Minimum 12V Voltage. When is it time to change?

What do you think the minimum voltage (MFD service menu measured) should be before changing out the OEM battery?

I know people like to toss around relatively high voltages like 12.6 to 12.8 volts, but that's open circuit for a new battery. In my experience the voltage measured on the MFD is a little lower than the true open circuit voltage (about 0.2 to 0.3 volts lower), and some people report never seeing more than about 12.3 volts on the MFD even from new.

So what do you think should be the lowest voltage before changing out the 12V battery? I'm thinking about 12.0 volts minimum for no load (accessory mode) and 11.5 volts minimum with the headlights on, (assuming car has been off for several hours at least).

I know those figures sound a bit low. However I'm trying to be realistic in terms of getting good service life out of the battery, and given that some people have reported voltages under 10 volts and their Prius is still starting (just).

Does anyone have any suggestions on exactly how low the measured (MFD) voltages should be before changing out the 12V?

 

Hi Harold, what sort of voltages are you seeing? (not straight after charging but at more random times btw).

I'm down to about 12.1 no load (acc mode) and 11.7 with headlights on. It's getting close to winter here now so the lower temperatures are probably influencing the readings a bit. I'm watching mine pretty closely as well and will probably be changing it out soon. I think if it goes below 11.5 with headlights then I'll take that as the final limit.

 

The voltage measured under hood or at directly at the battery terminal will typically be a little higher than the voltages measures at the MFD service menu. As the Prius must be at least in accessory mode to measure the voltage via the MFD then it makes sense that there will be some current drain on the battery and hence a slightly lower measurement.

In the past I've measures the no load voltage at both the battery terminals and under the hood and they are both in good agreement. At the MFD in accessory mode however I've typically measured 0.2 to 0.3 volts lower.

For the people who are quoting their no load measurements under the hood or at the battery terminal, could you please repeat these measurements in accessories mode so as to make it a fair comparison.

BTW. Whether or not you own a volt meter (I've got about 5) I still find the MFD service menu method to be the easiest way of keeping an eye on it with regular measurements.