Hello, I am building a hand crank generator from a DC motor to charge a lithium battery, which will provide power to a USB port that can be used to charge a phone (slow and inefficient I know). I am going to have a series of gears so that I can spin the DC motor fast enough.

The lithium batteries are typically 3.6 to 3.7 volts.

I am a bit confused on what rout to take for the DC motor. I am looking at those mini brushless and brushed DC motors - they come in all kinds of voltages: 3.3V, 5V, 9V, 12V, etc. They also come in different RPMs, ranging from 2,000RPM to 10,000RPM. I am assuming that I will need to spin the motor at the rated RPM in order to roughly output the rated voltage - half the RPM for roughly half the voltage, etc.

Here are my main questions:
I know that I need to regulate voltage to 5.0V for the USB port and 3.7ish volts for the lithium batteries. Is it going to be better to use the 3.3v version motor and boost this voltage up to the 3.7ish volts for the lithium batteries (I will use a battery management charging circuit for this), and then further boost the voltage up to 5.0V for the USB port? Or is it better to use for example the 12V version of the motor, and buck the voltage down to the 3.7ish volts for the lithium batteries?

Is it possible to do this so the faster and harder you turn the crank, the faster the lithium batteries charge? It seems like once you are turning the crank, and you exceed the 3.7V, that anything above that is basically wasted? I am a bit confused here.

Thanks and any help or advice is greatly appreciated!

EDIT:
For the sake of this discussion not getting offtopic, Lets ignore the phone charging, and lets just say I want the 5.0V USB port for charging a small USB thumbdrive style flashlight. Low power requirements.

 

I am assuming that I will need to spin the motor at the rated RPM in order to roughly output the rated voltage - half the RPM for roughly half the voltage

Correction: 1/4th the voltage.

Is it possible to do this so the faster and harder you turn the crank, the faster the lithium batteries charge?

Li-Ion batteries have a specific charge profile. Exceed the amount of current going into the battery will damage it or even destroy it. Upon destruction you're likely to have some excitement, smoke and possibly get to meet some new fireman friends.

 

This is a video I did some four years ago. The subject of the piece is on how free energy is not possible. You can generate energy by converting muscle energy into rotational energy and use that energy to do work. In this case I was simply lighting a diode. Not charging batteries. If I wanted to charge batteries using this simple arrangement I'd have worn a hole through my thumb down to the bone by the time I got any useful charge into a battery.

I am building a hand crank generator from a DC motor to charge a lithium battery, which will provide power to a USB port that can be used to charge a phone (slow and inefficient I know). I am going to have a series of gears so that I can spin the DC motor fast enough.

You're right. (Bold is my own edit to TS comment)

 

There are chips specifically designed to charge a battery properly from various sources including variable power such as a solar panel. I highly advise you incorporate such an IC into your project.

Driving a 5V output from a lithium battery is also a wheel already invented. In fact the IC above might even include that function.

 

If you want to turn a hand crank for 2 hours to charge a battery, have fun. It will make you appreciate power sources.

 

If you want to turn a hand crank for 2 hours to charge a battery, have fun. It will make you appreciate power sources.

For the sake of this discussion not getting offtopic, Lets ignore the phone charging, and lets just say I want the 5.0V USB port for charging a small USB thumbdrive style flashlight. Low power requirements.

 

I am looking at those mini brushless and brushed DC motors

You need a brushed type to use as a generator.

they come in all kinds of voltages: 3.3V, 5V, 9V, 12V, etc. They also come in different RPMs, ranging from 2,000RPM to 10,000RPM.

Pick the lowest RPM motor for the voltage you want.

know that I need to regulate voltage to 5.0V for the USB port and 3.7ish volts for the lithium batteries. Is it going to be better to use the 3.3v version motor and boost this voltage up to the 3.7ish volts for the lithium batteries (I will use a battery management charging circuit for this), and then further boost the voltage up to 5.0V for the USB port? Or is it better to use for example the 12V version of the motor, and buck the voltage down to the 3.7ish volts for the lithium batteries?

For best efficiency, pick about a 6-7V motor and then use buck converter(s) to get the voltage(s) you want from the motor voltage.

 

Here's just one example of the type of chip I recommend:
https://www.ti.com/lit/ds/symlink/bq25504.pdf
Texas Instruments has a range of such things, as do other chip makers.

 

If spinning a motor motor at its rated RPM produces its rated voltage, then the motor runs on 0V. The BEMF must be less than the voltage that will spin the motor at the same RPM.

 

Here's just one example of the type of chip I recommend:
https://www.ti.com/lit/ds/symlink/bq25504.pdf
Texas Instruments has a range of such things, as do other chip makers.

This looks like a really cool chip. I saw a few others that do something similar. This looks like it will do the trick - I picked up a BQ25504 breakout board / module to experiment with.

 

To get anything close to the rated voltage out of a PMDC motor you will need to spin it close to the rated speed. That should help make the selection simpler. Understand that the process is reversable

 

Understand that the process is reversable

Minus losses in both directions.

 

I neglected to mention losses, which are certainly present in both directions, motoring and generating.
What I see is that the TS probably does not know much about motors. Nor about "voltage changing circuits."
Whatever motor is selected, it needs to be a permanent magnet field type, not a wound field motor like most universal motors.
It might be that the motor portion of a BLDC motor could serve as an effective alternator, if it was used with a multi-phase bridge rectifier circuit.
I doubt very much that any sort of regulator will be required with the capabilities of most small motors.

 

MAX745 is a versatile lithium-ion charging chip; you can buy modules ready to use, with dipswitches for setting number of cells.

 

Consider that it is not reasonable to think that the TS will leave a charging battery unattended while cranking the generator, do you really think that it is needed to have an electronic regulator?? The TS can pay attention to the battery temperature while cranking the generator.

 

How could this work for charging supercapacitors instead? I understand that the energy density is way less, and that it will be more expensive, and much larger to use supercapacitors.

Is there a way to charge supercapacitors slower/faster depending on the amount of current that is available? e.g. depending on how fast you are cranking?

 

How could this work for charging supercapacitors instead?

In the end it all comes down to how much energy you expend charging a system, then getting that energy back in the form of useful electricity. The gains one way or another are always offset by the losses, no matter what the means or storage. A battery has greater energy density than a super-cap. More super-caps to gain the same amount of energy density still means the same amount of energy to charge them. Battery or SC - you're not gaining much, if anything.

Then there's the danger of a sudden discharge of a super-cap. It can be much more violent than a battery. And yes, batteries can be violent too. A dead short across the terminals could cause outgassing of hydrogen, which can be volatile as well.

 

CERTAINLY adequate care in the hardware assembly is required. Understanding the requirements and using adequate protection is the only way to avoid "natural selection."However, a series diode can easily prevent reversed current flow thru a problem with the charging circuit.

 

Supercapacitors are probably a little more efficient in terms of energy delivered for an amount of energy put in. I'm not sure about that, just a hunch. BUT, batteries have the wonderful feature of outputting most of their stored energy at a relatively constant voltage. The voltage across a capacitor drops predictably with every increment of energy output. A great deal of its stored energy may be at a voltage that's unusable by the load. By the time you compensate for that, you're likely worse off than using a battery.

 

Perhaps it will make a lot more sense if the TS is willing to explain the goal of creating a hand driven charging system of some kind.
Just like some of the other threads, this thread starts out asking how to implement a solution to some problem, instead of asking for solutions to the initial need.