12v low but it's a brand new battery

I just walked out to my Gen 3 and got 11 to 11.3 on the signal check screen under the same conditions (lights and brake lights on), and the car is behaving perfectly and I am not especially concerned about the battery.

Don't forget that there are multiple resistive circuit elements (fuses and fuse links) on the path from the battery to the headunit where that measurement is made, and a direct multimeter measurement at the battery posts would have been higher. I can't tell you how much higher, because I'm just not concerned enough to go pull up the deck panels and measure it.

-Chap

 

you can measure at the jump point under the hood.

 

try charging the new battery.

 

It could be a dud, a battery that sat neglected on store shelf for too long. Or it could very well be due to your measuring method. Battery acquisition is definitely an area where some foreknowledge and good tools are important.

Was it a local store purchase, or mail order? If local, ask them if they have an electronic battery tester, if they'd be willing to test it for you. This is a step up from simple voltage check, requires the user to input some data about the battery type and Cold Cranking Amps (or Cranking Amps), then does an electronic load test, displays the measured CCA, and gives a verdict.

On you own, for starters, I'd try charging, with decent 3~4 amp range smart charger, first. Then drive a day or two. Then measure voltage with a digital multimeter, with the car off, preferably after sitting overnight, post what you get. To get a better idea, look into something like Solar BA5 tester.

 

Though I don't have the wiring diagram in front of me at the moment, I suspect the front jump point is still separated by at least one fuse link from the battery terminals. The reading would be nearly the same with the car turned off, and only minimal current flowing. But under the OP's testing conditions (car in ACC, lights and brake lights on), I would expect the difference to be larger.

-Chap

 

agreed, and the battery would test lower at the posts as well. but the o/p's testing condones are a requirement of seeing the voltage on the screen, except for lights, which should be turned on to see the load vs no load drop.
i can't verify, but if toyota put a fuse between the jump point and the battery, that could become a major issue.

 

Or, if they didn't put a fusible link between the battery and a 20 foot length of wire running along metal surfaces all the way to the front junction box, that could become the kind of major issue that gets news coverage and damages awarded.

The fusible link at the battery end is built into the clamp that sits on the + battery terminal. You can see by inspection no other wire is attached on the battery-terminal side of it. There are more fuse links at the front junction box end of that cable, so the only thing I'd kind of need to see the diagram for would be to remember where the jump point sits in all of that, i.e., whether there is only one fuse link between those two points, or more than one.

-Chap

 

thanks, please let us know when you find out.
btw, if you check the static voltage at the jump point and the terminals, it's the same.

 

Right, the voltage will be the same on both ends of any resistance when no current is flowing through it. (There are small currents flowing even when a Prius is off, of course, but not enough you would notice a voltage drop with a typical meter.)

-Chap

 

If the brake fluid pump/actuator is running (that low clicking/buzzing sound) then it draws quite a bit of current and your 12V battery voltage will be depressed. Put enough load an any 12V battery, no matter how new it is, no matter how good it is, no matter how fully charged it is, put enough load on it and the voltage will drop below 12V.

What is the voltage in signal check mode with no accessories running (no brake pedal, fan and a/c off)? What is the voltage as above but with only the headlights on? (full headlights, normal beam, no brake pedal)

 

So what you know is that the voltage to the vehicles main 12v loads is low, not if the battery is. You need to check BATTERY voltage and compare that to the LCD screen, if they differ by more than 0.75v to 1.0v, you have a major wiring problem in the 12v circuit somewhere.

 

Hello, I replaced my 12v battery in my 2009 Prius about 4.5 years ago. I drive this purely as a commit car, 100 miles per day currently 210k miles. I checked the battery voltage of my 12v with system check and it was 11.8ish drop to 11.4ish with lights on. So I ordered the AC Delci battery on amazon rather than the optima yellow top I did last time. I used a cheap multimeter to check voltage of the new batter it was 12.55 which was what it was suppose to be. I put it in the car and did system check which showed like an 11.9V with no load. I thought it should be higher?? I then tested the old battery with the multimeter and it read 12.55...

Was my old battery fine?? Should the new batter be reading higher than 12v with no load in system check?