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. pasadena_commut

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    The 12V in our 2007 was replaced by a dealer a little less than 2 years ago. The car has not been driven much for the last year. I try to put at least 5 miles on it every other day. This may not be enough though. This morning my wife drove it to the post office about 1.5 miles away, parked it, and went to pick something up. When she returned there were all sorts of warning lights and it would not go into reverse. Eventually it did. The voltage of the 12V was measured in the fuse box under the hood maybe 15 minutes later and was 11.64V. Very, very low for no load. I then started the car (no warning lights, other than TPMS) drove the car about 7 miles and it was reading 12.29V at the same location. 150 minutes later it is down to 11.93V. None of the interior lights are in the "on" position. I think maybe we just didn't drive the car enough to keep the 12V reasonably charged. How much driving would that be?

    I have a 12V battery charger and will pull the battery and fully charge it. (I don't trust this cheap made in China charger not to glitch and blow up something expensive if it was used with the battery in the car.) Hopefully it will still hold a charge, but maybe not if it has been south of 11.64V.
     
  2. kenoarto

    kenoarto Senior Member

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    ARRRG!Z Another COVID19 victim.

    It takes 8-12 hours of continuous Ready mode (motor turns on and off), to fully recharge ... If your battery wasn't permanently damaged from being fully discharged.

    Get yourself a decent $30 smart charger and you won't have to pull the battery. I got a Battery Tender.

    Posted via the PriusChat mobile app.
     
  3. JC91006

    JC91006 Senior Member

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    If the lowest the battery got was in the 11.xxv range, then you should be able to fully charge that battery and continue to use it. You should use the GSM battery setting on your charger.

    While the battery is out, you should charge it and get it tested at your local autozone/oreilly's. Then you'll know if the battery is good or not. If it's not good, then bring it back to Toyota and get a warranty replacement.
     
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  4. pasadena_commut

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    Typo, GSM for AGM?

    The "Battery Tender Junior" we have will apparently only charge it up to 12.7V, but that will still be a lot better
    than what it is at now. Started it a few minutes ago, hopefully this should be pretty quick.

    Thanks.
     
  5. JC91006

    JC91006 Senior Member

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    AGM....damn my brain failing.
     
  6. Mendel Leisk

    Mendel Leisk Sunday driver DIY’r

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    You can DIY test with something like Solar BA9.
     
  7. MickyMatter

    MickyMatter Active Member

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    I'm with kenoarto:
    The DC/DC in the inverter charges a partially discharged battery with about 3 A to 5 A.
    So it lasts up to 9 h to 15 h to fully charge a 45 Ah battery.
    (An empty battery gets a higher current at the beginning, which decreases the charging time a bit.)

    [​IMG] [​IMG] [​IMG]
     
  8. ChapmanF

    ChapmanF Senior Member

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    That's right, as I understand it, for Gen 1 and Gen 2 (and therefore the right answer here, in the Gen 2 forum).

    Just for the sake of future readers who might land here from a search engine, Prius generations after Gen 2 do have a higher battery charging voltage and that reduces the time to recharge in those later generations.
     
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  9. sam spade 2

    sam spade 2 Senior Member

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    I think you are right......on all counts.

    But it is impossible to know how much driving is needed to keep the battery charged because there are too many variables.
    The most important being the health of the battery itself.

    The only really good solution is to get a new battery AND connect it to a battery tender when not in use.
    That way you don't have to worry about driving it regularly.
     
  10. sam spade 2

    sam spade 2 Senior Member

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    No, not true.
    Unless the tender is bad, it should "float" the battery at about 13.2. V.
    If it never goes that high, it is good evidence that the battery is indeed "bad".
     
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  11. pasadena_commut

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    Well something is way off, and it is probably that the battery has very little capacity.

    Yesterday attached the "Battery Tender Junior" directly to the battery at 13:35 through the open hatch. Originally I was going to take the battery of the car, but then I got lazy and just left it in and hooked up. With the charger active (showing a solid red light for "charging") measured 11.65V. Lowered the hatch (struts are nearly shot) onto a block of wood and let it charge like that until 22:30. The light was still solid red but after unhooking the charger the battery read 12.65V. That is 9 hours of charging at a nominal 750mA. No way the charge level should have changed that much for so 6.75AH of charge.

    A brief detour. How much current is drawn from the battery when the car is off but the hatch is open? I have a UT210 clamp on DC ammeter and this seemed like the right time to answer that question. So this morning I measured that, charger not attached, as 520mA. In another thread I had measured the current on a door sensor and it was over 300mA, so if the hatch is similar this is about right - 300mA for the hatch sensor and another 200 mA to drive whatever electronics wake up when a door is open. Then I folded myself into the hatch area, had my wife close the hatch, and kept measuring. Current dropped quickly to something in the 100 mA range (it was never steady) and then fell from there to 15mA where it stabilized. So the car does not have a large "off" draw, so long as all the doors and the hatch are closed. I also measured the DC current going into the battery when the charger was attached, which was about 800mA (but not quite stable). This tells us that yesterday's charge was even lower than expected, at a rate of only 300mA (800 in, 500 out), or
    about 2.7AH.

