Bicycle Power Basics

Download a PDF of this guide here

Electricity Basics

We’re not going to dig too deeply into definitions and equations here but to get to grips with the nature of electricity, we need to learn a little bit about what an atom is. All elements, such as copper and gold, are made up of atoms. Atoms in turn are made up of particles: protons (p), neutrons and electrons (e).

Protons & Electrons

In the simplest terms electrons are the smallest units of electricity. Protons & electrons have equal and opposite charges; electrons being negative and protons being positive. Like charges repel each other, unlike charges attract each other.  The visual representation of the attraction between protons & electrons is similar to that of magnets clicking together when we hold the opposite poles toward each other. While protons are fairly static, electrons like to move about. A good example of this is lightning which is when electrons travel between the clouds and the ground.

Moving the Electron

We’ve learned that electrons like to get about, but in order for them to be able to move from one place to another, they need a medium and this is where conductors come in. Just like we use bikes to get from A to B, electrons use electrical conductors. Conductors, such as copper and aluminium wire, allow electrons to move about within their structures when a charge difference is created across two parts of the material. If there is no difference between two points in the circuit then there will be no current.

Charge (Q), measured in coulombs, is the number of electrons that can be found in whatever metal we’re dealing with. The flow rate (just like you’d find in a river) of this charge is what we call electrical current (I), which is measured in amperes or amps i.e. 1 amp exists in a circuit when one coulomb of charge is passing a point every second.

Voltage or Potential

In the earlier section we mentioned charge difference, meaning a force that will push electrons from one place to another. This force is called voltage and it is measured in volts (V). If we use a water system as an analogy, then if we increase the pressure in the system then more water will flow through our pipe per minute. The same thing happens in our electrical circuit, the higher the voltage then the greater the current in the wires.

The Essential Mix

Once we combine electrons, conductor and voltage, we can create an electric current in a form the we can actually use. Electricity is created when voltage pushes an electric current through a conductor. Both conductors and voltage have an effect on how current flows through a wire:

-   increasing voltage increases the number of electrons flowing through a wire at any given time, leading to a greater current. If we think about our water system again then by increasing the water pressure, you increase the amount of water flowing through your pipe.

-   increasing the diameter of our conductor allows more electrons to flow through for a given voltage, therefore producing greater electric current. This would be similar to using bigger pipes in our water system, so that we can use the same pressure but still get more water flowing through the system.

Power

Now we have electricity, we can use it to power devices to produce sound, light, motion etc. For example, applying electrical current to a DC motor will cause its shaft to spin. Power basically measures the amount of work that an electric current does when running through a device or appliance. E.g. work needed to heat a filament in a light bulb is 60 watts. Power is measured in Watts (W). One watt is equal to one joule of energy per second (joule meaning unit of work or energy). So if one joule of energy is expended in one second then the power is one watt.

The electrical power of a device is the product of voltage across it multiplied by the current through it:

POWER = VOLTAGE X CURRENT

W = V X A


Resistance

Resistance is the opposition to a current and is measured in ohms. Resistance always results in the conversion of electrical energy into heat. All conductors, have some kind of resistance, and represents a waste of energy or a loss to the system. Resistance in a wire depends upon the type of metal, length of the wire and the diameter of the wire.


Currents that you may come across

-   DC (Direct current) is a simpler type of electricity and is unidirectional flow of electric charge. In simplified terms in a DC circuit, the current flows at a specific, constant voltage. This type of electricity can be produced by batteries, solar cells and dynamo type machine such as bike power generator. When we’re using a torch, pocket radio, portable CD player or virtually any other type of portable or battery-powered device, we’re using direct current.

-   AC Alternating current) is type of current whose direction reverses in cycles. The back-and-forth motion occurs between 50 and 60 times per second, depending on the electrical system of the country. Due to this cyclical motion, the voltage in AC current alternates. AC is the type of current that is delivered to our homes and businesses and we use it to power all of our appliances.

