li-ion vs aa batteries

Discussion in 'General Electronics Chat' started by adamclark, Dec 17, 2013.

  1. adamclark

    Thread Starter Member

    Oct 4, 2013
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    Hello. I have an rc car that has a 4 as battery pack that's makes it 6 volts, I have some 3.7v li-ion cell phone batts. So 2 of those in a series would be 7.4v with a combined current of 1800mAh. If I regulated the voltage down to 6v would they perform better and longer than the 4 as batteries? I was thinking I could build a charger to just plug it in too instead of having to take a bunch of the car apart to change batts. Thanks
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,086
    3,024
    A capacity of 1800mAh is no more than a good AA. I have some rated up to 2700mAh. And if you're talking about 2 x 900 mAh in series, the capacity is not additive.
     
  3. adamclark

    Thread Starter Member

    Oct 4, 2013
    472
    6
    I meant 1800 apiece. There's 2 of them. I was thinking series them and using a 317t to cut the 7.4v back to 6. I was also thinking that the li-ion was just better and it would take less space and I could build a charger to just plug it in. Half the car has to come apart to change the batts. my main concern is if they would work as well as the 4 aa that's there now. I have 4 of them, so I could series parallel them to add more capacity. How does the current work anyways? Does adding batteries increase it? Will the 2 li-ion last longer than the aa's? Or 4 of em series paralleled last longer?
     
  4. adamclark

    Thread Starter Member

    Oct 4, 2013
    472
    6
    I also have 2 3.7v 450mAh lipo cells from my rc quadrotor. How would the pair of those in series match up with the previous comparison. And just so you know, all these will run is a throttle servo and steering servo. The steering servo is a little stronger than the throttle.
     
  5. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    A diode drop may be enough. How sensitive is your circuit to voltage?
    Parallel 1800=3600
    Series 1800=1800
     
  6. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    Look up voltage specs for servo.
    Some are 4.5, some 6
    Two diodes maybe?
    Or switching regulator for efficiency, down to 4.5.
    Or up even from 3.7
     
  7. adamclark

    Thread Starter Member

    Oct 4, 2013
    472
    6
    Thank you for the help. Exactly what I needed to know. I don't have much experience with lipo's, I've heard they catch fire if not charged right. They are only 450mAh each. Will they compare to the 1800 may li-ions? I will try the diode. I have some 2 and 5 watt ones and a bunch of small ones. So the 317 is overkill? I need to build a small charger circuit for them to mount inside the car.
     
  8. adamclark

    Thread Starter Member

    Oct 4, 2013
    472
    6
    All I really know is that the receiver originally had 4 aa batteries running it. Its a hitec receiver and futaba radio but all the numbers are rubbed off the hitec. Wish I knew more.
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    The lithium batteries will probably have lower internal resistance than Ni-Mh, but if you use a linear regulator you throw all that away!

    To retain the advantage, you need a buck type step down regulator - giving less voltage out than it gets from the battery; its current draw from the battery will be less than you draw at the load.

    Its not completely trivial, but switcher chips like the MC34063 simplify things considerably. The 34063 can handle about 1.5A, if you need more than that you can add an external power transistors - there are online (and downloadable) calculators for the 34063 that pretty much do everything for you (inductor value etc) but only for the un-boosted configuration of the chip.
     
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