Checking 12v battery status "for Dummies?"

Possible issue with the linked product: the Prius 12 volt ports only have voltage when the car is turned on, and I think that's going to effect the voltage reading.

What works for me is a digital multi-meter, half decent ones are around $30~50. Check the voltage at the jumpstart point in the under-hood fuse box (with the car off). If it's in good shape it'll be around 12.6 volts or higher. 12.5 volt, or lower, and the battery's maybe starting to deteriorate.

Also, if you check voltage right after a charging session, it'll give falsely high readings: you want to drive it for a day or two before checking.

 

Only on the Gen II Prius could you check the voltage with the MFD. Don't know of a way to check voltage on any of the screens on the Gen III like you could do on the Gen II.

I agree with Mendel. Use a digital voltmeter. Check the voltage at the jump post. 12.6 is great. 12.2 during Winter is okay.

When you get below 12 volts though, that is getting pretty weak.

Consult the following chart for a good explanation of what the voltage represents. "Measure" voltages under a NO LOAD condition and compare your readings to the chart.

Anything 50% or below is pretty much toast in my book.

Ron

 

There's a place in the service mode of the Nav system (at least on 2010-2011, not sure about the later models) that gives the voltage... But it's wrong. First, because you're no longer measuring it at rest; you're measuring it with a load. Second, it's either measuring after a regulator or diode or something like that, or down some wire with some serious IR drop, or just using a terrible sensor; in my car, I measured 12.17V at the battery and front jump point, while the Nav said it was 11.5V at the same time.

I probably won't start thinking about replacing my battery until I either notice odd behavior (i.e. failure to turn on), or the voltage is under 12V (as measured with all doors closed and lights off, for at least 15 minutes).

 

I usually check mine when it is convenient for me, however I don't check it right after I have turned off the Prius and or just finished recharging said 12 volt battery.

I always try to check it after the car has set at least overnight. I usually check it about once a month. Real easy to do if you use the multi meter and touch off at the jump post under the hood.

Behind the fuse panel you will see one of the three strut nuts. Put the negative on the nut and the positive on the jump post. Hold everything firmly and then read the meter. Really easy to do. Hardest thing is getting the stupid cover off of the fuse panel, but once you figure out where to push on the plastic release tab, even that becomes easy...

Ron

 

First thing in the morning is great. I'd recommend popping the hood the night before (or 5 or 10 minutes before, at least) so that you can open the hood without having to open the driver's door - doing that will turn on the lights and the brake booster pump, which will cause the battery voltage to drop for a few minutes as it recovers from that load.