Recharging a car battery

[schmidtbag]

I was wondering if anyone could help me figure out how to make a 24V ~700W DC motor charge a 12V car battery. After some google searching, I surprisingly haven't come up with many useful results for this.

The quirky thing is the motor will not be at consistent speeds and it will also sometimes spin in reverse. I don't care so much about the motor charging the battery if it is in reverse, if that is a problem.

 

[duke37]

It depends on the motor.

If it is a permanent magnet motor, then it will need to be run at half rated speed and have a diode in series to stop reverse current.

If it has a wound field, then the field current can be varied to compensate fror speed. A car dynamo or car alternator do this very effectively. The control system should be set to 14V.

Running in reverse will need some cunning circuitry unless an alternator circuit is used which runs on either polarity.

Your motor is rated at 24V and 30A so the current limit is 30A. This is similar to car alternators and a controller from such could probably be used.

 

[schmidtbag]

Aren't all DC motors permanent magnet motors? Anyways, this definitely has permanent magnets because my multimeter gets up to 3v just by spinning it with my hand.

Are you saying the motor should only spin at half speed when in reverse or in general? I'm not sure of any easy way to deliberately slow down the motor from spinning too quickly.

I'm not sure what a wound field is or if the motor has one. I figured the motor running in reverse would pose a problem. I don't really care if the battery recharges when it is spinning in reverse, so is there a reactionary way to make reverse current ignored?

 

[BobK]

Actually, if you use a bridge rectifier, the motor can spin in either direction.

Bob

 

[duke37]

All old DC motors had wound fields and were used to drive trams, fairground cars (dodgems) or roundabouts. Large motors still use wound fields due to the cost of the rare earth magnets. Electric drills use universal motors which can run on AC or DC with the armature and field in series.

If your motor runs at full speed it will produce 24V and will not be controllable. If the motor is driven in reverse, the polarity will reverse.

To use this motor as a generator, the output should be passed through a bridge rectifier to get a constant polarity for either direction of rotation. a single diode will block reverse voltage if this is all that is required. A buck convertor can be used to reduce the voltage to 14V for the battery. The motor will need to run at above half speed to get more than about16V.

A car alternator has a large leakage inductance so that the output current does not vary much with speed, your motor will have no such limit.

What is the application and why not use a car alternator?

 

[schmidtbag]

Thanks for the input so far. My concern about using a single diode is if it burns up from the motor spinning too long. The bridge rectifier idea seems nice though.

So, are you saying that if the motor is spinning below half speed, it likely can't recharge the battery? If that's the case then fine, I can try using the EMF to power something else. I'm not very good with electronics so I'm not sure what it takes to recharge a battery. The application is a robot, and while the robot is coasting I wanted to let the EMF recharge the battery. The motor is controlled by relays, where in one of the relays the motor is connected to the "common" pins, the power source at the "normally open" pins, and I wanted to use the recharge circuit for the "normally closed" pins.

Considering the short spurts of drifting, I wouldn't consider an alternator a worthy value. But, if you think that recharging the battery isn't worth the effort then I won't bother.

 

[BobK]

Do you realize that there will be a braking effect when the motor is used as a generator? So the robot is no longer coasting. Perhaps this is desirable, perhaps not.

Bob

 

[schmidtbag]

Do you realize that there will be a braking effect when the motor is used as a generator? So the robot is no longer coasting. Perhaps this is desirable, perhaps not.

Bob

Yes, this has occurred to me and I personally don't see it as much of a problem. In fact I prefer it more than not, although I'm not sure how much it will slow down the unit.

 

[duke37]

A bridge rectifier consists of four diodes. One diode is not any more likely to fail as long as it is specified correctly.

I doubt if you will get significant energy back from regeneration, you have to consider the efficiency of the motor and the control circuit.

There are controllers for the job you have but I have no information on them. Wheelchairs use them and they provide variable speed.

 

[schmidtbag]

Thanks for the info, I may not be able to charge this battery but thanks to the bridge rectifier being brought to my attention, I might be able to figure out something creative and useful to do with the excess EMF.