Q: 12v AGM battery used to recharge 12v AGM battery

Discussion in 'General Electronics Chat' started by foolios, Jul 2, 2014.

  1. foolios

    Thread Starter Active Member

    Feb 4, 2009
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    I'm sure this question has been asked a billion times and I'm sorry my red eyes are tired of trying to find the answer in search.

    I have a 35 amphour 12v agm battery that is low on charge ~11.7v

    I would like to use a 24 amphour 12v agm battery that is at ~12.5v charge to bring up the low batteries charge level.
    At the batteryuniversity site I read in the comments section that this could be done but with some losses.

    I was told by an electrician that it would not work and that all it would do is drain the good battery.

    Thanks in advance.
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    You need 14.5V to charge the battery.
     
  3. foolios

    Thread Starter Active Member

    Feb 4, 2009
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    Ok, so the elevated level of charge I'm seeing isn't accurate. I must just be exciting the battery a little but not actually charging it.

    I must have to let it sit for a bit and then check the charge again and it will prolly drop back to precharge attempt level.

    Thanks so much.
     
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Ebay sells cheap adjustable "step up boost SMPS modules" which can be used to make 13.8v or 14.5v from your 12v battery.
     
  5. foolios

    Thread Starter Active Member

    Feb 4, 2009
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    Most excellent! Thanks for providing that info.
     
  6. alfacliff

    Well-Known Member

    Dec 13, 2013
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    charging a 35 amp hour battery from a 24 amphour battery isnt a good idea even with a booster, the batteries will even out with a little under a half charge on both. you cant make power with a smps, only step uup the voltage which reduces current available. take two caps, one discharged and one charged, paralell them, and IF they are the same capacity, they will each have a half charge at most.
     
  7. inwo

    Well-Known Member

    Nov 7, 2013
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    I don't follow your thinking.

    With a booster, the donor should produce until near discharged.
    Of course that will still not top off the dead one.

    Not saying it's a good idea, or even practical.:p
     
  8. alfacliff

    Well-Known Member

    Dec 13, 2013
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    as the battery powering the booster discharges, the booster will get to a point that it nolonger boosts. then you will be stuck with two discharged batteries. the booster cannot make power, if it is charging one battery at 14.5 volts and 4 amps, how much is it pulling from the donor battery? (hint, more)
     
  9. foolios

    Thread Starter Active Member

    Feb 4, 2009
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    Ok, so the consensus is battery to battery charging isn't worthwhile to explore.

    Thanks for all the input.
     
  10. k7elp60

    Senior Member

    Nov 4, 2008
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    alfacliff is correct. In my terms: Any dc power source that is slightly greater in voltage to a rechargeable battery will charge the battery to the level of the applied voltage provided the source will supply the current necessary to charge the low battery. In this case if the charged battery is connected to the discharged battery, the charged battery's voltage will reduce and at some point in time both batteries will have the same voltage. This voltage may indicate a partial charge, or both batteries will be below the cutoff voltage(point at which they both need to be charged). The battery university is correct.
     
  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Forgetting the booster has regulated voltage output?

    The only problem as the source battery discharges is that current drawn from it will increase. Output votlage (to the chatging battery) will remain at 13.8v etc.

    But like Inwo said; that is not saying it is practical, just that it can be done.
     
    inwo likes this.
  12. inwo

    Well-Known Member

    Nov 7, 2013
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    It comes down to size.

    To be practical, a larger source battery is a good idea. Higher AH and ideally higher voltage.

    There are consumer products on the market that work for their intended purpose.

    One is an inverter/battery pack that has ports to charge phone.

    Another is a little handheld nicad pack, used to charge auto battery thru acc. jack in "dead battery" situation.

    I disagreed with "batteries will even out with a little under a half charge on both".

    It is possible to run donor down to near empty. Final charge state of main battery depends on it's initial charge and energy available in donor.
     
  13. Engineerer

    New Member

    May 22, 2013
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    I believe that this charging is pretty close to ideal. Someone can verify, but I predict that over 80 % of capacity will stay on your use. Both batteries will charge to same percentual charge.

    [​IMG]

    Your 35 Ah battery is at 10% or 3,5 Ah. 25 Ah battery is at 75 % or 18,75 Ah. After connecting these batteries they would end up to something close to 11,9 V, 7 Ah in the smaller one and 11 Ah in the bigger one. I'm on my phone so I don't calculate accurately. Does someone think differently?
     
  14. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I think you missed the earlier suggestion to use a SMPS boost converter module between the two batteries? That allows the source battery to be run down to minimum, while maintaining 13.8v on the terminals of the charging battery.
     
  15. Engineerer

    New Member

    May 22, 2013
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    I didn't miss it. I just wanted to answer to the original question. SMPS boost converter sounds fine when you want the full capacity of the donor (switched DC/DC converter).
     
  16. MrChips

    Moderator

    Oct 2, 2009
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    Sounds reasonable.

    You are essentially connecting two 12V batteries in parallel to create one 12V 59Ah battery.

    Eventually, the two batteries will reach the same voltage of approx. 12V, both at about 35% charge. The 35Ah battery (A) will gain about 8.75Ah to move from 10% to 35% charge.

    The 24Ah (B)battery will lose about 10Ah to go from 75% to 35% charge.

    Just my estimates.
     
    Last edited: Jul 5, 2014
  17. foolios

    Thread Starter Active Member

    Feb 4, 2009
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    This is what I've seen, that the two batteries both became 12V. I used to think that it was working, then was told it does not. So I thought I was just seeing temporary states that would adjust back to what they were originally or as I was told, the higher charged battery would just deplete, nothing gained by the lower charged battery.
    But as I see it now, it is of benefit to be able to get a batteries charge level up a little by using another battery.
    This isn't something I would do in an everyday situation, it's just something I had tried when I was without power for a few days. I didn't want to leave one of the batteries in a very low state of charge during that time.

    Thanks for the info all.
     
  18. MrChips

    Moderator

    Oct 2, 2009
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    If you are just trying to maintain power in a power outage then you might just as well remove one battery and replace it with the fully charged battery.

    Then get some solar panels to recharge the depleted battery during sunshine hours.
     
  19. tcmtech

    Well-Known Member

    Nov 4, 2013
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    The most effective and efficient way to get more AH from the two batteries would be to simple use them in parallel right from the start.

    As far as charging one battery from another I do it all the time! At work we are constantly charging cell phones from laptops and then later charging the laptops from our service trucks batteries. ;)

    Whether charging one battery from another works or not is all on the details. :D
     
  20. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    I apologise. :)

    Thanks for clarifying.
     
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