Wei asked in Science & MathematicsEngineering · 7 years ago

# Will a 9v battery overheat?

Currently, my RC car, a project for school, is running on two motors. Each motor requires about 3 volts. Earlier, I was trying to run it on one motor but even with abundant gearing, it would barely move. Also, the two AA batteries and motor both were extremely hot and burned you if touched them. Soon later, I tried using a 9v with a 5v regulator to power my single motor, causing it to move even less and overheat the regulator. The regulator, even with a heatsink, burned you when touched. Most recently, I added another motor and tried using the 5v regulator again. This idea was easily discarded because I knew 5v wasn't enough to power the motors. I took away the regulator and the two motors ran perfectly on the bare 9v battery, even though I only kept it on for about 5 seconds. I am unlucky with this topic and I am too scared to run it for longer. Judging the circumstances, will the battery or motors overheat now?

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• Ecko
Lv 7
7 years ago

Electric motors draw a current proportional to the mechanical load, and the speed is proportional to the voltage across the motor. When starting or stalled though, the current is even higher than the full load current, so unless the battery can deliver this current it will not work as the motor pulls the battery voltage down like a short circuit. What makes the motor and the battery and the regulator hot is power, in watts, which is due to I * I *R. So the internal resistance in these components is the R, and the current must be more than an amp or so as it takes several watts involved to heat these things up. The motor has a current rating so it doesn't burn out, the regulator is rated around one amp but self limits. The 9V battery and the alkaline AA cells cannot deliver 1 amp properly.

Mechanical power is proportional to rpm and torque (the load torque, how hard it is to turn). That is the motor output. The electrical power input is more, depending on efficiency. With these motors it may need twice as much electrical power. That is V * I. Power is in watts.

Look at the motor spec, which might be vague for this kind of motor. The first step is to get a battery that has a suitable current capability for the motor maximum current. Then the voltage will not change much even with the high motor current. This means a rechargeable type like a NiCad or NiMH. Usually RC cars run on a 7.2V battery pack, which is 6 * 1.2V cells in series, so they get enough power. The cells could be AA size or bigger. However you have different motors I think, probably 3V motors that need several amps, so you probably need 3V battery with several amps capability.

The gearbox..

The motor speed is probably high like 12000 rpm. Most precision gears on the market are only rated up to 5000 rpm. Probably any gears you use will have poor efficiency and therefore reduce the motor power that arrives at the wheels by more than 50%. If the wheels are 50mm diameter then one rotation gives 0.157 meters distance travelled. Thus for a speed of 1m/s (3.6 km/h) there is 6.37 revolutions, and that is 382 rpm. That gives a gear ratio like 50:1 or more, depending on the maximum speed required.

If this arrangement still cannot drive the car, the car is too heavy or the motor is too small, or there is too much friction, assuming the battery voltage holds up. Two motors of course are double the power of one, and double the current from the battery. The motor would be drawing too much current if it cannot move the car. However measuring the motor current of more than 1 amp can be tricky, as a multimeter is not suitable most likely. Too much resistance in its connecting wires etc.

If the motor has a mechanical output of 1W it seems likely the power of the motor will not even overcome the rolling friction so there is no power available to accelerate it. If these are 3V motors that draw 300mA at full power they are definitely too small. That is less than 1W input power.

One watt can lift 1 newton = 1 / 9.81 kg = 102g weight one meter in one second, or 1 kg in 9.81 seconds.

The link below shows a suitable motor for certain model cars. It is intended for 3 to 5 series lithium polymer cells (3.6V each) so the voltage is around 11V. The maximum current is 70A and the no load current 3A. This is about 1 horse power (over 700watts). With the 11V supply the speed (no load) is around 21000 rpm. Of course you don't need anywhere near this power, this is for a 5kg off road "buggy" vehicle, but this gives some idea of why your motor is a bit sad.