Traction battery, strange balance after 4/5 weeks sitting still?

Discussion in 'Gen 2 Prius Technical Discussion' started by reallyreal, Apr 16, 2020.

  1. reallyreal

    reallyreal Junior Member

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    Hello all, I'm warning you that this post is somewhat speculative.

    My Gen 2 has been in my garage for 4/5 weeks, turned off.
    Few days ago I went to see the SOC of the traction battery (original one, car has twelve years, 2008, around 385000 km, ~240000 miles) but the 12v battery was dead (forgot to disable the smart key entry and to take away the Carista OBD 1 month ago.. :cautious:).
    So, being in lockdown and needing to wait at least a week to receive a new battery or cables to jump-start the car, I took away the 12v battery, charged it up with my hair trimmer transformer :eek: and some wires transplanted from some unused cables, and put it back in today. It worked. Great. I started the car and saw a SOC of about 48%, not bad considering the age and all those weeks off.

    While I was force-charging the traction battery, I noticed that something was different.

    First of all, the internal resistance is 19 milliohm for all blocks, while I remember well that it wasn't the case before. I remember seeing a difference of 1-2 milliohm between the bottom and the top value.

    Also, as you can see from the pictures, the outer blocks are the weakest in terms of voltage, while the others seem pretty even. Before, I had one internal block that was consistently the weakest of all. I don't remember whether it was block #4, #5, #6, or #7, but it was definitely there. Admittedly, I do not remember how the two outer blocks were doing.

    I didn't drive the car today so, when I will drive it, maybe, the situation will revert as flows with more current will go back and forth from the battery. Yet, my question is this. Is it possible that by slowly discharging in time without use, the traction battery somehow balanced itself?

     
  2. TMR-JWAP

    TMR-JWAP Senior Member

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    The 19 milli-ohm value is just the ecu default value that occurred due to loss of 12v power. The same occurred to the HV battery SOC. Both will be 'relearned' relatively quickly as the car operates.
     
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  3. reallyreal

    reallyreal Junior Member

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    Aaah, I see, there's the catch then. Thanks!

    Also, did you mean the blocks voltage distribution or the actual SOC?
    I remember seeing a SOC value around 60% as soon as I started the car, and then seeing it drop to 48% almost immediately. Do you think I can trust the SOC value around 64% that I have now after force-charging the HV battery?
     
  4. TMR-JWAP

    TMR-JWAP Senior Member

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    The actual SOC is what goes to default value. The voltages are 100% accurate. Once you started force charging the battery, the ecu algorithms would have started relearning the true SOC based on measured voltages and current levels, so it wouldn't take long for it to become accurate. It may need some driving (discharge and charge time) to become most accurate.

    If it was showing 60% initially, then dropped to 48% and then moved to 64% as you were force charging, I would say it's accurate within reason. Pretty sure the default setting is 60% for power restoration.

    This is why people sometimes get into trouble when the engine doesn't start due to running out of gas or something. The car tries to start a couple times unsuccessfully and then codes out (after partially draining the HV battery). Person then disconnects the 12v battery to reset the codes, not realizing it also reset the HV SOC to 60%. The engine tries to start again, and if unsuccessful, codes out again after draining the battery a bit more. Sometimes this gets repeated multiple times until the HV battery is completely drained with no hope of ever getting the car started, resulting in a dealership installing a new battery for $3000 and adding 5 gallons of fuel.
     
    #4 TMR-JWAP, Apr 16, 2020
    Last edited: Apr 16, 2020
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  5. Aaron Vitolins

    Aaron Vitolins Senior Member

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    Your battery doesn’t seem terrible when factoring in age and miles.

    It really isn’t smart to let this old of a hybrid battery sit unused for more than 2-3 weeks. NIMH chemistry likes to be used! Plus they do discharge over time.

    hybrids and electric cars using Lithium ion, they can sit for months even years without damage or hardly any discharge.
     
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  6. reallyreal

    reallyreal Junior Member

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    @TMR-JWAP Thank you very much for the clarification! As soon as I'll drive the car I'll also keep an eye on the voltage to see if its behavior will also go back to having a middle block as the weakest one.

    @Aaron Vitolins Exactly. That's the reason why I didn't want to wait an additional 7/10 days for a new 12v battery. I don't know if it applies to all kind of batteries but as a general rule of thumb I know it's better not to let any battery discharge too much to avoid 'deep discharge' issues.
     
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  7. dolj

    dolj Senior Member

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    What works best depends on the battery chemistry. Lead/Acid, for example, should be stored at 100% SoC. Li-Ion, I believe likes to be stored at around 30% SoC. NiMH on the other-hand can be stored indefinitely at 0% SoC as long as each of the internal cells is 100% in perfect working order. So you can see the dilemma as these batteries age and get out of balance.

    A good resource, if one wants to further their knowledge of different battery chemistries is Battery University.
     
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