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Same load one or three batteries

  1. Aug 4, 2012 #1
    I am drawing 12 volts at 1/2 amp for 6 hours a day off of one 12 volt car battery battery which is powered by a 2 amp solar charger voltage goes to almost 14 volts when charging I then added two more car batteries voltage is under 13 volts when charging with the same load is it better to use only one battery as voltage is higher when charging then with three or does this not matter?

    John
     
  2. jcsd
  3. Aug 4, 2012 #2
    What is your Batery's Ampere Hours ratings? That will determine how many batterys you require.
    Does your 6 hours use coincide with maximum Daylight (i.e. maximum solar Output)? If so, you could do with a much smaller battery.
     
  4. Aug 5, 2012 #3

    NascentOxygen

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    If the batteries were ideal and identical, then it would make no difference. But if you put real batteries in parallel, then the "strongest" one ends up charging the others, whether you intend that or not. It is rarely a good idea, but might be the simplest solution were you to have a lot more solar electricity available than is needed to charge a single battery.

    In your case, it sounds like you are draining fewer AHr than is available from your solar cells, so there is nothing to be gained by using multiple batteries.
     
  5. Aug 5, 2012 #4
    The solar cells are your voltage source and the batteries are the load. If you connect 3 batteries in parallel you get 3 times the load. Or in other words the resistance of 3 batteries is only 1/3 of the resistance of 1 battery. So therefore you get a lower voltage. When you put a lot of load on a voltage source the voltage drops.
    That is not a problem. You can connect 3 batteries to your charger, it just takes 3 times as long to charge them. Of course, no matter how many batteries you connect to your system, your average power consumption should not be higher than the average power output of your charger. And most importantly all batteries need to be rated for the same voltage and they should be the same type of battery.
     
  6. Aug 5, 2012 #5

    Averagesupernova

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    There is alot of OEM stuff out there going up and down the highways with batteries in parallel. Tell the big 3 that it is a bad idea.
     
  7. Aug 5, 2012 #6

    sophiecentaur

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    If all the batteries are of the same design and the same age then putting them in parallel is fine: it's more or less the same as having a single battery with larger plate area. If they aren't identical then they can have different charge /voltage characteristics and you can end up with only one of the batteries maximally charged for a given supply voltage. When you remove the charger, as mentioned earlier, then one may discharge into another. (A pointless exercise)
    If you are measuring lower volts when the batteries are charging in parallel, it may just be that they are not getting enough charge in the time to make up for the total discharge. Are you sure the AHr drain is no more than the available AHr on charge?

    The ideal thing to do is to decide on the total Ahr that you need for your job and then choose a single battery which will have something more than twice this nominal capacity. (Lead Acid cells can suffer when discharged to lower than half their rated capacity - I think that is a ridiculous way to specify a battery but that seems to be how manufacturers actually do specify the capacity.)
     
  8. Aug 5, 2012 #7

    Averagesupernova

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    I should have probably mentioned in my other post that parallel batteries can have their issues. What can happen is with age poor connections may develope on one of the batteries and this is not necessarily noticed. Then when you really need the capacity that a second battery gives you it isn't there. But, like sophie says, the same design and age in a pair of batteries is little different than a single with larger plates.
     
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