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Batteries in series - charging and center tapping

Discussion in 'Power Electronics' started by istvanb, Mar 14, 2013.

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  1. istvanb

    istvanb

    18
    0
    Dec 10, 2012
    Hi,

    I have two batteries of the same type connected in series (2x12V). I also have a 24V charger which can charge my battery pack. The issue is that sometimes I need 12V battery voltage and sometimes I need 24V.

    Can I use the circuit I have attached? Please explain me if there is anything wrong with it.

    Thanks,
    i-
     

    Attached Files:

    Last edited: Mar 14, 2013
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    The problem is, if you use the 12V setting and discharge the bottom battery, the upper battery will be overcharged while the lower battery gets charged. This is bad for the top battery.

    Alternatively, if you discharge the bottom battery (say 50%) and then switch to 24V and drain both batteries, you can easily over-discharge the bottom battery (which, again, is not going to do it any good)

    lead-acid batteries are somewhat tolerant of overcharge, but that margin gets lower if you're using sealed batteries, and will increase the maintenance effort required if you're using flooded cells.

    Depending on your load at 12V, I would consider a 24V to 12V DC-DC converter.
     
  3. Electrobrains

    Electrobrains

    259
    5
    Jan 2, 2012
    Steve explains well what is wrong with your circuit.

    Here is a way to solve the problem:
    Remove your SW2 and use a DPDT switch or relay as seen on the diagram below.
    In the seen position the batteries are connected in series, in the other position they will be in parallel.
    The condition is of course that the batteries have the same voltage and can be paralleled.

    As security, you could fuse both batteries or at least the right one (as indicated).
    This would be to prevent short circuit if (especially in a relay) a contact would stick in one position.
    Also, in the far future, the batteries will get old and if the aging is not synchronous, it might be that suddenly too high current would rush from one battery to the other when switched in parallel.

    [​IMG]
     
  4. istvanb

    istvanb

    18
    0
    Dec 10, 2012
    Thanks both of you for the detailed description! It helps me a lot. I can use the setup suggested by Electrobrains.

    Actually I can have 2 x 12V charger. Can I use the circuit I suggested earlier if I have a 12V charger parallel to each batteries? Can I center tap them in this case without damaging the batteries?

    If I use the circuit suggested by Electrobrains I guess I should connect the batteries in parallel and then connect a 12V charger in order to charge them properly.

    Please let me know your thoughts!
     
    Last edited: Mar 14, 2013
  5. Electrobrains

    Electrobrains

    259
    5
    Jan 2, 2012
    1. If you would use two chargers (!) that are a bit intelligent (sense when the batteries are full), your circuit would probably work well.

    2. If you want to charge two 12V parallel connected lead batteries (like in the circuit above), I would suggest to add fuses to both batteries.
    That would give some advantages:
    a) protection against short circuits as described above
    b) the inner resistance of the fuses would help splitting the charge (and supplied) current evenly between the batteries.
     
  6. istvanb

    istvanb

    18
    0
    Dec 10, 2012
    Thanks. What do you think about the new circuit diagram attached? (I have 'intelligent' chargers, so actually they dont have to be disconnected while the circuit loads the batteries)

    Let me know.
     

    Attached Files:

  7. Electrobrains

    Electrobrains

    259
    5
    Jan 2, 2012
    I suppose it would work.
    If you use that configuration, you would not need any fuses at all (for the reasons I mentioned before).
    If you still want a fuse, then put it in the conductor at the very top.
     
  8. istvanb

    istvanb

    18
    0
    Dec 10, 2012
    Kudos, man!

    thanks for helping me out!
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    Before you wire up the chargers, just make sure that neither output is connected to earth. Whilst I think it is unlikely, if they do, you will short out one of the chargers (bad) and one of the batteries (worse) when you connect it up.
     
  10. istvanb

    istvanb

    18
    0
    Dec 10, 2012
    Steve! I am not about to connect the batteries to earth or so, but can you explain this again? (I guess connecting an earth gnd to bat2- would be ok. Is that true?)

    thanks!
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    You show 2 chargers. It is not inconceivable that 1 terminal of each is grounded (assume it's the negative lead).

    If that is the case, connecting 2 of these chargers in series essentially shorts one of them out (and consequently also shorts out a battery)

    Before you try this you need to be sure that both outputs of your chargers are completely floating (i.e. not connected to ground or any other point common to both chargers)
     
  12. istvanb

    istvanb

    18
    0
    Dec 10, 2012
    I see what you mean now... I wont do that :) thanks for your help!
     
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