How much driving to fully charge the 12V?

If you can supply the model number, I will try to look up the accuracy specs.

The first RatShack DMM specs that came up for me were +/- 2% of reading +/- 0.8% of fullscale +/- 1 digit. But that is very unlikely to be for your particular model.

I'm also hoping for a battery name and model number, so that I can look up its spec and application sheets.

 

Which will be meaningless for that specific meter.
It may or may not be in spec.

 

That's right, never ever waste money on a +/- 0.1% or better meter, because +/- 5% meters are far cheaper. And you don't ever know if the 0.1% device is actually operating within its spec.

 

The "battery university" folks suggest, as one way to handle surface charge, "... or allow the battery to rest for a few hours."

So I'd think "Sitting for five days, OFF the charger" might have that covered.

While waiting to look those up, can this be worked from the other direction too? How far is the reading you would have expected from the reading being shown on those meters? Supposing they are off by that much, what would that translate to as a % accuracy spec?

The two meters make it interesting. They're within 0.2% of each other. Supposing that make and model of meter had very loose specs, a wide distribution of reading offsets between different meters of that model, those two seem to have come from very near each other out of that wide distribution.

I recently had a pleasant surprise involving my 30-year-old never-recalibrated Fluke handheld. I had a chance to compare it to a new instrument with up-to-date calibration certificate and its DC volts were well within spec. (On AC, slightly out of spec, nothing that would get in the way of anything I ever use it for.)

Granted, that's a Fluke, not a Radio Shack. But then I also used it to check a sort of equal-vintage Kikusui bench power supply (also spot-on), and a used no-name Chinese power inverter (spot-on once I reset its user-tweakable calibration offset to factory default!). It might be that reasonably decent DC ADCs are sort of mature technology nowadays.

Interestingly, though I wrote that less than a week ago, I am already noticing a difference since doing the solar panel mod. My aux battery used to sit at a lowish SoC (especially during lockdown), which Sam has scolded me about before, and that may be why I so often saw the converter going up to 14.7 to charge it faster.

Since I've had the solar hooked up for a couple weeks now, the battery is at a much better SoC, and I just noticed while driving today that the converter went only to 14.4-ish at first, and backed off to 13.6 when I shifted into gear. (It was definitely reacting to gear selection; it went back to 14.4 when I pressed P, and back to 13.6 when in gear again.)

I had seen others describing that behavior, but can't remember ever seeing it myself, back when my usual battery SoC was lower.

The Gen 3 New Car Features manual does say the HV CPU adjusts the output voltage setpoint of the converter, but really only mentions doing that based on the aux battery temperature sensor. Clearly there's more than that going on.



I am not certain how it is judging the SoC of the battery. Gen 3 doesn't even have the small voltage sense lead running back there that Gen 1 and Gen 2 had. The temperature sensor isn't touching the battery, just sort of hanging in the air above it. Does the HV CPU just observe the voltage at start-up time and estimate SoC from that? Or is it occasionally telling the converter to reduce output for a split second to get another snapshot of the battery voltage?

(Don't laugh, I have a blower motor in my furnace that works something like that. It regularly cuts motor power for a split second and takes the slope of the RPM decrease, and calculates from that how much air it's moving in CFM. It's imperceptible in operation.)

 

Another relevant item from "Battery University":

"Lead acid comes with different plate compositions that must be considered when measuring SoC by voltage. Calcium, an additive that makes the battery maintenance-free, raises the voltage by 5–8 percent. In addition, heat raises the voltage while cold causes a decrease. Surface charge further fools SoC estimations by showing an elevated voltage immediately after charge; a brief discharge before measurement counteracts the error. Finally, AGM batteries produce a slightly higher voltage than the flooded equivalent."



A battery manufacturer and model number could help track down datasheets that reveal plate chemistries that produce the higher voltages.

In my industry years, I learned that the distribution of specified properties don't need to be a wide Gaussian distribution around the center and filling the tolerance range. They can be very narrow and piled up in a tight range well offset from the center. Or bimodal. Or wide but with glaring gaps where selected units were plucked out for other use. Or different from batch to batch. Or ...

 

Fair point.

 

I believe the question was: The actual resting voltage of an AGM battery, after a full charge.

Note: Includes "surface charge".

 

No it doesn't.
Because by definition, "surface charge" disappears very quickly when any kind of significant load is applied and does not come back without a charge being applied again.
IF.....a battery measures 13.8 immediately upon being disconnected from a charger (or even hours later) but then drops to 12.8 when a load like headlights is applied for 15 seconds or so............
Which of those numbers would you consider to be the "actual resting voltage" ?
If you say 13.8 you would be WRONG.

 

I meant MY measurement included the surface charge. My bad.... Your post above is right on the money.

 

How are you measuring that amperage?

 

20 minutes is for old cars. Prius takes 8 HOURS.

 

It's probably been mentioned earlier in the thread, but of course driving isn't necessary. The 12 volt battery gets charged whenever the READY light is on, whether you are driving or parked.

Conventional cars with a belt-driven alternator would charge the battery faster if you were driving (or revving the engine). That was them.

 

AND.....if those voltages he is quoting are with it in READY mode, that likely means that the inverter that is supposed to do that charging has failed. While charging, the voltage should be at least 13.5 and more likely 14.5.
All that assumes that he is really measuring the actual battery voltage and not some point deep in the electronics somewhere.

 

LOL! Enjoy your time pissing into the wind, Einstein!