Spooky Tooth Cycles Essential Guide to Understanding Electric Bikes

When speaking of a battery or battery pack we use the word nominal (in name only) to refer to the voltage, "a nominal 12 volt battery", or, "a nominal 36 volt battery pack."

The reason for this is because a 12 volt battery is rarely ever 12.00 volts. Most batteries have a working range of 11 volts as a low and 14 volts as a peak. If a battery goes below or above these numbers, permanent damage will occur.

The working range of a battery is specific to each battery type and important to know for proper battery maintenance. Remember back to if or when you accidentally left your car lights on over night. Of course you can jump start a car and bring the battery voltage back up from 0 volts, but your car battery will never respond the same. Its life span has been compromised so it becomes harder and harder for that battery to retain a full charge of 13.9 volts. All electric bike controllers have low voltage shutoffs to prevent damage. On the other side, if you over charge, a battery could either swell and or explode. Battery chargers have come a long way and this is not a concern any more.

Charging a 12 volt battery occurs when a source of higher voltage comes in contact with a supply of lower voltage. Think of charging as the syphoning of gasoline from a car's gas tank. The higher supply flows into the lower supply. Sealed Lead Acid (SLA) batteries are very straightforward and easy to work with because they will accept measured voltages easily, i.e. solar panels, alternators, generators, other batteries. A Nickel Metal Hydride (Ni-MH) battery charges using a complex Y-Delta algorithm and only accepts input from a "smart-charger". Lithium Ion (Li-Ion) batteries also use "smart chargers" but on a different algorithm, and occasionally Li-Ion chargers will charge each cell of the pack individually.

In the electric bike world, we have 2 ways of defining battery size, by voltage and amp hours. Look on the side of an electric bicycle battery and you may see something like this: SLA 12v12ah. This means that the battery is a Sealed Lead Acid rated at 12 volts and 12 amp hours. Voltage is frequency and can be roughly thought of as speed. Generally when more voltage is applied to a motor, speed increases. It is possible to take a 24 volt motor, add a 3rd battery to the pack and increase the speed of an electric bicycle by a somewhat proportional amount.

Important side note for the hot-rodders out there. Increasing voltage beyond a motor's specific rating is fun, but voids all warranties and can either harm the motor, controller, or wiring harness. Later, I talk about how to do this safely, so keep reading...

Our other important measurement for battery size is amp hours. An amp is a unit of measurement for electrical power. Amp hours refer to how many amps the battery will release per hour. A common 12 volt 10 amp hour electric bike battery will release 10 amps at 12 volts for 1 hour, or 1 amp at 12 volts for 10 hours. A rough translation is that voltage can be thought of as speed, and amp hours as range. The full definition for power output is not complete, however, until watts are brought into the conversation. We will look at that shortly.

Cold cranking amps are a different measurement than amp hours. Automotive starter motors need a lot of amps to start a car or truck. Cold cranking amps refer to how many amps a battery will put out at 0 degrees Fahrenheit. A 12 volt deep cycle battery has the benefit of being able to draw power from below the common threshold of 11 volts and be able to return normally. Electric bikes do not use deep cycle batteries because, again, low voltage cutoffs are in place to keep battery voltages from going below 11 volts per battery and therefore are overkill for our purposes.

The Battery Pack

To create a battery pack we can either run the batteries in parallel or in series. A parallel circuit is when you connect positive terminals to positive terminals and negative terminals to negative terminals; just like when you are jumping a car's battery. What this accomplishes is the retention of voltage, but gives an increase to the amount of amp hours. A battery pack run in series means that the positive terminals are connected to negative terminals in such a way that voltage is increased while amp hours remain the same. Let's look at what happens when we use 3 batteries each with a 12 volt 10 amp hour rating. Ran in parallel, the like terminals are lined up and we come out with a 12 volt pack with 30 amp hours (3 x 10 amp hours = 30 amp hours). Ran in series, we would use the positive of battery 1 as the main positive, the negative of battery 3 as the main negative, and then connect battery 2 positive to battery 1 negative and battery 2 negative to battery 3 positive. The result is a 36 volt pack with 10 amp hours (3 x 12 volts = 36 volts).

