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Low-current batteries?

Discussion in 'Power Electronics' started by moeburn, Aug 2, 2013.

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  1. moeburn

    moeburn

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    Jun 25, 2012
    Is there such a thing as batteries that can't give more than 500mA of current? Or some way of preventing a circuit from drawing too much current from them?

    I recently posted about trying to get my MintyBoost 550mA DC-DC boost converter working with my Atrix phone that tries to draw 800mA and overheats the circuit. I thought of using solar panels, because I know they can't give more than a certain amount of current, but then the only way to use them that way would be directly connected as a power source, which would probably drop their voltage from the high current draw and make them turn on and off all the time (I tried, they did)
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    at a given voltage a circuit will only draw the current it needs .... Ohms Law

    if the circuit requires say, 10V and it has an overall resistance of say 100 Ohms
    then it will ONLY draw 0.1 A ( 100mA).
    if you phone requires 800mA (at 5V presumably) then you need a beefier DC-DC converter
    simple as that :)

    Dave
     
  3. moeburn

    moeburn

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    Jun 25, 2012
    Rats. The beefiest one available in a DIP through-hole mount, according to DigiKey, is 600mA, the one I'm using (550mA measured).

    Although if there are beefier SMT ones, I have some left over SMT to DIP adapters, and I have had success soldering 8-pin SMT ICs to the adapters before... TO THE PARTS DEPOSITORY!
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    No, that's a regulator designed for reducing voltage to 5V
     
  5. moeburn

    moeburn

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    Jun 25, 2012
    I don't think so...

    "Type - Step-Up (Boost)"

    It has a bonus feature that it works well if vIn > vOut. It's the LT1306. The one in the Mintyboost currently is an LT1302.
     
    Last edited: Aug 2, 2013
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Hahaha, yeah. My mistake. (after you open in a new tab you should look at the right tab:))

    That looks better, but you'll also probably need a different inductor (it will at least need to handle the higher current without saturation). You also may be very close to thermal limits so if you can attach a heatsink to the device it may help (just epoxying a piece of metal to the top may help -- they do make small heatsinks for DIL chips that you may be able to cut down)

    Also verify that the minimum startup voltage does not exceed the battery voltage, and that the device can operate from the voltage of a battery when the sevral amp load is placed on it. You may have to consider a lithium battery rather than a disposable battery, or maybe a D cell instead of an AA cell.
     
  7. moeburn

    moeburn

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    Jun 25, 2012
    Yeah, the LT1306 does specifically state that it is designed for being used with a li-ion source, whereas the 1302 says to use AAs.

    According to the max load current vs input voltage chart, when Vout is 5v, vIn must be around 3v to get at least 800mA out. vIn of 2.5v looks like it maxes around 700mA, and 2.2v around 600mA. So I might need to use three AAs, instead of 2.

    And thanks for the tip about a heatsink, I didn't think of that. I actually have some old heatsinks lying around, back from the days when they were made of copper (!) instead of aluminum, so they would still perform very well if I chopped them up with a dremel into a little piece.

    What I want to know is, can NiMH batteries handle such a high current load? They certainly got very hot when the LT1302 tried to draw 550mA; it hurt to hold my thumb on them for more than 2s, but they seem to still work fine even after they got so hot. I know they might not be as efficient as Li-ion at high currents, but will they be damaged?
     
  8. BobK

    BobK

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    Jan 5, 2010
    If a NiMH battery gets too hot it vents and loses capacity or even dies. If it is getting too hot to touch, you are in the danger zone. You realize that two NiMH batteries is only 2.4V and go get 5V out at 500mA, you will need over an amp from them? I would use 4 NiMH and forget the boost converter.


    Bob
     
  9. moeburn

    moeburn

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    Jun 25, 2012
    I did not realise that, I forgot that it would be double the current because it is half the voltage. No wonder they were getting so hot.

    But how would I go about using 4 NiMH to charge a cell phone without a boost converter/regulator? Because when they are fully charged, they are 1.4v, x4 is 5.6v, too high for 5v USB. And then when they level off at 1.2v, 4x will be 4.8v, which might be too low for USB.

    Could I still use the boost regulator, but with two pairs of AAs in parallel? This would split the current draw between them in half, right?
     
  10. BobK

    BobK

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    Jan 5, 2010
    The USB standard is 4.75 to 5.25V. The batteries, when fully charged are unlikely to produce more than that when any significant current is drawn. I think they would be safe to use with no regulator.

    Bob
     
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