permanent magnet dc motor used as a generator

amybkant

 Connected a dc motor to a bicycle as a generator and would like to charge my 4 100ah agm deep cycle batteries when my solar panels are not providing power. what kind of charge controller can be used? it produces up to 60 volts and a steady 30 volts would be easy without much of a work out. 12 volt battery bank. 

mike95490

Your battery bank is the regulator.  When you load the motor / generator down, the voltage drops, and the shaft becomes harder to turn.

Pumping the bicycle, you might be able to generate 50 watts for an hour.

amybkant

Thanks Mike. can you be more specific as to which charge controller can take that much variation of the input voltage. 
would a dc permanent magnet wind turbine charge controller
 work?

BB.

amybkant said:

what is the best charge controller to use for a permanent magnet dc motor connected to a bicycle and charging 4 100 ah deep cycle batteries. the motor is rated at 90 vdc 10 amps.  measured 50v at fast speed peddling without much effort. just a project for when my solar panels aren't charging. thanks for any help.

Amy,

A question about how much money you want to sped, and how good of cyclist you are...

As Mike says above, the battery is what really regulates the battery bus voltage (as long as the battery is large enough to accept the charging current--Typically around 10% to 20% of the Battery AH capacity... I.e. 100 AH battery * 10% = 10 Amps "optimum" charging current).

More or less, one way to regulate the battery state of charge is to simply stop pedaling when the battery is full (roughly 14.4 to 15.0 volts held for 1-6 hours -- Depending on the battery type/capacity/how deeply discharged).

Then there are charge controller options:

• When battery is full, the charge controller simply cuts off the current in a rapidly pulsing manner. Not great for regulating a generator--But may work. https://www.solar-electric.com/c40.html
• You can take the same C40 charger, and place it in "dump mode"... When the battery nears/reaches full charge, the controller "dumps" or shunts current to a load bank (resistors, a 100 Watt 12 volt filament lamp, etc.). This keeps constant current (on average) from your generator and keeps the pedaling force constant.

And there are MPPT and various other charge controllers/energy conversion devices... And depends on how much wattage you can output when pedaling, your motor specifications, how much you want to spend, etc.

More or less, your genset is probably rated at 10 amps from 0-30 or 0-60 volts (approximatly). The "torque" of the generator is roughly proportional to the amount of current being drawn from the generator (say 0-10 amps). Above 10 amps, the genset will probably overheat or simply not output much more current. Say you can pedal at 100 Watts for 30-60 minutes... The current output by the genset would be:

• Power = Voltage * Current
• I = P/V = 100 Watts / 14.0 volts charging = ~ 7.14 Amps

At this level, there is not much reason to use a DC Converter to "match" the motor output to charging the battery bank.

However, if you can pedal at 200 Watts with your motor:

• 200 Watts / 14 volts = 14.3 Amps

Or more current than the genset can output at 14 volts.

So a solution would be to use a MPPT (Maximum Power Point Tracking) solar charger or a DC to DC converter to take higher voltage/lower current form the genset and efficiently down convert to the voltage needed by the battery bank... For example:

• 200 Watts / 10 amps (maximum torque) = 20 Volts at the genset (and 10 amps output)
• 200 Watts / 5 amps (1/2 torque) = 40 Volts at the genset (and 5 amps output)

A DC to DC converter is a "buck mode switching power supply". And can down convert to battery charging voltage and help the genset output more power than a "simple" connection to the battery bank.

https://www.solar-electric.com/mosumpsochco.html

For MPPT type charger (internally, it is a "buck mode switching power supply"), the controller above should be a good fit to your application.

The other possible option is to use a DC to DC buck mode converter--Probably set for current regulation mode:

https://www.amazon.com/Converter-DROK-Voltage-Regulator-10V-75V/dp/B071LGTTRN

You could program it to both voltage regulate (14.4 volts for charging AGM batteries) and adjust the maximum current (to let you adjust the power harvest from the genset (pedal faster with lower torque, pedal slower with higher torque, etc.)... I am not sure if you would be happy with this unit, but at $30, it is "relatively cheap" to experiment with.

In the end, is the cost worth it? Assuming you can to 10 amps into 14 volt battery bank (140 Watts) for 1 hour (that is typically more than the "average" person can probably do... That is ~10 AH of current into your 400 AH battery bank or ~2.5% ... To even charge by 10%, you would need to pedal very hard for ~4 hours...

Are more solar panels or even a gasoline genset an option?

-Bill

mike95490

with 400ah of battery, and pedaling like mad, for 1 hour, 50watt hours @ 12V, would give about 4ah of charge, barely more than a trickle charge for 400ah of battery.  No risk of overcharging, unless you have 10 bikes and 100 bikers to rotate pedaling them.

The question is ,

how to pedal fast enough to have the motor be a generator of power, instead of being driven by the battery as a motor ?

706jim

You can charge the battery in theory. In practice I think you will tire of this quickly.

amybkant

Thank you all for your help. Mike I was going to use a diode to keep the voltage going in one direction. After getting on the bike and seeing It could get up to 48 volts without much effort I was concerned that the batteries would be damaged. then looking at regulation needed a controller that would accept up to 60 volts and not blow up. also not wanting to blow up a $200 dollar mppt controller so I will try the regulator and see what comes out the other end before I connect it to my batteries. Thank you BB for the information on the DROK-voltage regulator. I hope to replace this with a small wind generator  and  2 more 100 watt solar panels next year. 

Estragon

Just to clarify terms a bit... voltage is a difference, like pressure, not a flow.  Current (amps) is the flow.

