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Thread: Max amp load on LiPo battery

  1. #1
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    Max amp load on LiPo battery

    Hey everyone!

    I'm running into an issue where I have some servos hooked up to an Arduino, but they aren't drawing enough amps from the LiPo battery. I can hear and see them act much faster when I hook it up to my desktop power supply. The reading on the desktop power supply is telling me it's consuming around 5Amps, but when I use my Lipo battery and try to test it with my multimeter, the max amperage is 1.25 (also it feels like testing amperage with the multimeter is making my servos noticeably slower, do I have a shitty multimeter that is introducing more resistance than normal??)

    I'm using a 7.4v 1800mah 25C LiPo battery which from what I read on the internet, it should have a max amp draw of 45?

    Capacity * Discharge rate / 1000 = Max amp draw
    1800 * 25 / 1000 = 45 amps

    I'm powering my motors separately than the Arduino so that shouldn't be a problem. I tried switching to a battery with a higher discharge rate of 65C and everything seemed to work fine, but I'm wondering why the other battery can't output as I expected...

    Thanks for the help!

    -Alonso

  2. #2

    Re: Max amp load on LiPo battery

    What voltage are you using on the benchtop power supply? If you're at 8V with the battery, but 12V with the benchtop, then yes, the servos will run a lot faster on benchtop! The solution at that point is to use a 3S (11.1V) LiPo battery.

    Btw: I have a dual 5A benchtop supply, and when I turn on my servos all at the same time, the current limiting makes it all reset. 5A just isn't that much when a lot of servos are running simultaneously.

  3. #3
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    Re: Max amp load on LiPo battery

    Just to rule it out, your battery is charged, right?

    What type and how many servos are you using?

  4. #4
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    Re: Max amp load on LiPo battery

    Thanks for the fast response. I can always count on you two =)

    Yes, my battery is fully charged. ( I checked it using the multimeter)

    I'm using a variable desktop power supply which I'm setting the voltage to 7.4 and the amps to max. I'd have to go home and check what the max amps for the unit was, but now I'm starting to guess it's 5amps
    Click image for larger version. 

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    I'm using 3 of these high voltage servos:

    http://hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=18399

    -
    Alonso

  5. Re: Max amp load on LiPo battery

    Quote Originally Posted by alonso View Post
    do I have a shitty multimeter that is introducing more resistance than normal??)
    You can also use the voltage mode of the multimeter to check the voltage drop without putting any extra resistance to the circuit.
    Quote Originally Posted by alonso View Post
    I tried switching to a battery with a higher discharge rate of 65C and everything seemed to work fine, but I'm wondering why the other battery can't output as I expected...
    I haven't used Zippys, but of course it's possible that the stated discharge rating is just way more than the battery is actually capable of. Does your battery circuit have thick enough cables, that they aren't adding any significant resistance?

  6. #6

    Re: Max amp load on LiPo battery

    Also: Are you running the battery power through the Arduino board to the servos, or are you going straight from battery to servos for power? (Or perhaps through an UBEC?)
    The Arduino board uses thin traces, and the 5V regulator on it only does about 500 mA, so that's not good for driving anything other than the Arduino itself and some LEDs and whatnot. Even if you don't use the 5V output, the thin traces makes for significant voltage drop (and heating!) on the Arduino board if you power motors through that.

  7. #7
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    Re: Max amp load on LiPo battery

    Hukka, I'm not sure if my cables are thick enough and creating some resistance. I haven't noticed them getting warm or anything, but I will buy some thicker cables and try it out.

    Jwatte, because the servos are high voltage (7.4v) I'm running the servos straight from the battery (and connecting the ground to the Arduino). I also tried using a separate battery for the Arduino than the servos, but that didn't help any.

    The only thing that has worked so far is using a battery with 65C discharge rate rather than 25C, but I can't figure out why the 25C battery doesn't work. =(

  8. #8

    Re: Max amp load on LiPo battery

    25C and 1800 mAh should allow you to do 45 A draw, and three servos should not come close to that.

    If it works with a 65C battery pack, my guess is that the 1800 mAh pack isn't actually 25C. Either it's misleadingly marketed/sold, or it's old and worn out. LiPos can wear out as quickly as 200 charge/discharge cycles; this is one area where LiFePO4 is better, as it can do 5-10x as many cycles.

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