AC rapidly depletes battery

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by pasadena_commut, Aug 4, 2020.

  1. pasadena_commut

    pasadena_commut Active Member

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    Our 2007 Prius has been in the dealer's shop several times to repair the A/C (under the extended warranty they provided when we bought it used.) They finally seemed to have fixed the leak on the last trip when they replaced the evaporator. Maybe it was a larger subunit than just the evaporator itself, in any case it was the big piece that sits buried in the center of the dash. Since then the A/C has worked. (They somehow broke the air redirection though - it only comes out the dash vents and will not redirect up or down.) Anyway, it has been hot lately and I have noticed that when the A/C runs it rapidly depletes the battery. That does not happen when driving, perhaps because the motor compensates for it. But I have had to park a few times and wait for she who must be obeyed to run an errand, and during that time the state of charge indicator would go from nearly full to nearly empty in just a couple of minutes and the motor would restart. With the AC off just running the radio while parked does nothing of the sort.

    Is that sort of rapid depletion normal? I never thought to test it previously when the A/C was working (always briefly).

    This is still the original HV battery, so it well could be getting old. Yet when the A/C is not in use the SOC has not been observed doing anything at all odd, no sudden drops or increases in the number of bars. My main concern is not that the big battery might be going, I expect that sooner or later, but rather than that the compressor might have been damaged by the repeated instances of running with no refrigerant, so that it now might be drawing excessive current. I have a clamp meter which can measure DC current up to 100A which could be used to measure the current, but I do not know where to clamp on, nor what the normal current is. Anybody have this info?

    Thanks.
     
  2. JC91006

    JC91006 Senior Member

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    The problem is the hv battery
     
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  3. Austin Longenecker

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    I don't know if its "normal," but it happens to me as well. On hot days at least. Keeping the cabin cool on a hot day does take energy. Energy drain depends on how hot the air temp is outdoors, how cool you want it inside, battery health, etc. Probably a mix of all three for you and I both.

    Now that doesn't necessarily mean your battery is going bad, just means it ain't what it use to be over a decade ago.
     
    #3 Austin Longenecker, Aug 4, 2020
    Last edited: Aug 4, 2020
  4. ChapmanF

    ChapmanF Senior Member

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    @lech auto air conditionin has posted several threads and videos about the way a system running without the correct charge can consume a great deal more power at the compressor than when correctly charged, a situation that may reveal itself through temperature readings at the compressor discharge and across the condenser.

    Now, I wouldn't be expecting improper charge in a system fresh from an A/C repair at the dealer. But the temperature measurements might be worth making.

    @Austin Longenecker, yes, the compressor is variable power, so to some extent it is normal for the A/C to drain the battery faster under more demanding outside conditions. But in the case where the power drain now seems noticeably worse than in the same car before, it might be worthwhile to check whether the compressor working harder than it should.
     
  5. Austin Longenecker

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    You're right. I've noticed the AC take much less strain on my battery after getting my compressor replaced. MPG is still the same, but when "idling" AC takes longer to make a noticeable difference.

    OP may still have AC issues, or its just the battery showing age.
     
    #5 Austin Longenecker, Aug 4, 2020
    Last edited: Aug 4, 2020
  6. ChapmanF

    ChapmanF Senior Member

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    The key is not to stop at a list of maybes, but to look at what can be measured to distinguish between them.
     
  7. pasadena_commut

    pasadena_commut Active Member

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    After a 20 minute drive yesterday a wait with the A/C on seemed to be draining the battery more slowly than before. Perhaps one or more cells in the HV battery have a high rate of discharge so the pack becomes unbalanced and cannot supply the compressor for long. After those weak cells have charged back up it can do better. I should run some diagnostics on the cell voltages after the car has sat and after it has been driven for a while, there are instructions for doing that in this forum somewhere or other.
     
  8. ChapmanF

    ChapmanF Senior Member

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    Could also connect an OBD device and monitor the battery current. That's easy.

    Under different conditions (temperature and humidity, which can vary day to day, and also refrigerant charge, variable over long stretches or recharges) the A/C can draw anywhere from an amp from the battery to ten, maybe even twenty.

    If you compare the current being drawn from the battery at the times it seems to go down slowly and the times it goes down quickly, and it goes down more quickly when more current is drawn, that's no cause for concern. :)
     
  9. bisco

    bisco cookie crumbler

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    normal for a 2007. wait until it throws the battery code, then put a new one in
     
  10. ChapmanF

    ChapmanF Senior Member

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    Even the most perfectest battery you can have will drain faster when the current being drawn is higher.
     
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