    At 9:10 this morning I measured the voltage again, it was 12.09V with the hatch open and the charger not attached. Hooked up the charger through a window (showed "red") and closed the hatch. So the car should only have been drawing 15mA from the battery instead of 520mA. Came back at 10:00 and the charger LED was green ("fully charged"), which should not be possible. Couldn't get back to it for a while and then made these measurements in quick succession:

    10:40 (at the battery)
    12.81V (hatch open, charger on)
    12.68V (hatch open, no charger)
    12.57V (hatch closed, no charger)
    12.65V (hatch clased, no charger, 10s after the preceding)

    10:48: (at the test point in the fuse box under the hood)
    12:40V

    In short, the 12V battery seems to have almost no capacity.

    I noticed a small black tube which comes off the battery near the positive electrode and disappears down into the car. What is that?
     
  12. Mendel Leisk

    Mendel Leisk Sunday driver DIY’r

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    Not lazy, smart: as long as you're careful with polarity, not need to disconnect, regardless of what Owner's Manual says.

    Easier/simpler to hook up at the under-hood jump point.

    Yeah time to replace.
     
  13. sam spade 2

    sam spade 2 Senior Member

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    You went to a lot of trouble to "prove" what we already knew. :)
    But that's good experience.
    That tube probably is a "vent".......which on a sealed AGM battery is overly cautious.
     
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  14. pasadena_commut

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    I'm going to replace this battery next week. In the meantime, a few more measurements (all in the
    fusebox under the hood)

    3/6/2021
    11:45 12.34V.
    (Drove 11.4 miles in about 25 minutes)
    12:22 (measurement odd, car made a whirring noise and the voltage dropped below 12V and then came back up,)
    12:23 12.39V
    17:00 12.27V

    3/7/2021
    10:45 12.14V
    11:45 12.12V
    (drove 10.3 miles in about 25 minutes, pop hood, lock car, wait 3 minutes)
    12:15 12:54V
    12:20 12:57V

    So yesterday the battery didn't charge up a significant amount in the same amount of driving (distance and time) that today resulted in a reasonable amount of charging. Bizarre.

    AGMs fail if they are overcharged. How does the Prius keep from doing that?
     
  15. kenoarto

    kenoarto Senior Member

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    Yup, you have proven that 25 minutes of running the engine will not charge your Prius's drained battery. 8-15 hours is a tad longer.
    Posted via the PriusChat mobile app.
     
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  16. sam spade 2

    sam spade 2 Senior Member

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    The same way that they keep from overcharging any similar battery.
    The charging system must FAIL in order to "overcharge".......which is a rather rare occurance.
    An AGM is perfectly happy with the charging designed for a wet cell automotive battery.

    Note: Pretty much all common battery types will fail if they are overcharged.
     
  17. pasadena_commut

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    It shows that the battery does not charge consistently. In one case, not at all, in the other a shift of about 22% SOC, from roughly 62 to 82%. That last one is way out of spec too I think. For a 45 AH battery a change of 22% SOC would be 9.9 AH, and that was around .37 hour, which would need 26.8 A. No way the inverter was sending that much to the battery, so the actual capacity is much, much lower than 45 AH.
     
  18. sam spade 2

    sam spade 2 Senior Member

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    You are assuming way too much from simple voltage readings.
    Especially when you don't know what load, if any, is present at the time of your measurements.
     
  19. dbstoo

    dbstoo Active Member

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    I did not see this point addressed.

    The number of hours needed to charge a battery depends on how much energy was drained from it. You'll often see a charger labeled as being a certain number of AMPS. They do that by changing the voltage until the battery pulls that number of amps. A 45 AH (amp hour) battery is one that will charge from dead to full in 45 hours at the rate of 1 amp. Double the amps and you cut the charging time in half. Charge at 10 amps and the battery will go from empty to full in 4.5 hours. These are rough numbers, of course.

    If the car draws 1/2 amp at all times, the 45 amp hour battery will need to be charged long enough to replace those 12 amp hours. Again, The numbers are simplified for ease of illustration and are not really valid numbers. :)

    Dan
     
  20. ChapmanF

    ChapmanF Senior Member

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    That's pretty close to how it is. One subtlety is that the measurable amp-hour capacity of a battery is not completely independent of how fast you discharge it (the way the size of a bucket is independent of how fast you drain it). So battery amp-hour ratings are usually given with a certain discharge rate or interval in mind, such as 20 hours. A 2.25 amp current would discharge a 45 Ah battery in 20 hours. Mileage may vary slightly with lower or higher currents.

    When charging, you have to put more back than just what works out to the amp-hour label capacity. There are losses involved, so you might have to charge at 4.5 amps for 15 hours, even if from the amp-hour rating you would have thought 10.
     
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