# Question on DC motor

1. Jun 1, 2009

### dvn

Hello, I am new to this forum. I hope this can help me with my curious question.

I know that the DC motor can be run by electricity, but also it can create electricity by torque force.

The question I would like to know if this works or not?

One 1.5v DC motor is powered by one rechargable 1.5v battery. The DC motor is attached to another 1.5v DC motor where there is a rechargable 1.5v battery but that battery is not feeding the power to that DC battery. Can the DC motor recharge the battery for the another DC motor's battery?
Will there be even feed from one battery to another battery?

thanks for your help with this question.

2. Jun 1, 2009

### mgb_phys

Yes you can do this.
A rechargeable battery is normally 1.2V, this will (just about) run a 1.5V motor. If you connect this motor shaft to another 1.5V motor it will output 1.5V which will charge another 1.2V battery.
Obviously using up the whole of the first battery won't completely charge the second one because energy is lost in the motors.

3. Jun 1, 2009

### dvn

Is there any sort of formula to work out how much the battery has been recharged after the first battery loses all its energy and in-between energy loss transfer?

4. Jun 1, 2009

### likephysics

Batteries takes different time to charge depending on the charging current. You can measure the current going into the 1.2v battery and get an estimate of the amount of recharge.
Batteryuniversity has some useful info on recharging batteries and their chemistry. Generally more current=faster charging.
Your input power will be 1.5V * current going into motor
output power will be 1.2v * current going into the battery

Pout/Pin will give you the efficiency of your ckt. loss = pin-pout

5. Jun 2, 2009

### Lok

Usually a dc motor has about 80% eff, if run cold. Accumulators are around 90% at discharge and 60% at recharge, and of course the dc motor as a generator that has a very low efficency, as it was never designed to do that. So for a very gross overall efficiency:

0.80*0.60*0.90*0.50=0.216 => 21% efficiency

I've been overly generous with the values and still most of the energy is lost.

Btw a DC motor as a generator might not achieve the necessary voltage to charge the second accumulator.

6. Jun 2, 2009

### getitright

Lok hit on the very point. That is why you don't see electric powered electric generators. They would operate at low amperage and would require constant changes of the batteries to keep a radio running unless you're using a gel electrolyte battery and you're not going to find those on anything but a nuclear sub.

7. Jun 2, 2009

### dvn

So this means the energy transfer from one battery to another battery will only achieve about 20% or less recharged?

8. Jun 2, 2009

### dvn

Are there any special DC motors or designed to make electricity like the Hydro plants that creates electrity from water flows or the wind generators?

9. Jun 2, 2009

### Lok

From an energetical point of view yes. Very little energy will get transmitted.

But an accumulator needs a recharge voltage of 1.2 (read the label) to actually recharge. The chemical reactions within cannot start if less. A normal recharger works with a voltage slightly more that the accumulators.

But your assembly will not be able to produce 1.2, as voltage is lost on the way so it might be that the second accumulator will not feel a thing. And all the energy will be lost as heat.

10. Jun 2, 2009

### dvn

Are there any ways to make the battery recharge and not to lose energy? Or is this not possible?

What if I use 12v DC motor run by 12v battery and feed into 1.5v dc motor and recharge the 1.5v battery. Will that work?

11. Jun 2, 2009

### Lok

It comes down to the electric energy being transformed into motion and back into electric. The first setup might have worked as you had two DC motors of same size and make. Using a bigger motor is a thing (energy will get transfered )but the motors need to have a similar rpm range. If not the end voltage might be too high or too low. Measure it before attaching the second accumulator.

It's easier to recharge the accumulator electrically. I've done it with a nintendo and a mobile phone charger (cut and split the wire) :P

12. Jun 2, 2009

### mgb_phys

The generators in hydro plants are effectively large AC motors, in fact you have to be careful when running a generating plant that you keep the correct speed - otherwise if you get behind, the rest of the grid will drive your generators like a motor.

Motor-generator pairs used to be a common way of converting AC to DC to power things like subway trains before high power electronic converters were possible.

13. Jun 3, 2009

### dvn

Are there any good web site to research for the permanent magnet DC motor for their power supply output and input as well rpm and voltage usage?

thanks

14. Jun 17, 2009

### dvn

Thanks Lok as your information is what I needed also to everyone who replied. I got the answers I needed.

Cheers

15. Jul 22, 2010

### luncay

yes, but the voltage of second motor must be higher than 1.5 volt of first motor.Then you must use the pulley system just like bicycle which mean the first motor use big pulley that attached to second motor which use smaller pulley to double the rotation.Then you get the higher output.