Introduction: Solar Bike DIY 5V
Hi, today we will make a lightweigh and completely sun powered solar bike, of course it will not be so much powerful as common bikes, often you have to pedal but you don't need to charge or move heavy batteries or motors.
First thing you need is an usb 10W flexible solar charger from Aliexpress (About 20 Dollars 2017), this have about 21% cell efficiency, (in future you can upgrade with a more powerful solar panel).
For saving space you have to cut some borders, i used a hot iron, be very careful, use glasses and do this outside.
Don't smell the smoke, pay attention because you could also damage the solar panel.
Make some holes in passive areas for fixing the solar panel on the bike it with some zip ties.
You have to remove the charging circuit box, it consumes about 7mA and for now you don't need it.
The panel gives about 2A of current at about 5,6V so it's about 10W and it's ok.
You need a cable, you can find it in old broken electronics devices.
Now you have to solder wires and fix with some silicon, do this outside, use glasses and do not smell it, let it dry for some days.
Now you need a low size, low weight, low power and low kv outrunner brushless motor, this is about 760kv and 200W and weighs less thann 100 grams, you can find it in rc model shop or online like hobbyking.com.
kv means turns/(min x Volt), it's very important to choose the right motor, it's depends on your usually travel speed.
Low kv gives you a high torque at lower rpm but can slow you when pedaling, high kv gives you low torque at high rpm but can overheating the motor.
An other important parameter is the out motor diameter, it should be small for having a good torque from the gear ratio wheel/motor, in this case with a 28" (690mm) and 28mm is about 22.
The revolutions of the wheel are for now (760r / (minV)) x4V / (60s x22) = 2.3 r/s (revolution per second).
About 22 of the ratio between 670mm of the wheel and 29mm of the engine, this means that the wheel will have 22 times less the motor's speed but 22 times more the motor's torque.
With an outer wheel diameter of about 69cm we have a theoric about 2.3r/s x0.69m x3.14 = 5m/s x3.6 = 18km/h, this in theory, at load will be a little less.
To fix the motor on your bike you can use the dinamo support, some 3d printed parts, Inox nuts and bolts.
You can find the files here, you can use Sketchup Make 2017, it's free for makers.
You should get something like this, use a rubber band to keep the motor in pressure to the wheel.
It's very important to fix the motor direct to the wheel, if it is askew, it will loose efficiency and could become hot.
Use a thin and big diameter bike's rubber air chamber to maximize the friction between the motor and the wheel.
The dirt will not reach the motor in this configuration.
You could use a zip tie to disconnect the motor for any problems but the motor braking is very low, you can set the type of brake in the esc if you want, I use none.
You need a brushless 40A skywalker ESC and you have to fill the holes with hot glue for Watertight because the rain will damage the electronics immediately.
You have to set Ni-Mh instead of Lipo, and disable the cutoff, so you can use it at extremely low voltage (from 3V)
It's true this solar bike does not use batteries, but shadows could stop or reset the ESC, so for sureness and for giving a bit more power, we need two 100F 2.7V supercapacitors, from Aliexpress.
Remeber to protect the pins with some hot glue.
Supercapacitors have a lot of discarge current and cicle lifes than Lipo batteries.
Remeber that two similar capacitor in series have half capacity than a single one, so don't exaggerate with the series.
The two supercapacitors have a similar capacity but not exaclty the same, so when charged in series, one could charge more than the other and can explode, so you need a protection balancing module circuit, unlukly in my case, the total voltage from those modules reached only 5,3V, not enough for the ESC.
The ESC needs 5,4V to start, so we have to hack a bit those modules.
We have to find two resistors with high values, in my case 12k and 178k, these control the threshold voltage for shunt the current when the charge is complete.
Now you should solder in parallel a high value resistor, in my case 500k (I used 2x1M each module), for get some additional millivolt, if you see that the voltage decrease after that, you have choosed the wrong resistor.
Don't exaggerate with this, because you can exceed the nominal tollerance value.
I used an Lemonhobby wattmeter to check the power, remember to cover the display from the sun, otherwise some pixels can become black.
When you solder cables, use glasses, because some tin bubbles can come out easily.
The sensing resistor is between negative wattmeter's cables, so there is no need to supply a big cable for the positive wattmeter terminal, source is the input, load the output.
You can use a switch NC for program or reset the ESC if there are problems, be sure the nominal current will be enough.
At first I used a servo tester to control the speed, but It is not comfortable driving with a potentiometer, so I used an Atmega328p or called Arduino stand alone, with 4 buttons, each button set a speed: 0W, 2W, 8W, 22W.
The ESC is like a servo motor except you have to control it from 1000 to 2000µs.
Here's the code:
You can protect from rain the microcontroller with a plastic box or you can 3d print it.
The esc once exceeded the 5.4V does not emits more sounds and will allows the engine to run, the engine will also runs under 5.4V, up to 3V approximately if voltage drops below 3V then even if after the voltage rises it will start to emits beeps until the voltage will exceeds 5,4V and so on.
The only thing you have to do is to check the wattmeter sometimes, for not going under 3V but when the voltage drops the ESC output power will be automatically reduced, if you want you can set a Ni-Mh cutoff in the ESC.
If you like playing with the code, you could add an assisted pedaling, you will need a reed switch and a little magnet.
You could also add more capacitors for having more energy and checking less times the voltage, is there also a plug for programming Arduino.
The guide is a bit strange, initially you have to charge the supercondensors, then you can use the buttons and set the power, depending on how much sun is there and how much help you want.
Once a speed has been set, the motor like a generator generate a voltage, this is used as signal feedback to be sent to ESC, in this case a sensorless feedback position.
Not always the engine delivers the power set, sometimes less, for example if you start to ride slowly and then you incease the pedal's speed, you will see the motor's power output decrease as it approaches the set speed, this is most noticeable at maximum power.
This is because like a generator, a voltage is generated in the engine, and oppese the voltage produced by the ESC some sort of balance, there will be only the friction losses to be fed.
The solar panel is 5V to avoid problems with the shading of cells in series, as well as having fewer cells and more surface, there is less chance that a cell is completely shaded, and in case of shading the voltage is quite low so does not cause shock to the panel.
The motor should be about 80% efficiency, leaks in cables at 22W are about 1%, the wattmeter cables should be long enough to avoid breaking the wires and turning the handlebar not so hard, but at the same time not too long to avoid leakage.
A person is only efficient from 5 to 20% and to reach those powers should burn about 200W of power contained in the food, the solar panel consumes part of the sun's energy so the more you use this bike, the more you cool the planet and not chemical pollution is generated or greenhouse gasses like CO2 and water.
I hope you like it, happy pedaling and happy sun.
More info at: http://electrogreen.altervista.org/bici-solare-2/