If using the drok buck converter, I'd use a stable input source (eg 3 x 9v batteries wired in series =~18v) to adjust output voltage (~14.4v).

amybkant

I will be using a elliptical bike with a 20 inch rim going to a 1.5 inch pulley on the permanent magnet dc motor. I know it sounds crazy but this is how I educate myself trial and error. I have another question. Why would 4 100 amp hour agm batteries go down to 75 percent in 20 hours when I am only using 1 amp from and inverter and motorcycle trickle charger? 13.6 volt yesterday at 3pm and 12.5 volts at 12 pm next day. If this is a stupid question could you please answer it anyway? one cloudy day and I am dreaming up bicycle charging systems using discarded motors from my job. LOL

BB.

With "small" generators on a "large" battery bank (like you have)--You will never overvoltage the bank unless it is 100% full. You can just watch the bank with a voltmeter and make sure you don't go over 14.4 volts (for typical AGM). If you do, time to stop pedaling.

Regarding the drop in voltage--In theory:

• 20 AH / 400 AH capacity = 0.05 or 5% of capacity drain.

With AGM batteries, it is difficult to accurately estimate State of Charge... Hot vs Cold batteries, how long they have been resting (typically 3+ hours of no load/no discharge for "resting voltage") can all have an effect.

And since you cannot (easily) measure Specific Gravity, you are left with load testing and resting voltage.

And if the batteries are old and/or sat for a long time (>6 months) unused/uncharged, then there could be sulfation, with does make "big AH" batteries "smaller/lower capacity".

-Bill

amybkant

it is very easy to go over the 14.4 volts with the bike.  purchased the regulator and will experiment. the batteries were purchased 4 months ago. I have contacted the seller with this info. hope they have a good answer.

Estragon

It will be very easy to go over 14.4v with the bike, with no load (open circuit).  Once the bike generator is connected to the bank and gets over the battery voltage (eg 12.5v), the bank charging becomes a load.  Unless the bank is full, it should not be easy to increase voltage.   

As Bill noted, voltage isn't especially accurate in judging state of charge, but 12.5v could be ~85-90%.  At 90%, that's about 40ah x 12v = 480watt-hours.  You'd be in pretty good shape put out 100-150watts on the bike, and keep that up for ~3-4hrs to charge 480watt-hours.  

What size inverter is running the 1a load?  Is that [email protected] inverter output, or [email protected] inverter input?    

amybkant

So even if the bike is at 48 volts the battery would adjust to it until it is charged? I was going to use a inline fuse and diode with no charge control until I saw the 48 volts as soon as I started peddling. The inverter is a 1500 volt Energizer. I didn't need that much I know. It draws about .7 a and with my motorcycle trickle charger it goes up to about 2.5 and then settles back down to 1a. On a sunny day I am using some lights and other charging devices also. But it is raining so when the batteries go down to.12.5 I shut off the inverter. Thanks for all the good information.

Estragon

Yes, the battery should sort of anchor the voltage until it gets close to fully charged. 

When it gets close to fully charged, the battery takes less current.  With regulation, voltage would be held at ~14.4v by the regulator as load current drops off.  Without regulation, the voltage could continue to rise to potentially damaging levels, so you'll want to slow/stop pedalling if voltage rises above ~14.4.  It won't get as full that way (unregulated), but still pretty close.

mike95490

"Most pro cyclists produce about 200 to 300 watts on average during a four-hour tour stage. The recreational rider, on the other hand, might be only able to sustain this wattage during a 45-minute or hour-long spin class."

I believe that the risk of damage to 400ah of batteries is minimal. It's more likely the batteries are already damaged from chronic undercharge.

Estragon

I agree the risk of overcharge is small, but with unregulated charging it's still risk. 

Taking the low end of the 2-300w range, that's still ~14a (7% of 400ah Capacity) vs a ~1% of C regulated absorb end amps.  Enough current, I think, to raise unregulated voltage too high?  If the bank is sulfated from undercharging, the effective capacity could be significant smaller, increasing the 7% current number.

amybkant

the charging from the bike is only a little project that I am enjoying very much. I am learning from people like you all who are generous with their time and knowledge. the batteries are being charged with 2 100 watt solar panels and someday a couple more of the same with a small wind generator but will have to save up for that.

amybkant

amybkant

some things i have learned from this project. the batteries are definitely the only regulator I needed. I am not physically capable of making this charge my battery bank in the amount of time I have to spend in my shed. if I could spin this motor a 1725 rpm i would get closer. I will look for a low rpm motor and keep at it.

amybkant

amybkant

was 12.74 before I started pedaling this took about 3 minutes

softdown

I remember the difficulty of powering the light bulb on my ten speed and I was a strong biker. But you can't go wrong with biking. 

flightrider

Interesting discussion. I am testing two configurations. One self-starting car alternator with a built-in regulator, the second with a 350W DC motor-generator. I am using a cycling powermeter to determine mechanical input power as well as a volt-amp-power display, similar to Amy's to measure output and charging parameters. I am using a PWM controller with a blocking diode in the motor supply line (extra precaution) and a capacitor in the motor output line to smooth out the input to the controller. I have a more reasonable 70Ahr battery. The car alternator turned out to be too big and inefficient. I can cycle over 200 watts for over an hour (64 yr. old cyclist and triathlete) but the output was in the 25-45 watt range. I am waiting for additional equipment before trying the DC motor. I have developed a cadence - motor RPM table to calculate the gearing and pulley ratios so I can tell what motor RPMs are for my target cadence range of 80-100 for each of 20 gears. Doing this as a hobby project and for use in school STEM projects.