Know your revolutions

Revolutions per minute (rpm, RPM, r/min, or r·min−1) is a unit of frequency. It simply means the number of full rotations completed in one minute around a fixed axis. It is most commonly used as a measure of rotational speed of some mechanical component, in our case the bicycle wheel.

For more information check out these books:

Off The Grid, Managing independent renewable electrical systems Duncan Kerridge,

CAT Publications, ISBN: 978-1-902175-56-0

Advanced Electrical Installations – C Shelton, Addison Welsey Longman, ISBN: 0-582-24618-0

Electronics for Dummies – Gordon McComb & Earl Boysen, Wiley Publishing, ISBN: 0-7645-7660-7

The 12 Volt Bible For Boats – Miner Brotherton, McGraw-Hill, ISBN: 0-07-139233-5

SYSTEM DESIGN

Designing any off-grid system is a question of matching up your supply of energy with the amount of power you will need. A single bicycle generator’s capacity for energy production is dependent on the power produced by the cyclist and the condition of the bicycle itself. There are many ways to put together a bike generator, from cheap to super expensive, from cheap to more complicated. All of them have pros and cons and so the choice is really up to you. The generator itself is not the whole story so we have to look at the bigger picture.

The areas we need to take into consideration when we are building our system are:

  • Assessing our loads and the potential for reducing these where possible
  • Storing power with a battery
  • AC or DC or both
  • System control
  • Distribution of power
  • Safety precautions as electricity can be dangerous
  • Deciding upon your budget and thinking about a system that you can afford

In this workshop we’ll follow the above sections in more detail with some examples of different generator setups at the end. Below is a simple diagram to help guide you through the process:

1. INITIAL ASSESMENT

  • Existing Situation
  • What you need to and want to power
  • Resources available to you
  • Your budget

2. LOAD ASSESMENT

  • Appliances
  • How we’re going to use them
  • Priorities
  • Load reduction

3. SYSTEM DESIGN

  • Stationary/portable
  • Distribution
  • Regulation
  • Ac/DC
  • Storage
  • Monitoring
  • Safety

4. COSTING

5. SYSTEM


Here is a complete list of the components we use to build a single bicycle generator. You may not necessarily need all of the items listed below (depending on your budget or requirements) but it’s good to get an idea of all the bits and pieces that can go into the system.

  • Bicycle (ideally with slick tyres)
  • Bicycle training stand
  • DC permanent magnet motor
  • Plate or wood for mounting motor
  • Skateboard wheel or roller
  • Multimeter
  • Cable & connectors (30 amp Anderson connectors or mains 3 way connectors)
  • Screws, nuts and bolts
  • Regulator
  • Inverter
  • Capacitor
  • Heat sink
  • 12V connectors x 4
  • 12V sockets
  • Fuses & fuse holders
  • RCD
  • Earthing system
  • Battery
  • System housing e.g. wood box
  • Tools e.g.
  • Crimp tool
  • Soldering iron & solder
  • Terminal blocks
  • Electrical tape
  • Electrical screwdriver
  • Spanner
  • Needle nose pliers
  • Wire cutters and strippers
  • Allen keys
  • Plug in power meter
  • Diode (zenner)
  • Battery Charger
  • Dump Loads

Assessing our loads and the potential for reducing these where possible

Load usually refers to the equipment that we need and wish to power with our bicycle generator. A good first step would be to find out just how much power i.e. its power rating (our load) the different appliances require to do their job. Most appliances have stickers on the bottom of them that tell us either how many watts, volts and amps they use. We can use volts and amps to calculate watts via using the equation W = I x V . Loads are usually measured in watt-hours e.g. a 30 watt DVD player consumes 30 watts every hour it is turned on.

