charging multiple used 12v car batteries

Discussion in 'The Projects Forum' started by knighta, Jan 15, 2010.

  1. knighta

    Thread Starter New Member

    Jan 15, 2010
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    Hi all,

    Is it possible to charge approx 24 x 12v wet flooded batteries that have been fitted to vehicles (some are more discharged than others) using one battery charger connecting the units in parallel? The units range from 40amp to 100amp. If this is possible how would i determine the size of charger required, what charge rate would be used and what safeguards would have to put in place?

    Or is this only possible when refreshing units at around the same voltage and size? Would the only safe way be to use a multi bank charger and if so would any one have any suggestions the best place place to locate one of these chargers.

    Many thanks in advance
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    You can't connect them in parallel without some means of limiting the current flow between them. If there is a significant difference in the battery voltages, very high currents will flow. This can result in heavy damage to the batteries.

    It is very common for used batteries to have heavily sulphated cells. One way to help desulphate the cells is to subject the battery to a series of charge/discharge cycles, but you really don't want the internal temperature to rise a great deal.

    Charge them individually at first. If a battery measures less than 10.5v after a brief period of charging (say, an hour or so), it has one or more shorted cells. You probably won't be able to do much with it. If you attempt to put such a cell in parallel with other batteries, you will overcharge any remaining good cells in the battery, while discharging the other good batteries that are in parallel with it; you will wind up with a lot of dead batteries.

    A typical charged auto battery will measure anywhere from 12.6v to 12.9v at around 25°C internal temperature; variations may be due to individual battery chemistry. There is a negative temperature coefficient of about 3mV per cell per 1°C, or 18mV for the six cells.

    Depending on how knowledgeable you are in electronics, you might be able to construct a "round robin" type of charge controller that would select one battery at a time for charging. It would be a rather involved project to be a really good one though.
     
  3. knighta

    Thread Starter New Member

    Jan 15, 2010
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    0
    Ok that I understand. I have located a shumacher multi bank charger (eight bank at a time ) at around $800.00 would you suggest this reperesents decent value compared to looking for a company to build a suitable device as you have mentioned. As going forward this is not a one off requirement I am looking for a cost effective solution in terms of finacial outlay and time spent in producing test results for these units.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    COTS (Commercial, Off-the-Shelf) is often the most cost-effective way to go.

    You will still require man-hours to rotate the battery stock, even with the 8-bank charger.

    I have not read up on the capabilities of the charger that you pointed out, except it briefly states on one commercial website that I found that it is microprocessor controlled, and can select automatically between 12A and 2A rates, with overcharging protection. I have no idea if it is capable of evaluating and reporting on the condition of the batteries.

    If you wish for something like those capabilities, you will probably have to resort to having someone engineering a unit or units capable of providing those features.
     
  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Sgt, maybe you can recall a Sr Members thread dealing with automatic charging of a bank of flodded cell batteries, All +s tied together, switching done on _ side; do not recall the outcome.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Hi Bernard,

    Yep, I remembered that thread yesterday, it was iONic's thread, from a year ago. I'd completely forgotten about it.
    http://forum.allaboutcircuits.com/showthread.php?t=16266

    I'm afraid I never did get around to helping him finish it.

    It could be useful for a hobbyist project, but of questionable value for a business, where they would be more interested in low turn-around times rather than saving money on a charger.
     
    Last edited: Jan 18, 2010
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