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Good HV battery balance

Discussion in 'Gen II Prius Care, Maintenance and Troubleshooting' started by Britprius, Apr 30, 2012.

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  1. Britprius

    Britprius Senior Member

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    Do you think it would it would prolong the life of the HV battery to do a rebalance and perhaps a module shuffle at say 100k miles at the end of guarantee on an otherwise healthy battery? Possibly one for Seilerts.
  2. GrumpyCabbie

    GrumpyCabbie Senior Member

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    If it ain't broke.....


    How long do you intend keeping the old girl? I'd be tempted to have your 12v checked and/or replaced. Was washing mine just now and noticed my interior lights went noticeably duller when I switched the car from Ready to off.
  3. jdenenberg

    jdenenberg EE Professor

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    This has been discussed before. I think waiting for the failure and then rebuilding the battery is a lower overhead plan.

    JeffD
  4. Britprius

    Britprius Senior Member

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    I replaced the 12v battery earlier this year with a battery that I believe you should look at, with the extra load of your taxi equipment. It is a Lucas AGM deep cycle battery with a amp/hr capacity of 55amps at 10hr rate. Batteries in cars are normally rated at a 20hr rate this would bring it up to 60amp/hr capacity, considerably more than the OEM or the Optima and cheaper. The battery is the same physical size as the OEM.

    I intend keeping the car a good while yet its just coming up to 4years end of May and am at 82,000 miles.

    As a retired electronics engineer I have the knowledge and time to service the HV battery. My field of expertise is not in battery technology hence the request for technical opinion.

    John.
  5. Britprius

    Britprius Senior Member

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    If by overhead you mean cost, it will cost me nothing but time that I have plenty of, and if it extends the life of the battery it must reduce cost per mile of running the vehicle as well as improving reliability.
  6. seilerts

    seilerts Battery Curmudgeon

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    I wouldn't do anything with chargers, due to the labor involved and the equipment requirements. There is also the risk of making matters worse any time that you attach a charger to an individual module.

    On the other hand, decay in capacity is generally parabolic, with outer modules usually maintaining almost all of the original capacity and inner modules showing the largest loss. A reshuffle could help prolong life. So one approach could be a pair-wise shift of inner modules to the outer and vice-versa. Always keep pairs together as well. If you consider that the battery has 14 pairs, then exchange #1 and #14 for #7 and #8, exchange #2 and #13 for #6 and #9, and finally #3 and # 12 for #5 and #11.
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  7. uart

    uart Senior Member

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    Just a random dumb question here. When people talk about re-balancing the modules, are they talking about internally re-balancing the individual cells (6x1.2=7.2) that comprise each 7.2 volt module, or are they talking about balancing the modules with each other?

    Sorry for the noobness of this question, but I don't know much about the internals of the HV pack. :eek:
  8. Britprius

    Britprius Senior Member

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    Thanks for your valuable input on rebalancing. I have ten of DC6 Super Mate chargers bought as a job lot some time ago for next to nothing and much more electrical test equipment, just thought I might put it to good use. I will probably just shuffle the modules as per your advise. I can monitor the module voltages as I run Priidash
  9. Britprius

    Britprius Senior Member

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    It's just balancing each 7.2volt module for amp/hr capacity and voltage.
  10. uart

    uart Senior Member

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    Sorry I'm still not clear on that. Balance 7.2 volt modules internally or externally?

    That is.

    1. Balance internal cells of module with each other?

    or

    2. Balance modules against other modules?
  11. Britprius

    Britprius Senior Member

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    Balance modules against each other. Unfortunately the connections between the individual cells are not bought out to the surface of the sealed modules.
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  12. uart

    uart Senior Member

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    Thanks Britprius. That's what I'd always assumed to be the case. However as I'd never actually seen a module close up, I just wanted to be sure. :)
  13. jdenenberg

    jdenenberg EE Professor

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    Rebalancing the 6 cells in each module is done externally by 3 times doing a full discharge and recharge of the module. The cells with a higher charge burn off their excess charge as heat while the cells at a lower charge level get charged up.

    Each module should then have about the same capacity and series resistance to maintain long term balance. Then you make sure that all of the modules are at the same voltage when reassembling the battery.

    JeffD
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  14. Britprius

    Britprius Senior Member

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    See picture below

    Attached Files:

  15. Ed Carmack

    Ed Carmack New Member

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    Hey John,
    Realize that thread is an antique but I am in the process of rebalancing a battery that failed and had a duct tape repair. Replaced a cell without balancing and it worked Ok for 9500 miles. After I learned more about the issue. More as in a little as compared to nothing. I decided to do a better fix. Bought a donor battery swapped it with the duct taped battery and proceeded to rebalance the old battery. Have done 17 cells so far. First off it is a slow go. You will be without use of the car for a couple of weeks . Assuming you have the capability of doing 4 cells at a time and are not distracted from the job. However back to your original question about extending life by doing a preventative rebalance. Based on what I have seen from my pack so far I believe that doing an balance that included balancing the individual modules would prolong the life. Reason I say this is that I am truly amazed that the pack was functional based upon the differences in capacity of the cells. A couple have initial capacities that are half the others. Also I have determined that as found individual cell module voltage says little if anything about capacity. So leveling the modules voltages against one another is not going to help. Also reschuffling the modules from outside to inside based upon position probably won't either. Reason I say this is that position in the pack is not a guarantee of condition. The two really bad ones I have seen so far are 7 and 11. Reschuffling needs to be done based upon capacities and possibly also cell resistance. Have not done resistance checks yet.

    I believe that the answer is that a preventative rebalance if done properly will extend the life. Is it worth it? It is not a lot of work. Took me a little under 3 hours to swap out the packs and reassemble car. Second time I've done it. The balancing part is not a lot of work either but is slow. Slower than a prius with a fully failed battery even. You cannot just set up a charger/discharger and tell it to cycle the battery three times. There is baby sitting involved. You maybe could if you had a smart enough charger but I don't know that one exists at least for the handyman.

    I doubt it is worth rebalancing prior to failure. There is also the potential of causing a problem by disassembly/ reassembly. However if you have the time and inclination it might be.
    .
  16. Britprius

    Britprius Senior Member

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    Hi Ed, There are a number of chargers (made for the batteries used in models) That can not only be set to do three cycles by them selves, but will record the AH capacity on each discharge cycle so that any improvement can be monitored.
    The real point of the exercise is that it may be possible to stop a module with a cell in a low state of charge "compared with the others" from being reverse charged and destroyed.

    John (Britrius)
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