However, it is best to measure appliance consumption with a plug-in power meter (see parts description for further details) as that will give us the most precise information. This is especially important when we’re thinking about powering sound equipment, as the equipment specifications don’t reflect its actual consumption. You may find that sound equipment often runs at much lower wattage with higher peaks, depending upon the levels that the equipment is set to. For example, a sticker on the amp may read 200W but the amp may only use 30W at the level you require it at. On the other hand some loads may work on three times their operating power for short periods when turned on. All of this is information is rather important if we don’t want to damage our equipment and blow fuses.

Since powering loads with a bicycle can often be lot of effort and many loads take more than one bike to power, we need to think about ways that we can reduce our loads if possible. For example, many people would like to power a projector but it’s not possible with just one bicycle as most consumer projectors require somewhere between 150W to 350W. One way to deal with this situation is to compromise the size of our screen/image quality and use an LED projector that only requires 30W.

Many loads are not possible to power with a bicycle generator as they require too much energy, these are mainly heating loads e.g. AC kettles require about 3000W, DC kettle about 200W.

DC appliances are lot more efficient as you avoid losses due to inversion of electricity from DC to AC. However, DC appliances are more expensive and harder to find. However this may be outweighed by the savings on not having to buy an inverter and fuses and RCDs and is also much safer than AC, which works at 240V.

If we’re planning to plug in more than one appliance at a time into our system we need to be thinking about out total peak consumption, i.e. adding up all the watts, to make sure that our load fits within the capacity of our system.

Storing power with a battery and supplementing bike power

Batteries come into play when we wish to store power for using later or when we’re supplementing bike power with some battery power (hopefully charged up by energy from renewable sources). Bear in mind that when planning to use batteries that they are hazardous, expensive and represent a big cost in the system, as they need replacing more often than other components. Therefore, they have to be well looked after to assure that we make them last as long as possible. Badly treated batteries will work inefficiently and the electricity we’ve been sweating for will be wasted.

Batteries can only store DC electricity and come in 2, 6, 12, and 24V types. The most commonly used batteries are lead acid batteries. For smaller system designs you can use lithium ion battery, which are smaller and lighter, and hold a charge much longer than other types of batteries. They are less environmentally unfriendly and much lighter but still a bit more expensive.

The capacity of batteries is worked out in amp-hours (Ah). A 70 Amp-hour battery can supply 1 amp for 70 hours or 70 amps for 1 hour.  Batteries can be collected in parallel (+ to + and – to -) to increase amp-hours but keep the voltage the same. This is a bit advanced and should only be done with batteries of the same type and size. To work out how much power we can store in a battery we can use our old equation W = V x I

70Ah x 12V = 840Wh  => This means we can use a 70W juicer for 12 hours or 100W item for 8.4 hours.

When supplementing bike power with battery, it can provide extra energy and allow for larger loads. When using a battery, make sure that the battery has the opportunity to fully charge between loads. Deep discharges should be avoided if possible. When charging a battery you want to have your voltage output between 13.8V and 14.4V.

We have to remember that most batteries cannot withstand being completely discharged. Depending on the battery they should only be discharged to 50% to 80% of their capacity. If you have a battery that is 70 Amp-hours than you really need to see that available power as 50% (leisure battery) to 80% (deep cycle battery) of 70 depending upon the battery. The best batteries to use with bike power are deep cycle batteries as they are designed to regularly discharged between 30% to 80% of their capacity.  Well-treated deep cycle batteries will last much longer than a leisure battery. When buying a new battery, the more expensive it is the better the quality.

Car batteries are not suitable to use with bike power, as they were made for short bursts of high current. They never get discharged much in the short time it takes to start a car and they are recharged immediately. Therefore deeper discharge of car batteries damages them.

‘On sealed-top batteries, the state of charge can be determined by checking the battery’s base or open circuit voltage with a digital voltmeter or multimeter. This is done by touching the meter leads to the positive and negative battery terminals.  A reading of 12.66 volts indicates a fully charged battery; 12.45 volts is 75% charged, 12.24 volts is 50% charged, and 12.06 volts is 25% charged.’

