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20A Discharge and Voltage depression detection/ determination

Discussion in 'Gen 2 Prius Technical Discussion' started by terramir, Sep 15, 2018.

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  1. terramir

    terramir Member

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    Ok I built a 20 A discharger 10 12V light bulbs eBay sourced GT power "power analyzer" down to 6v a while ago based on Steve's suggestions. Been slapping the light bulb dis-charger after and then grid charging. Well something I noticed while some modules actually it seems like the higher capacity ones basically fall off a cliff around 6.66V (yeah I'm a old metalhead so I would notice that lol) on the separate multimeter, others trudge only slightly accelerating toward 6V while others crawl towards 6V some have a cliff at 6.2. they all differ a little and they all have different capacities then again there an eBay batch.
    But here is the question: at what voltage should these modules "fall off the cliff" if they have little to no voltage depression???
    terramir
     
    #1 terramir, Sep 15, 2018
  2. TMR-JWAP

    TMR-JWAP Senior Member

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    If you see a sudden "cliff", what you're seeing is one of the six internal cells running out of juice and being reversed. remember, each module assembly is made of (6) individual 1.2v NiMH cells placed in series. On discharge, a perfect module would approach 6 volts very smoothly. One weak cell changes that into a cliff. This is easy to see using a discharger that can graph or one that measures capacity.
     
    #2 TMR-JWAP, Sep 15, 2018
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  3. terramir

    terramir Member

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    I'm not so sure about that, because it seems like we are discussing two different cliffs. The higher the current capacity of the module measures the longer it sticks above 7V even at 20A discharge. The lower the capacity is at least on these the more of the capacity discharges below 7 V like one of these modules I just discharged like 4800 mAh and only 2500 above 7 V and then 5100 one where 3500 discharges above 7 and a 6200 one where 4500 discharges above 7 V while a 3700 one discharges more that 2700 below 7 V . I am thinking that over the cliff was too dramatic a term. I'm just saying I have seen this during testing several times and I dun think it's a reversal it's just that some modules have more voltage depression than others. Because it is still a curve but the higher the capacity the steeper it seems to be.
     
    #3 terramir, Sep 15, 2018
  4. TMR-JWAP

    TMR-JWAP Senior Member

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    The higher the current capacity of the module measures the longer it sticks above 7V even at 20A discharge.

    Yes, because all 6 cells are in good condition and of near equal capacity

    The lower the capacity is at least on these the more of the capacity discharges below 7 V like one of these modules I just discharged like 4800 mAh and only 2500 above 7 V

    Yes, because one or more cells are weak and have lower capacity than the others. Once the weak one drops out, the remaining 5 continue to discharge, but at a lower voltage because one cell is no longer adding its 1+ volts

    then 5100 one where 3500 discharges above 7 and a 6200 one where 4500 discharges above 7 V

    Yes, because all 6 cells are of fairly equal capacity, with none dropping out until deep into the discharge

    while a 3700 one discharges more that 2700 below 7 V

    This one is a POS that has at least one cell barely clinging to life

    Here's an NiMH discharge curve for a single cell.

    NiMh curve.gif
     
    #4 TMR-JWAP, Sep 16, 2018
  5. terramir

    terramir Member

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    I am starring at that curve and I see it with a few of the modules, but others have an odder behavior. Maybe I am explaining it all wrong. It's just a lower voltage main bulk on some and mostly it's the ones that give me a lower capacity reading but not always it just seem like some modules would now drain another Ah or even 2Ah at like a 500 mAh drain while others would cough up 200-300 mAh and be at 6V again mind you after a 20A discharge already. I have worked with this b4 and I have a sneaking suspicion that some of these modules are suffering from voltage depression. Maybe I am looking for patterns where there are none
    terramir
     
    #5 terramir, Sep 16, 2018
  6. Hybrid Battery Exchange

    Hybrid Battery Exchange Member

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    For this reason I cycle every single module/pack that goes through my place. On a typical first discharge (measurements taken @ 195v under load) most of the modules would be around 7v while some would be @ 5.5. After cycling the pack down to about 140v, followed by a balance charge to 0.3 delta, repeat at least 3 times the whole pack begins to work much better, capacity is improved, self discharge is improved and the pack remains balanced discharged down to 190v.
     
    #6 Hybrid Battery Exchange, Sep 23, 2018
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