Question re: 12-volt battery, please?

This is the current version of my favorite meter, note the accuracy spec is improved to 0.05% for DC voltages.
Amazon.com: Fluke 87-5 Digital Multimeter: Home Improvement

If the price of a new meter is out of your range, but you can appreciate the utility of having this around, you might consider a used unit listed on eBay.

 

Hi Julia,

1. How long did you leave the car READY to charge the battery?

2. There is potential error in the MFD voltage readings. Therefore I prefer to use a good digital multimeter.

3. How accurate is the meter that is available to you? Here is a rough calibration metric: Take a new 9V battery and measure its voltage. It should read 9.6V. A new 1.5V battery should measure 1.6V.

4. A new 12V AGM battery, fully-charged out of the box, should read 12.9V. Do not expect to see that voltage reading when using the MFD voltage tests since the battery is under some load.

 

Hi Julia,

1. The amount of READY time that you reported should be sufficient to replenish the 12V battery charge. (It is unnecessary to actually drive the car during that period, for purposes of battery charging.)

2. Test your meter using new 9V and 1.5V batteries.

3. If you find it is accurate, then measure voltage across the 12V battery after the car has been sitting unused overnight and report what it is.

 

Everything sounds good. Not sure about the Optima though. Test it in another week and see the results. My opinion only! H

 

I know Patrick is guiding you, my thought is that the battery is simply not at a full charge. 12.57 V is pretty good imo. If you want 12.9 V, you should invest in a good battery charger.

Based on your measurements with the 1.5 V and the 9V batteries, I think your meter is Ok for testing of your 12V battery.

Respectfully, I honestly think you are good to go at this point and should just drive the Prius.

Ron

 

I agree that Julia's Prius is good to go. Her 12V battery may not be fully charged but has sufficient charge for now.

I also agree that periodic overnight use of a battery charger will be helpful to keep her 12V battery in good condition.

 

I have a 2007 prius we bought brand new in MO. we have since moved to Arizona for the last 2 years, I tried the 12 volt battery test it shows my battery to be good. 5 years out of a factory battery is that unheard of? also the only thing we notice more now is the battery display on the mfd seems to go low pretty quick but it does charge back up. could that be telling me the 12 volt is getting weaker?
thanks

 

1. Five years service life is not unheard of. When I lived in California my practice was to replace the 12V battery at six-year intervals and the replaced battery still provided sufficient power to start the car.

2. The MFD battery display relates to the traction battery, not the 12V battery. You can expect that the traction battery will discharge pretty quickly when the AC compressor is in operation, and I expect that is working pretty hard during the Arizona summer. As long as you do not see any warning lights, there is no reason for concern.

 

Thanks Patrick, a little less worry is great these days!

 

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The winter in maine can be rough, my wife is from Bangor. cold meets hot sorta thing haha did the change out go smooth? I am assuming you used the yellow top optima?

 

The correlation between voltage and fraction of battery charged is non-linear. Anybody care to guess where Julia's battery is at charge-wise at 12.6 volts ?

 

75%

The Optima page suggests that charging current of up to 10A and voltage up to 15V is OK. It also sets the expectation that charging time at that current level may be as long as 12 hours:
Optima Battery Charging - AGM Battery Charging