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How much driving to fully charge the 12V?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by pasadena_commut, Mar 5, 2021.

  1. fuzzy1

    fuzzy1 Senior Member

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    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.
     
  2. sam spade 2

    sam spade 2 Senior Member

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    Which will be meaningless for that specific meter.
    It may or may not be in spec.
     
  3. fuzzy1

    fuzzy1 Senior Member

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    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.

    :rolleyes:
     
  4. ChapmanF

    ChapmanF Senior Member

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    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.

    vlo.png

    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.)
     
  5. fuzzy1

    fuzzy1 Senior Member

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    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."

    upload_2021-3-22_18-26-20.png

    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 ...
     
  6. ChapmanF

    ChapmanF Senior Member

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    Fair point.
     
  7. Albert Barbuto

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

    Note: Includes "surface charge". :whistle:
     
  8. sam spade 2

    sam spade 2 Senior Member

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    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.
     
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  9. Albert Barbuto

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

    pasadena_commut Senior Member

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    The now roughly 2 year old 12V showed up as 11.4V in the MFD test today. (I looked because the MPG has been low recently.) Ran the car for a while and measured under the hood, to find 12.16V. Sort of a huge difference, no way it charged 50% in that short period. Maybe a temperature thing though, it was supposedly 37F last night, which is very cold for us.

    So I repeated the above test. The car was warm from the sun by this point. This time the same ammeter (properly zero'd before measurement) measured 406 mA with the doors closed, but not locked. It was 416 mA with the doors locked. This is after several minutes in each state, with the car not having been run or had the fob in the ignition for several hours. The only car specific difference I can think of to account for this is that I had enabled the alarm system with techstream (in the mode where it comes on after explicitly locking the doors, but does not autolock the doors). The fob was in my pocket though, and that may have been enough to raise the discharge rate. (This car has the basic fob interface, not the fancy one where it doesn't need to be put into the slot.) Note, there was no "door open" indicator shown on the dash, the only light was the flashing car symbol for the alarm. Anyway, I should probably figure out where the change from 15 mA to 416 mA is coming from.
     
  11. Mendel Leisk

    Mendel Leisk Senior Member

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    How are you measuring that amperage?
     
  12. pasadena_commut

    pasadena_commut Senior Member

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    With the same UT210 clamp on DC ammeter as was used for the original measurements, as in post 11 near the start of this thread.

    It has been raining for days, and there are some really dark clouds going over, so I don't want to run an extension cord out to the car to trickle charge the battery. (Can't get it in the garage at the moment.) So I drove it for a little over 20 minutes to put some charge into it. On returning home the MFD read 11.9V. That was with the cabin blower going at one setting under the middle. Turned the blower off, then it read 12.1 V. Went back and forth and it was consistent, 11.9 V with it on, 12.1 V with it off. If that is all battery internal resistance, that would be, let's see .2V at maybe 7A? So .028 Ohm or so? That sounds ball park but a little high for a normal lead acid battery, and the input values have big error bars. (The MFD display truncates numbers as I recall, so 12.1 could be anything from 12.10 to 12.1999999, and the blower current was a purely hand waving number. So the numerator might be closer to 0.1V.)
     
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  13. kenoarto

    kenoarto Senior Member

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

    pasadena_commut Senior Member

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    Well, I'm not going to drive 8 hours just to charge the battery. Have to pull it during a long break in the rain and charge it outside the vehicle. (I'm not one to mix rain and 120V extension cords.)

    Made another short trip (just 4.6 miles round trip.) On returning did the cabin blower test again, but this time on full (11.5/11.6 alternating) and off (12.0 stable). Since it was alternating 11.5/11.6 it is really close to 11.60, assume 12.0 is 12.05, so .45V, and guess the blower uses 20A, giving .0225 Ohms internal resistance. Given the error bars, that is compatible with the previous measurement.
     
  15. ChapmanF

    ChapmanF Senior Member

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    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.
     
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  16. pasadena_commut

    pasadena_commut Senior Member

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    Good point. However, I would rather drive around than sit in the driveway for that long. I needed to go out and get a newspaper in any case.
     
  17. sam spade 2

    sam spade 2 Senior Member

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    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.
     
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  18. pasadena_commut

    pasadena_commut Senior Member

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    They are not. Checking the 12V voltage in READY mode provides no useful information about the 12V battery. Well, other than it is at least good enough to get into READY mode.
     
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  19. pasadena_commut

    pasadena_commut Senior Member

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    No rain this morning so I set out to find the source of the parasitic draw - and it was gone. Without having turned anything on, just opened the hatch and then tricked the hatch sensor by stuffing in the wide end of a carabiner, and the clamp on ammeter was reading only 18 mA after it settled. Put the fob in the front seat and nothing changed, so that apparently isn't a variable to worry about. Tried turning the car on (READY) and let it sit for a while, then turned it off. After a longish time (like 10 minutes) it was back to 18mA. That is all good (other than the parasitic load which comes and goes at random!)

    What is bad is that the battery now appears to have very little capacity. Yesterday the "Battery Tender Junior" was attached at the jumper post from 12:30 to 22:00. Solid red LED at the start, solid green at the end. UT210 voltage measurement on the post was 12.51. The post usually reads about .2 below the battery itself, so that would have been 12.71V at the battery. The right value, but the wrong time. Before charging it was like 11.6V at the post with the car off - no way that a .75A (max) charger should have fully charged the battery in 9.5 hours. In any case, after charging, putting the car into IGN-ON showed only 11.8V on the MFD.

    Current measurements in IGN-ON mode. No cabin fan 9.01A, Full fan 20.75A, Medium fan 16.54A, Lo fan 10.37A. (So, assuming the car wasn't randomly turning things on and off at the same time, the cabin blower uses 11.74A/7.53A/1.37A in Full/Medium/Lo.) That's actually quite a lot of current for a 46Ah battery, it is .196C, let's just round it to .2C. I could not find a table for that, but there is one for .1C here:

    https://sunonbattery.com/agm-battery-voltage-capacity/

    Car off, 12.33V measured at battery with UT210.
    Put the car into IGN-ON mode through an open window (so 9A from battery, cabin fan off). Exited car and immediately made these measurements with the same meter:
    at Battery 11.60V
    at post 11.41V
    at Battery 11.57V
    On MFD 11.4V.

    Seems like too much voltage depression from the 9A load. I recall measuring the voltage of this battery when new in IGN-ON and it was 7.x V (don't recall x at the moment).

    I need to get this battery load tested.

    Edit: After charging again with Battery Tender Junior (Solid Red to Solid Green in < 2 hours), the voltage was checked on the MFD:

    ACC mode 12.3V
    IGN-ON mode 11.8V

    I forgot that this test worked in ACC mode too. Unfortunately the hatch area had been reassembled, so the current draw could not be measured. It must be less than 9A though since the voltage depression wasn't as great.
     
    #99 pasadena_commut, Feb 28, 2023
    Last edited: Feb 28, 2023
  20. kenoarto

    kenoarto Senior Member

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