AC or DC or both

We mentioned DC power in an earlier paragraph about load assessment. If you can use DC, do it. It is safer and you’ll get more out of the cyclist and get more sound if you thinking of building a sound system. In the case of sound systems, the wiring would be same as you would use in car amplifier and speakers setups. For other appliances, DC plugs and sockets are the same as the you’d find for a car cigarette lighter, in caravans and on boats.

Regular household appliances run on 230 volts AC at frequency of 50 Hz. An inverter converts low voltage DC electricity to mains power AC. The size of inverter ranges greatly from 20W to 10KW and more. If you are wishing to use AC mains powered appliances you will need to purchase an inverter. Inverters are designed to work with batteries so they usually work with voltages between 9 and 14 volts.

There are inverters with different types of waveform, which essentially try to mimic the mains supply.  A pure sine wave inverter creates a smooth AC output, which is indistinguishable from mains.

There are also square wave and modified wave inverters but some devices such as motors may not work with these inverters. However, they are generally the cheaper option.

Different loads will have different tolerances to inverter characteristic and different inverters will have different tolerances to load characteristics. This is important to consider when buying an inverter e.g. some inverters can’t deal well with peaks in sound equipment. Also, all inverters work inefficiently at low loads.

The things to look out for in inverter specifications (example at the back of the hand out):

  • Power output
  • Input voltage
  • Waveform
  • Efficiency
  • Standby
  • Surge ability
  • Input Voltage
  • Load sensing
  • Interference suppression
  • Protection mechanism
  • Cost
  • Recommendations

System Control

Without control mechanisms our electricity would be all over the place. Controls make sure we don’t blow things, set them on fire and hurt ourselves and/or other people. Controls also get rid of fluctuations within our system. The voltage that person produces on a bicycle is variable and can be as high as 70V and that has to be kept in check by regulations.

The sorts of controls that we need to know about with bike power are:

  • Reverse input current blocking can be achieved with a diode. Our DC motor is a two-way device, something must be in place to keep the current from going back the way it came, causing the motor to act like a motor and not like a generator (a motor performs the opposite job of a generator when it takes in current and converts it into mechanical energy). The diode will go between the generator and the battery or regulator/converter, with only the fuse between it and the battery/regulator/converter. A diode has both a positive and negative terminal. Make sure you connect positive-to-positive and negative-to-negative or the device will not work. Look at the information that came with your diode to determine which terminal is which, and if you are still having trouble, ask the supplier. 

  • Voltage regulation can be achieved with:

A. Step down buck converter. Converters are generally used in communications technology and their use in bike power is novel but brings some disadvantages.  Buck (or step down) means that the converter can only produce a voltage lower than that supplied to it.  So, if you require an output voltage of 12V then the converter will need to be supplied with a voltage constantly equal to or greater than 12V in order to work.  The voltage created by a permanent magnet (PM) motor is directly proportional to the speed that the motor is rotating (RPM).  The scooter motors we use in our system are designed to provide a 24V output at 2850RPM, so in order to create a voltage constantly higher than 12V the motor must rotate at a speed of at least 1425 RPM.  How fast the motor rotates depend on:

  1. The speed the cyclist is spinning his or her legs (faster = greater motor RMP)
  2. The size of the rear wheel of the bike you are using (larger = greater motor RPM)
  3. What gear the bike is in (downhill gears = greater motor RMP)

The disadvantage of the converter is that with the motors we currently use the voltage can sometimes get a little bit too high if a person jumps on a bike and spins. If not careful this can blow the board. This can be overcompensated with larger roller or smaller motor. The converter also requires a heat sink attachment to dump excess power as heat. Running the converter at high loads without a heat sink can blow the board.

B. Capacitor. A capacitor can be used to smooth the output voltage from the motor and has the added benefit of providing a reservoir of energy for bass notes in sound systems for the amplifier to use on demand. As a rough estimate every 400W of sound requires 0.5 farad of capacitance.

