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Replacing a battery with a different type

  1. May 9, 2007 #1
    Where I work, we have some equipment which uses custom batteries. I opened one of these batteries up, and inside, they're just three D batteries in series.
    We want to make lighter ones of these batteries, so I tried replacing the D batteries with three AAA batteries, hoping (quite doubtfully) that it would work, simply with a shorter battery life. Unsurprisingly, it didn't work.
    The three AAA batteries were producing a much smaller current than the D batteries, even though the voltage was the same, and I assume this is why it didn't work. As far as I understand, this must be because of the internal resistance, but I'm not really sure about that.

    Is there any way that I could replace the D batteries with AAA batteries and still get the same current, or is there any other way I could make the batteries lighter? (It doesn't matter if the battery life is greatly reduced. The only important thing here is weight)
    As you can probably tell, I'm quite clueless about this :frown:
     
  2. jcsd
  3. May 9, 2007 #2

    russ_watters

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    How about AA batteries?

    A AAA battery (alkaline) has a capacity of around 1,000 mah.
    A AA battery has a capacity of around 2,000 mah.
    A D-battery has a capacity of around 12,000 mah.

    You can draw quite a bit of current from a regular battery without damaging it. I've seen battery packs get pretty hot if discharged in 10 minutes or so, so lets say that's the fastest you can reasonably discharge a battery. So for a AAA battery, that would be .166 amps. The same amperage from a AA battery would give you 20 minutes and from a D battery, 2 hours.

    What kind of device are you using and what amp-draw is it?
     
  4. May 9, 2007 #3
    AAA batteries have high internal resistance. Since you need 3 AAAs, which is approximately 4.5V, you can try using a single fully charged Li-ion battery. A fully charged Li-ion battery is about 4.2V. It has a much lower internal resistance and is lighter.

    caution: If you charge a Li-ion battery to more than 4.5V, it might become volatile!
     
  5. May 10, 2007 #4

    berkeman

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    What is the application? If it's just a motor start current that is keeping the smaller batteries from working, you might be able to put a large capacitor in parallel with the AA/AAA batteries to help provide the motor start current surge....
     
  6. May 10, 2007 #5

    Danger

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    I'm not sure whether or not this is relevant, but is this to be a rechargeable pack? The reason that I ask is that when the indwelling NiCads in my first cordless phone packed it in, I replaced them with a supposedly compatible aftermarket set (they were AA size). The damned thing caught on fire the first time that I tried to recharge it. :grumpy:
     
  7. May 10, 2007 #6

    berkeman

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    It's a little-known PF history legend that Danger chose his login name the day after this incident. Probably just a legend, though..... :blushing:
     
  8. May 10, 2007 #7

    Danger

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    There's a spark of truth in that, but this was not, in fact, the fire in question... :uhh:
     
  9. May 10, 2007 #8

    berkeman

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    Um, you mean there's been more than one? :bugeye:

    J/K. Not that any of my projects have ever caught on fire before. Pretty funny story about the time my R&D VP's head popped up over the cubicles one weekend when I accidentally arced a 25kV CRT anode button to a PC terminal chassis....
     
  10. May 13, 2007 #9

    It's a laser tag vest.
    I took some readings with a multimeter, but I don't know what the actual current was because it seems to move the decimal point around depending on the setting I choose :confused:
    I set it to 20 mA and it read 0.25 most of the time, then jumped up to around 0.65 whenever the pack made a sound.

    It is just the larger currents that are causing the problems (when I tried the AAA batteries, it worked fine until the laser game started) but these larger currents can come up to four times a second (the fastest shot rate) when the game is running, so a capacitor probably wouldn't work here, would it?

    The batteries we have are rechargable, so it would be nice to replace it with a rechargable battery, but it's not necessary.
    I'm trying to make a lightweight, smaller pack specially for younger children to use, so it won't be used too often and using non-rechargable batteries for it won't be a problem, so long as the batteries aren't too expensive.

    I looked up some li-ion batteries in an electronics catalogue after reading that, but there weren't very many, and they were all either 3v or 7.2v
     
  11. May 13, 2007 #10

    NoTime

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    That doesn't make sense.
    An AAA battery can easily handle > 1 ma.
    I suspect, with your mention of the DP moving around, that you had the meter set to the 20A scale.
    So your currents would be 250 ma and 650 ma.

    Those currents would be way too large for a normal AAA battery.
    You could try a C size battery.
    Or some of the low internal resistance AA size batteries used for electric race cars could work.
     
  12. May 14, 2007 #11
    An easy way of getting a single Li-ion battery at 4.2V is by fully charging your hand-phone battery (Chances are it is a Li-ion battery). After fully charging it, if you measure the voltage at the most-right and most-left terminals, you should get about 4.2V. Those Li-ion batteries rated at 7.2V, have 2 cells in series inside.
     
  13. May 14, 2007 #12
    It was definately set to 20mA. The multimeter I used doesn't even have a 20A setting.
    I tried the AAA batteries again, since you said it should work, and it did work.
    I don't know why it didn't work before. I used new batteries both times =/

    I'll try a few different batteries now, to try to get the longest battery life I can, while still keeping it light.

    My phone's battery is a Li-ion battery, but it's only 3.7V. I'll buy a 7.2V Li-ion video camera battery and try that.
     
  14. May 14, 2007 #13
    The 3.7V rating is the normal rating when the Li-ion battery is about 50% charged. If you leave your phone for a few hours in the charger, then measure the fully charged open circuit voltage, it should be around 4.2V.
     
  15. May 14, 2007 #14

    NoTime

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    If the current draw of > 1ma is correct then you should get a 800 to 1500 hours of operation from standard alkaline AAA cells, depending on who made the battery.

    Can't imagine why they used D cells in the first place.
    That current draw works out to just about shelf life for a D cell.
     
  16. Nov 10, 2010 #15
    Here's my problem... I use a Jazzy wheelchair from the Scooter Store... It has 2 12v deep cycle batteries to run the chair. The battery charger is a 1amp trickle charger that takes 8-12 hours to give a full charge to these batteries. After about 6-8 months the strength of the batteries start to deteriorate to the point that I cannot go as far on a single charge as if the batteries were new.

    I saw a report on the Lithium Metal Polymer batteries taking a car 375 miles on a 6 minute charge. This type of technology should be rather easy to develop for a power wheelchair of any type. Someone please help me make this happen. I'm a computer technician by trade for the past 20 years so electronics is not a strange subject to me. Thank you in advance.
     
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