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Thread: Battery voltage reduction

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    Default Battery voltage reduction

    Hey all
    ive picked up a little 2.4Ghz remote control 4WD toy which runs on a small 4.8v rechargeable AAA battery pack. Performance off road is good but run time tends not to be too long and its recharged by a USB adaptor which takes forever to top up the cells.
    what i was thinking of doing is running the beast on 2 x 18650 batts mounted inside it. I know they are 3.7v each making 7.4v , which is probably too much for the electronics inside the car , but reducing the voltage down to something more like 5v which it should handle. What are my options here ? a DCDC converter or some sort of low value ceramic resistor? I'm not worried about performance so much , more want to increase the run time and have precharged batteries in the field.



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    I'd look for a small stepdown /buck/ solar controller module.

    They come in many different sizes and flavours, a basic one you can set and forget would do you. They are adjusted with hair trigger trimpots but would do. You will just have to watch the amps. Then again, if the thing is normally powered by AAA rechargeables, it can't be pulling too much power. You may also want to run a decent cap to smooth out the pulses for the power feed. Bit of experimentation involved. I'd also look at running a pair of the 18650's in series. They wouldn't voltage drop as much as I think the smaller pack would and the car may be happy on that although you may loose a little speed. Alternately you could also look at boost converters rather than buck converters and parallel the larger cells and ramp the voltage up to what you want rather than down. Again, the amps may be the thing.
    Often these little converters will run higher amps if you cool them and many ratings in Typical Chinese Bullshit inflated number descriptions like a rating with additional cooling.

    I think the biggest challenge here may be fitting everything in to the car body.

    Resistors would be smaller BUT, they will need to be larger Ceramic's to handle the power ( may not be that much smaller all up come to think of it) and will waste a lot of the power and runtime you get and will almost certainly need to be cooled as well.

    This is the sort of thing I'd look at:


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    ive picked up a little 2.4Ghz remote control 4WD toy which runs on a small 4.8v rechargeable AAA battery pack. Performance off road is good but run time tends not to be too long and its recharged by a USB adaptor which takes forever to top up the cells.
    what i was thinking of doing is running the beast on 2 x 18650 batts mounted inside it. I know they are 3.7v each making 7.4v , which is probably too much for the electronics inside the car , but reducing the voltage down to something more like 5v which it should handle. What are my options here ? a DCDC converter or some sort of low value ceramic resistor? I'm not worried about performance so much , more want to increase the run time and have precharged batteries in the field.
    Depends on the current requirements of the motor, any idea what that is? IF it's not too much, you could do 3.7 -> 5 via boost converter ...or like you posit, 7.4 -> 5 via buck/linear regulator.

    Now the caveats and gotchas if it's a brushed motor --- if you increase the weight of the vehicle, it will pay out on the motor brushes/commutator (and gear-train) and result in shorter life of these components....physics applies equally to models. If you increase the constant running time of the motor, this will also result in more heat, which again pays out on the motor brushes/commutator and shortens the motor life.

    In fact, some of the brushed motors in these models are so small & fragile, the 'evident example' of for instance 10minutes runtime/50minutes charge-time, is a reflection of the duty cycle that results in the maximum motor life. You come to learn this if you have such a model, and a dozen spare charged batteries to throw at it. with the plan being to get an hour or more near uninterrupted playtime on the thing 'coz no recharging time - swap and go. Sounds like a plan, until around the 3rd or 4th battery swap you notice the motor's lost power -- it's done, time for another motor. If you open them up to diagnose the failure, it's usually a commutator sector failure resulting in loss of an armature winding -- in 'pseudo-brushed' motors where the brush is actually just a sprung wire, the commutator segments get literally cut in half by the lathe action/heat of the sprung wire pressing against them...it's like mega electric motor carnage & damage at a micro scale =)

    I did all this nonsense in the air, not on the ground... that's had it's fun moments when more than 1 motor decides to fail at the same time ...but whatever, brushless is the go...more grunt, better efficiency, longer life..and you would run 7.4v direct

    Even my nano helicopters all got me shoving on a brushless conversion ...much better idea.
    Last edited by wotnot; 18-09-20 at 11:16 AM.

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    thanks for that , i might go with the 18650s in parallel , then a buck converter. But first i'll hook up a digital ammeter in line just to see what the car draws , i dont think it will be much.

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    Btw, what's the toy exactly? I've got to service my nephew's in coming days, coz his brushed->brushless upgrade has destroyed the plastic driveshaft uni yolks =)

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    Its just a little 4WD Jeep. uses cheap brushed motors at each axle. goes ok for the scale speed and jumps over obstacles and gutters no problem. I pulled the motherboard out this afternoon and looks like both motors are controlled from the one CMOS chip. The board is marked XN-1657R-2.4G V4 .

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