Voltage regulator for bike generator

Question

If one wants to build a bike generator to charge a 12V battery, a voltage regulator seems to be a smart choice. It makes sure that pedaling too quickly does not damage the battery.

But what about the current? Depending on the charge state of the battery the required torque will vary if I understood this question correctly Controlling alternator load for pedal bike

Would it be smart to use a controlling circuit that limits the output voltage and the output current to be put into the battery?

My thought process is that with such a limiter the controller circuit could be e.g. configured set to something like "load with 150W!". Pedaling more slowly would then increase the required torque, pedaling more quickly would reduce it.

Is such a circuit possible / feasible / efficient? Does it have a name I could look up on the internet?

Answer 1

The simplest and most effective way to make a bicycle generator would be to use a rear wheel with an ebike hub motor as a generator. This avoids all the gearing and transmission headaches, and it will be more efficient than a car alternator.

The motor should not be geared, and it shouldn't have a freewheel. So you can use the simplest type of hub motor, direct drive, no freewheel.

You will need a stand to support the bike frame, because if the rear wheel rolls on something it will add rolling resistance.

These motors are almost always brushless three phase. You don't need more than a 250W motor. These run on 24V to 36V batteries. This means the motor should produce a voltage close to that when running at the design speed (around 25 km/h) which is also the best efficiency point. If the bike has multiple gears, the user can select a proper gear to make sure the rear wheel rpm is optimal no matter what their favorite pedalling cadence is.

This means you can probably avoid the MPPT.

You need a three phase rectifier bridge to turn the three phase AC from the motor/generator into DC. I'll suggest Schottky diodes for lower Vf to reduce losses.

If you want to charge a 12V battery from a source higher than 12V (24-36V from the motor), then you need a buck converter. It should have a constant current output.

The output current needs to be controlled depending on battery voltage to avoid overcharging it.

I would also suggest controlling the current depending on the rear wheel rotation speed: if the cyclist gets tired and slows down, reduce the current to make pedaling easier. If they put in more effort, increase the current to make it harder. That's basically the opposite of your "constant power" idea: instead of setting the power using a knob and then trying to produce it, the cyclist just produces whatever they want/can and the charger will use it, without needing adjustment. This should make it easier to use and keep the wheel RPM at the generator's optimum efficiency point.

Since generator voltage is proportional to rpm, you probably don't even need to measure wheel rpm. A buck converter that increases output current as input voltage increases should work nicely.

Also it needs a "score counter" on the handlebars to keep the cyclist motivated, for example a Watt meter and Watt.hour counter.

Answer 2

Don't reinvent the wheel (oy). Consider using a standard automotive alternator, with a standard voltage regulator (often intrinsic to the alternator) for 12 V lead-acid batteries. These taken into account current and voltage regulation, and the needed temperature compensation for those settings.

A new 12 VDC 30 A alternator/rectifier/regulator, for example, can be found for ~US$70... or try automotive used-parts store.

As for the human interface, let the person on the pedals shift gears to determine the optimal pace, while watching a current meter to see the effect of gearing. BTW, you might want to make an averaging meter, integrating over a few seconds, to avoid sprinting and burnout of the organic component.

Answer 3

• You would never be able to start the bike from a standstill if the power was constant at 150 watts because zero speed requires infinite torque.
• Hence, you need to think about ALL the scenarios in their entirety and decide how much wattage your circuit transfers to the battery.
• Then, you need to factor in what happens when the battery reaches peak charge and how much that power should be backed-off.
• There is no one-size fits all.

Would it be smart to use a controlling circuit that limits the output voltage and the output current to be put into the battery?

Yes, but the devil is in the detail as implied earlier in my answer.

Is such a circuit possible / feasible / efficient?

It certainly is but, you need to refine how it works.

Does it have a name I could look up on the internet?

Think about how a solar charger works; the circuit needs to maximize the power (as a fraction of the pedalled power) in order to charge the battery. It's called a Maximum Power Point Tracking but, with the hindsight that you only want to remove a small percent of the available pedalled power.