats why you do all the tests at the front jump points. Your testing that whole circuit at the same time. It doesn't matter what it reads in the trunks it matters what the car see's and the car see's whats at the front jump terminal. The main heavy cable from the battery bolts right to the jump vertical tang. Its main B+ input to the system. You will quickly discover all trunks issues right there. Or not.
Yes I know that but there are two reasons why I wrote that: The problem might be intermittent. Even to the point where it works perfectly when it’s working. Or maybe the OP had the 12V battery tested. And that’s why the OP is so against it being the problem here.
Last summer I had a 12 V battery tested at Advance Auto and they use a modern tester that measures internal resistance and told me it was good. A week later I was having trouble with it again and put it on my battery charger and took it to Sam's Club and the guy said it had a bad cell. I measured the voltage before I took it to Sams Club and it was dropping fast coming off the charger and was down to 10.5 V. Strangest battery I ever saw.
Simple load test with headlights on and car off will tell you alot. My very healthy 12 drops .5 volts with high beams on and car off at the jumps. If intermittent 12 it should be real intermittent with that load on it at the jumps.
I did another test, sitting 12 on my car is 12.6 With lights on high beam and car off drops down to 12.1 volts. I left the lights on for 5 minutes to see how it would do and the 12.1 Never moved. After 5 Minutes I turned the lights off and checked sitting and it was 12.55. This is a very easy and quick test to do. All you need is a dc voltmeter and measure the jump point. If your battery is radically worse than mine replace it. Charging it is temporary and won’t fix it. Lately my car has seen nothing but around town so I have a very good quality battery living under bad Circumstances use wise.
