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Charging battery with battery

  1. Jul 3, 2012 #1

    I was just on vacation and forgot my camera's battery charger. I thought a lot about building some sort of homemade charger, but just didn't dare try. Luckily it ran out on the very last day, but I'd like to know if it would've worked.

    Basically, the camera has a 7.4V Li-ion battery, which I'm guessing is just two cells/batteries connected in serial. Its charger is an external one, the camera itself has no power input.

    Afaik, a Li-ion battery's voltage is higher the more it is charged, and it's charged by just applying a higher voltage that "forces" current in the opposite direction.
    My idea was to connect two fully charged 3.7V Li-ion cell phone batteries in serial to make another "7.4V" battery, and then connect + to + and - to - between these two 7.4V batteries. The fully charged one should with its higher voltage slowly charge the discharged camera battery until their levels are about equal. Fully charge the phone batteries again and repeat if needed.
    This should never be able to charge the camera battery beyond 100%, so it should be safe.

    Would this have worked, and how safe would it be?
  2. jcsd
  3. Jul 8, 2012 #2
    Nobody knows, or was the text too long?

    Short version: Can I serially connect two 3.7V Li-ion batteries, and use that to charge a 7.4V Li-ion battery?
  4. Jul 8, 2012 #3


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    Staff: Mentor

    Hi Jeremy87. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    Unfortunately, it is not safe to charge (or discharge) Li cells this way. You have no idea of the level of current involved, and have no control over it. An exploding Li cell is dangerous. https://www.physicsforums.com/images/icons/icon8.gif [Broken]

    Better to buy another charger and keep your eyesight so you can appreciate your holiday snaps.

    Feel welcome to post more questions, or join in answering some, here on Physics Forums. :smile:
    Last edited by a moderator: May 6, 2017
  5. Jul 9, 2012 #4
    Well I was thinking that the batteries might have some internal resistance that limits the current for the small voltage difference between them. At the time there was absolutely no electronics store (or a town/city) within 500km so buying a new charger was out of the question.
  6. Jul 9, 2012 #5


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    Science Advisor

    Your conundrum probably explains why you a few manufacturers (at least Canon and Nikon) still make AA-powered cameras. Doesn't help you any, but food for thought for my next camera! :smile:

    EDIT: And welcome to PhysicsForums!
  7. Jul 13, 2012 #6
    I'm still losing sleep over this. So the only obstacle is that the current is/might be too high? Would it help to rapidly connect/disconnect them (like 0.5Hz) and/or feel the temperature of the batteries?
  8. Jul 13, 2012 #7
    These ideas are pretty terrible. Li-Ion accumulators need some intelligence in the charging circuit, or they are likely to explode. When empty the voltage across them is very low, and the internal resistance is tiny, so one uses constant current to charge them, you cannot achieve constant current with a second battery alone. Connecting and disconnecting with some duty cycle lowers the total current depending on the on off ratio, I don't know if this would work, or if the short very strong current bursts might break something. Checking the temperature might also work, but you don't know the temperature profile inside the battery, it might get hot inside before you notice outside. Inside the Lithium Ion batteries there is some kind of diaphragm, if you manage to break it, while the battery is carrying charge it explodes (apparently this happened on that famous burning laptop incident). The explosion can happen on the supplying battery side or on the receiving end. Once the battery is holding some charge, the voltage basically rises close to the final voltage and one can charge it with constant voltage. So it you happen to have some current limiting device or an amp meter and a potentiometer to do it by hand you could try, but I myself would not try to charge lithium ion batteries in some improvised way as the energy density is too high for my taste.
  9. Jul 18, 2012 #8
    More questions. I may never dare try these anyway even if they worked in theory, just curious.

    - If I have two cell phone chargers, and they don't use a grounded 3-pin plug, do they still have a common ground that prevents serial connection? If by some miracle it would work, would it be able to (more) safely charge a 7.4V battery?

    - Maybe more realistically, would it be possible to serially connect a charger and a fully charged battery? Would the charger limit the current for the whole setup?

    - And a more simple question: if the camera has a 3.7V battery instead, would a cell phone charger be able to charge it safely?

    By a cell phone charger, I mean using the internal plugs inside the phone that actually charges the phone battery, not the power cable that may not even have the correct voltage (5V USB).
  10. Jul 18, 2012 #9
    i think it should not affect camera battery and cell phone one too.since cam battery is fully discharged (at 0V(approx.))now when charging it with source of 7.4 V(3.7v+3.7v) so after a long time due to slow motion of e- cam battery will get charged little below 3.7V till both side(source side & reciever side) gets equal potential diff..ur cam will work but with less power output
  11. Jul 21, 2012 #10
    More stuff:

    1) Two identical chargers connected in series might work, there is no connection of the output to the line because in many countries the power plugs are not keyed, and you don't want the phone charger plug to carry line voltage, relative to earth even if relative to one another the contacts have some low dc difference.

    2) It depends a lot on the charger, but it might break. Usually the charger would limit the current by lowering it's voltage. But lets say we need 1 Volt initially for charging, and in the end we need 7 Volts. If you connect the full battery and the charger to the empty one, then the charger will drop to 0 volts, but the series voltage is still 3.5 Volts and the current would still be too high. Charging with one charger until the battery is "half full" (actually until half the voltage is reached, which is something different), and then connecting a full one in series with the charger might work but that is speculation.

    3) This is all guesswork, usually all the Li-Ion batteries have multiple contacts, not just two. They have some intelligent electronics to control temperature, filling status, overload conditions and such things to prevent them from exploding and to give some feedback, maybe even the charging logic can be embedded. Of course all of that depends on the manufacturer, so I wouldn't really hope that using a charger on a battery that it is not designed for would work all that well (or be safe for that matter). Back in the days when all batteries were still made out of potatoes life was much easier.
  12. Jul 22, 2012 #11
    1) I still believe the minus sides of the chargers could be somehow linked, but I'll have to look more into that.

    2) I was under the impression that a Li-ion battery will never be completely depleted, it will just appear so when it reaches some minimum voltage (2V or something). Going below that would make the battery never work again. I have a multimeter so I could check that the next time I've depleted a battery.
    That 2V would correspond to 4V on a "double-battery". Maybe I should start with a half-empty cell phone battery to limit the voltage a bit and give the charger some headroom?
    I'm still a bit confused about how serial connections work. Can a battery output "infinite" current when serially connected with another source with limited power? Or is it just that 0V that allows for the "infinite" current?

    3) Still, it might start charging the battery, and it's obviously incapable of too high currents anyway. I don't even need to fully charge whatever battery I'm charging, just enough so it will work for a while.
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