There are a couple of disadvantages in using a capacitor. The capacitor doesn’t supply a constant output voltage. Once charged it corresponds to the voltage output from the motor, which (as discussed above) is relative to its RPM.  If you are using an inverter (designed to work between 9 and 14V) or sensitive 12V equipment you will need to keep an eye on the voltage across the capacitor and adjust your pedalling speed or the gear you are in to keep the voltage steady.

Capacitors can be damaged or may cause damage to those nearby if not treated with respect.  The voltage rating of any capacitor will be written on it.  Once the capacitor is full of charge, if a voltage higher than its rating is supplied to it can damage the capacitor. Some capacitors come with their own built in voltmeter, making monitoring of voltage easier.

C. Battery Charger. This is a slightly more advanced system that still requires the use of a capacitor in combination with a battery charger charge in diversion mode setting. The capacitor is usually wired after the bikes followed by the battery charger. Every battery charger is different and has different specifications that have to be followed when deciding about how it will work in the system. This option also requires the use of dump loads such as metal coils that heat up in the case of excess power is produced. Using battery chargers can be expensive.

  • Fuses. If there is a surge in the electrical system that causes too much current to pass through the fuse, it pops, which creates disconnects the circuit (known as an open circuit). This break prevents current from continuing its path and keeps the device from working. It also keeps the devices from being destroyed by too much current. For the price of a fuse, we can protect our equipment. When we use batteries, 25 amp fuses are needed to protect our wiring should our system short circuit. It should be placed immediately adjacent to the positive terminal of the battery.

  • Earth leakage trip detects the difference between current leaving and the current returning. If this becomes bigger than 30mA (the maximum safe level) it switches of the power. Earth leakage trips are RCBOs, RCDs, RCCBs and ELCBs. Read the additional pages about RCDs that are part of the hand out.  

  • Earthing. Earthing creates a link between our system and the ground. It is standard practice to earth the battery negative and the negative of the inverter. Inverters may require different earthling. Always follow the inverter manual guidance on earthling! Earthling assures no current can flow through our bodies in case we touch a live part of our system, as current will want to run to the ground. In most cases we’ll require a galvanised rod of min length of 1.5m that we can run into the ground. We need to connect the earth rod to our battery and appliance negative using a copper conductor, with our conductor being at least as thick as the largest one in our circuit.

Distribution of power

Cables for our system are generally chosen on the basis of current-carrying capacity. Cables need to be able to carry the required current without overheating.

DC cables will need to be much thicker as the current in a DC system is much higher than 240V AC. Cables also need to be thicker because of voltage drop. Voltage drop is the loss of power as the current runs down the cable. This is due to the resistance in the copper. The loss of power occurs as heat. Voltage drops will be worse for cable stretching long distances. Voltage drop can also possibly damage appliance that are sensitive to the voltage they receive. The most efficient systems will have the smallest voltage drop. The formula for working out suitable cables is:

Volt drop (V) = 0.04 x cable length (m) x current (amps) / cable cross sectional area (mm2)

It’s also possible to knock out 25A on a bike for bursts, so our cable should be able to handle those sorts of currents too!

Please take safety precautions as electricity can be dangerous

Please take all safety precautions mentioned in this hand out such as earths, fuses, RCDs, and more! Read the guidance on working with batteries and be aware that the power coming out of an inverter is 240V.

This is not to scare you, but electricity when handled wrong can be deadly. Even though a 12V system doesn’t pose an electrocution hazard, there are still other hazards such as electrical fires from too much current. Keep a fire blanket and a small extinguisher near by if possible. If you’re not sure of anything ask an electrician. If you intend to take you generator to a public event get a certificate from a qualified electrician, which will assure that your installation is safe!

Deciding upon your budget and thinking about a system that you can afford

In the simplest terms you will need a person, a bike, a way of supporting your bike off the ground, a motor, an energy smoothing system and an inverter if you want to use your bike generator to power mains equipment.

