Efficiency of a small generator

[software]

I have a small device that I would like to power with harvested mechanical energy. There is not much force available.
The plan is to have a rectifier and connect it it directly to a few AA NiMh cells.
I am trying to pick the most efficient geared motor to charge these AA cell batteries. For the time being I discharge cells, and count how many spins it takes take to up the battery voltage by 0.1. Very primitive and not scalable method.
I am looking for a way to measure the motor amperage, in mA. I will do my best to produce a constant rotation of the gear shaft.
Thanks in advance.

[Zero999]

A brushless motor will be best efficiency wise.

Use a Schottky rectifier for low on voltage losses. If it's a brushed motor, a single diode would be more efficient than a bridge rectifier, but it would only work when the shaft is turning in one direction.

Generally, the smaller the motor/generator, the less efficient it will be and the smaller it is, the higher speed it needs to run at, to get the best efficiency.

[software]

@Hero999, thank you for your advice. I have that. Now I need to measure mA it produces. When I connect the multimeter from the rectifier into the battery the results are all over the place, I'm not quite sure this is the correct way to measure the current the motor produces.

[MrW0lf]

If you want so squeeze absolute max out of it experiment with impedance matching. To get the general idea you may connect variable resistor to driven brushless motor (=generator) and observe effects of changing resistance at given fixed RPM. Forget the multimeters. For energy consumption you use oscilloscope. Low value inline shunt resistor (<=0.1ohm for example) for amperage and voltage between lines on second channel. Use math function to calculate power. When you get proper hang of it may move to optimizing actual complex load.

From https://meettechniek.info/measuring/power.html

With shunt resistor
circuit power measurement with an oscilloscope and shunt resistor
Fig. 10: Power measurement with an oscilloscope and shunt resistor.
The probe for the voltage measurement is connected directly to the load. The current is measured indirectly via a shunt resistor. Because the grounds of both scope channels are internally connected, the ground terminals of the probes must be connected to the same potential. Because of this the current is measured with inverted polarity. It's therefore necessary to invert the signal coming from the shunt.
By multiplying the voltage and current signal a third signal is created that represents the power as function of time. To take the average of this power signal over one full period the average power is determinate.

[Zero999]

@Hero999, thank you for your advice. I have that. Now I need to measure mA it produces. When I connect the multimeter from the rectifier into the battery the results are all over the place, I'm not quite sure this is the correct way to measure the current the motor produces.

The current into what load?

If you just set the meter to current, then it will be the meter's current shunt resistance, which will be very low, possible much lower than that of the winding resistance, so can be treated as a dead short. You'll probably also notice that the motor's shaft becomes more difficult to turn, with the output shorted. This is because the current flowing is trying to turn the motor in the opposite direction.

[Gyro]

Small coreless DC motors with precious metal brushes can be very efficient....  https://www.eevblog.com/forum/chat/post-a-guess-on-the-min-starting-voltage-for-this-brushed-dc-motor/

[MrW0lf]

Small coreless DC motors with precious metal brushes can be very efficient.... 

Also self built. I've done motors that run on <100mV no prob (reed switch commutation, near lossless ferrite cores for coils). For generators when impedance matching can approach unity pretty close.

[software]

This is helpful, thank you. The goal is to compare several motors, apply about the same amount of force to each and see which one is more effective to charge the two AA MiNh batteries connected directly to the rectifier. So I was trying to calculate the amperage of the motor as it relates to charging the battery. Not sure if I am asking the right question, though.

[Zero999]

Small coreless DC motors with precious metal brushes can be very efficient....  https://www.eevblog.com/forum/chat/post-a-guess-on-the-min-starting-voltage-for-this-brushed-dc-motor/

They run at high speeds though, which would require lots of gearing which will result in a considerable loss.

[MrW0lf]

They run at high speeds though, which would require lots of gearing which will result in a considerable loss.

Gearing could be electronic - potentially much more efficient. It is important to present adequate load to gen. For example with brushless one of most useless modes of operation would be just clipping top of the sines with bridge to charge some load sitting near gen output voltage. Alternatively one could present to gen low impedance / matched load that would make use of full available torque / gen voltage and then raise the voltage using electronic means for actual end-load.
I have never played with DC motors but conceptually low voltage buffer before high voltage actual load should work there also.

[Zero999]

They run at high speeds though, which would require lots of gearing which will result in a considerable loss.

Gearing could be electronic - potentially much more efficient. It is important to present adequate load to gen. For example with brushless one of most useless modes of operation would be just clipping top of the sines with bridge to charge some load sitting near gen output voltage. Alternatively one could present to gen low impedance / matched load that would make use of full available torque / gen voltage and then raise the voltage using electronic means for actual end-load.
I have never played with DC motors but conceptually low voltage buffer before high voltage actual load should work there also.

I don't see how electronic gearing would be any more efficient, than mechanical gearing. The problem then becomes converting a very low voltage, to a much higher one. The power output would still be limited by the winding resistance, which would have greater losses, at high currents. Spinning the shaft faster, using a gearbox would generate more voltage and the current could be lower, thus reducing the winding losses.

I agree that using a power factor corrected rectifier would be much more efficient, than a basic bridge for a brushless alternator.

With a DC motor/dynamo the rectification has already been done by the commutator, so I don't think much can be done, other than perhaps use an ideal diode controller as the rectifier, although that won't work below a certain voltage.