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Alkaline batteries in parallel?

Discussion in 'Electronic Basics' started by George, Jan 24, 2008.

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  1. George

    George Guest

    I just bought a dirt-cheap wireless lapel microphone with
    transmitter and receiver. But, I think there may be a problem with
    battery life.

    The transmitter is powered by a 9V alkaline battery, and I rigged it
    up to test the current draw, which was 38 ma. The receiver is
    powered by a single AAA battery, and it draws 52 ma. In both cases,
    that's with the power-on LED indicators disconnected to save power.

    Back in the day I've used an external battery pack to extend the
    life of a digital camera which didn't operate well on rechargeables.
    The pack was just a Radio Shack holder for four C cells, plugged
    into the AC jack on the camera, to take the place of the usual four
    AA's. The C's lasted a long, long time, at not much more cost than
    four AA's, but at the cost of "wearing" the battery pack in some
    position.

    With the wireless mic, there may be times when I want to use it over
    an extended period of several hours without having to stop and
    change batteries, and without having to wonder if it's gone dead.

    In the case of the receiver, I could attach a twin AA holder to the
    back, and power it from that, but with the holder wiring modified so
    that the batteries are in parallel. That would give me two AA's in
    the place of the one AAA, which should significantly extend the
    life.

    The transmitter is a bit more of a problem. The best I can think of
    is to mount two external 9V's on the back, probably with tape, and
    wire the connectors together in parallel, possibly also leaving the
    internal 9V still connected, which would give me three 9V's in
    parallel.

    Well, I have a couple questions:

    1. Is there a better way to do this?

    2. Is connecting the batteries in parallel going to cause any
    problems? They will remain connected to each other even when the
    power switch is off. Will the batteries do bad things to each other
    connected that way for extended periods even when no current is
    flowing to the external circuit?

    I don't know yet how much voltage drop the devices will tolerate and
    still operate. I'm testing that now. The answer might give me
    other options.
     
  2. Tom Biasi

    Tom Biasi Guest

    Usually if you parallel batteries of identical type the internal resistance
    is enough to prevent them from "doing bad things to each other".
    You could rig up a belt pack to supply the needed voltage at greater
    capacity.

    Tom
     
  3. gearhead

    gearhead Guest

    Don't mixed used and new batteries when paralleling. Use all new
    identical batteries (same size and brand). No problemo.
    I took apart a 9 volt battery once. It has six individual cells,
    wired in series. It's kind of cute, they look just like the A type
    batteries but much smaller. Cute, but inefficient. The nine volters
    cost far more per unit energy than a single cell "battery" (misnomer,
    a single cell is not a battery of cells). I calculated it and you get
    way more bang for the buck (calculted per joule) with single cells.
    You were contemplating weighing yourself down with more nine volt
    batteries in parallel, but you would do better in my opinion to put
    six single cells in series, AAA or whatever you want. Clip it on your
    belt and run some 20 or 22 gauge wire to the mic. Get a 9 volt
    connector clip to connect to the mic.
     
  4. ehsjr

    ehsjr Guest

    You might want to try a 9V lithium battery.
    http://www.batterymart.com/p-ultralife-9v-lithium-battery.html

    Ed
     
  5. Bob

    Bob Guest

    Why don't you run the reciever from a mains power supply?
    A single D cell would be better than paralleling two AA's.

    For the tx there are lithium PP3 batterys with more than
    twice the capacity of a typical alkaline PP3. A few suppliers
    have PP3's they specifically recommend for long life in
    radio mikes.

    Personally I prefer rechargable batterys. Since I have a suitable
    charger (and take precautions just in case they do catch fire
    while on charge) I'd use three 18650 cells and a couples of
    diodes to drop the voltage a bit as 11V from a full charge
    might be a bit high, a little polyfuse in case the wires get
    shorted and insulate the whole thing in heatshrink.
    That would not be much more bulky than a single extra external PP3.
    2200mAH 18650 cells would in theory give 58 hours at 38mA drain.
    18650 cells are used in most laptop battery packs, cheap and
    high capacity but you have to charge then properly and not short
    circuit them so I'm allways slightly hesitant to recommend them
    for projects.

    Bob
     
  6. default

    default Guest

    Connecting batteries in parallel isn't a good idea, if you do it make
    sure they are with the same date code and both new and replaced as a
    pair.

    If you are going to strap batteries on the outside . . . go to radio
    shack and get a holder for 6 AA batteries with a 9V snap on the case
    and use that. That would increase the capacity by about 3.5X

    A 9V battery should last a good while in the transmitter.

    The alkaline 9 V should have a capacity of about 700 milliamp hours
    and at 38 ma that's 18 hours . . .

    A single AAA has a capacity of about 1.25 amp hours or should last
    around 24 hours . . .

    Maybe the switches are being left on?
    --
     
  7. George

    George Guest

    George says...
    Thanks very much for all the suggestions. I had to
    interrupt my battery-life test, but at 15 hours both the
    transmitter and receiver were still working. With the power
    on, the 9V was at 5.47V and the AAA was at 1.20V.

    What I was trying to find was the voltage at which the
    circuits stop working, since of course that could have a
    substantial impact on how long the batteries will last. And
    so far the numbers are better than I expected.

    I'll finish the test, but I think if I need better life than
    provided by alkaline batteries, I think using lithiums is
    the best option rather than adding on external packs.
     
  8. Wim Lewis

    Wim Lewis Guest

    Yup, those are AAAA cells --- as far as I know, they're only ever
    found inside 9V batteries...
     
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