Cost is an issue for many people. When we first stated generating electricity with bikes we were on a really tight budget. As opposed to buying stands that would cost us £50 with a bit of welding we made our own bicycle stands that came to about £15 per stand. Instead of using an aluminium roller we used old skateboard wheels.

There are many ways to do things and it’s all about being resourceful. You can find motors, from old washing machines, car windshield wipers as long as they are permanent magnet 12V DC motors and within you parameter you should be fine.

You plate for mounting could be old bit of wood that you may find laying around. Stand can be put together from old discarded bikes. Many, many, many ways…………

Suppliers of parts

Bicycle (ideally with slick tyres)

Local bicycle store

Gumtree – www.gumtree.co.uk
Ebay – www.ebay.co.uk

Bicycle training stand

Cycle Store – www.cyclestore.co.uk
Cycle Sports UK – www.cyclesportsuk.co.uk
On Your Bike – www.onyourbike.co.uk
Minoura – www.minoura.co.jp

DC permanent magnet motor

Ebay – www.ebay.co.uk – Part no. MY1016
Conrad Electronic – www1.conrad-uk.com
Science Shareware – http://www.scienceshareware.com/
Campaign For Real Events – http://www.c-realevents.demon.co.uk/

Plate or wood for mounting motor

Timber merchants
Steel stockists
DIY store

Skateboard wheel or roller

Skate shop
Ebay – www.ebay.co.uk

Multimeter

Maplin – www.maplin.co.uk
Screwfix – www.screwfix.com
DIY store

Regulator

RS – uk.rs-online.com
Maplin – www.maplin.co.uk

DC to DC converter

part 445-9787 from RS or part L22BR from Maplin.

Inverter

Maplin – www.maplin.co.uk
Outdoor GB – www.outdoorgb.com

Capacitor

Car audio store
Passion Auto – www.passionauto.co.uk
Halfords – www.halfords.com

Tristar Battery Charger

Mysolarshop.co.uk – www.mysolarshop.co.uk
Wind and Sun – www.windandsun.co.uk

Dump Load Coils

Mysolarshop.co.uk – www.mysolarshop.co.uk
Kaieter – www.kaieteur.uk.com

Heatsink

RS – uk.rs-online.com

12V Connectors x 4

RS – uk.rs-online.com

Outdoor GB – www.outdoorgb.com
12V Shop – www.the12voltshop.co.uk

12V Sockets

RS – uk.rs-online.com
Outdoor GB – www.outdoorgb.com
12V Shop – www.the12voltshop.co.uk

Fuses & Fuseholders

Maplin – www.maplin.co.uk
Halfords – www.halfords.com
Outdoor GB – www.outdoorgb.com

RCD

Wickes – www.wickes.co.uk
RS – uk.rs-online.com

Earthing System

Canford – www.canford.co.uk
Screwfix – www.screwfix.com

Battery

Halfords – www.halfords.com
Outdoor GB – www.outdoorgb.com

System housing e.g. wooden box

DIY store
RS – uk.rs-online.com

Cable & connectors

Maplin – www.maplin.co.uk
RS – uk.rs-online.com

Screws, nuts and bolts

DIY store

Tools e.g.

• Crimp tool

• Soldering iron & solder

• Terminal blocks

• Electrical tape

• Electrical screwdriver

• Spanner

• Needle nose pliers

• Wirecutters and strippers

• Allen keys

• Plug in power meter

For more information check out these books:

Off The Grid, Managing independent renewable electrical systems Duncan Kerridge, CAT Publications, ISBN: 978-1-902175-56-0

Advanced Electrical Installations – C Shelton, Addison Welsey Longman, ISBN: 0-582-24618-0

Electronics for Dummies – Gordon McComb & Earl Boysen, Wiley Publishing, ISBN: 0-7645-7660-7

The 12 Volt Bible For Boats – Miner Brotherton, McGraw-Hill, ISBN: 0-07-139233-5

Page last updated on February 16, 2011 at 3:07 pm
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