If you are assembling a battery pack yourself, you will need to take a few steps for safety. Soldering your connections is best. Use male-female connectors and heat shrink when you can. Never touch the main positive directly to the main negative of any battery or pack. Buy a cheap voltmeter and continually check your connections for positive/negative flow ("DC Volts") or continuity ("Ohms"). Touching low voltage battery terminals won't kill you. But, at 72 volts and above your skin is conductive and you will get shocked. Try your best to hook up like batteries. You will want to match like amp hours and like voltages. For instance, a 12 volt 10 amp hour battery and a 12 volt 7 amp hour battery ran together in series or parallel will destroy the lesser size battery. Likewise, match battery voltages so that you do not have 1 low voltage battery at 11 volts tied to a pack with batteries in the 13 volt range. You could cook the higher voltage batteries. Plan the operating range for your battery pack and find a charger and controller to fit. If you are working with a 48 volt battery pack, then your working range will be from a low of 44 volts (4 batteries x 11 volts = 44 volts) to a high of 56 volts (4 batteries x 14 volts = 56 volts). Your charger then should put out no more than 56 volts (yes, you can measure a charger with your new voltmeter's leads), and the controller should shutoff at 44 volts. If you need to, you can purchase a Watts-Up Meter to run inline on the battery to controller lead which will tell you instantly your amperage draw, voltage, and many other things.

Watts are the true definition of a motor's output. For instance, a common configuration for Izip electric bikes is a 24 volt 10 amp hour battery pack with a motor rated at 450 watts. One horsepower equals 750 watts. Therefore, 450 watts is almost 2/3rd of a horsepower. Side note - horsepower ratings for electric motors are never that accurate because they do not take into account the amount of torque (turning or off the line power) for specific motors. Brushed motors have a ton of torque compared to brush less motors which have lower rolling resistance and better top speed. The Izip motor only has 450 watts of power, however, most of that power is located at the lower rpm ranges to provide excellent hill climbing.

The consideration of watts in relation to volts and amps is important for many reasons, the most of all range. Here is a good formula for you to memorize: AMPS = WATTS/VOLTS, or WATTS = VOLTS x AMPS, or VOLTS = WATTS/ AMPS.

Let's say we want to compare 2 electric bikes, an Izip Mountain Trailz with a 24 volt 10 amp hour pack, 450 watt motor and a top speed of 16 mph, and a do-it-yourself (D.I.Y.) brush less electric front hub motor kit with a 48 volt 10 amp hour pack, 1,000 watt motor, and a top speed of 30 mph. Using the above formula for amps, the Izip uses 18.75 amps at full throttle (450 watts/24 volts = 18.75 amps). Given the battery size at 10 amp hours with a full power draw of 18.75 amps, the Izip will travel at 16 mph for 32 minutes ((10 amp hours/18.75 amps = .53) x 60 minutes = 32 minutes of travel). This means that at full throttle without pedaling you will be able to travel 8 miles. However, Izip designs higher efficiency into their electric bikes by encouraging pedaling and installing quality components. By adding pedal power to the (hybrid) electric bike, it is very easy to double the range. Many Izip bikes also have the option of adding an extra battery pack, which is nice. The D.I.Y. kit, on the other hand, draws 20.83 amps (1000 watts/48 volts = 20.83 amps). With a battery size of 10 amp hours and excluding hills or wind resistance, the D.I.Y. kit will run at full throttle for 29 minutes (10 amp hours/20.83 amps = .48 amps) x 60 minutes = 29 minutes of travel). Riding at 30 mph for 29 minutes means that the D.I.Y. kit will travel 14.4 miles without pedaling. Remember that a 24 volt battery pack uses 2 12 volt batteries and a 48 volt pack uses 4 batteries. If 1 12 volt 10 amp hour battery weighs 7.5 pounds then the Izip pack will weigh 15 pounds and the D.I.Y. kit 30 pounds. A pound for pound comparison of the 2 systems would show that the Izip, using both 24 volt battery packs, will travel 16 miles at 16 mph without pedaling and have 30 pounds of batteries (2 24v 10ah battery packs at 15 lbs. each = 30 lbs.). The D.I.Y. kit will travel only 14 miles but at a whopping 30 miles per hour with 30 pounds of batteries (4 12v 10ah batteries in series = 48 volts = 30 lbs.). There is lower wind resistance at lower speeds therefore it is fairly easy to help the Izip motor along. At 30 mph there is a lot of wind resistance and therefore much harder to pedal assist. Izip bikes have easily detachable battery packs which makes transporting and charging them very easy. D.I.Y. kit battery packs are usually hard mounted to the electric bicycle.

Motor Types

Electric motors are becoming increasingly complex with newer and better technologies. If you would like an in depth explanation on motor dynamics, here are a couple of links to good articles:

http://en.wikipedia.org/wiki/Brushless_DC_electric_motor

http://en.wikipedia.org/wiki/Brushed_DC_electric_motor

Simply stated, electric bicycles implement 2 main types of motors, brushed and brush less. Both run on direct current (DC) and have their own unique benefits.

In the center axis of many brushed motors is a rotor, or commutator, which spins. Brushes, mechanically, form an electrical circuit between the rotating commutator in the center and the surrounding electromagnetic coil windings, or stator. Electricity flows in a firing sequence from the stator to the commutator via the brushes. The number of brushes a motor has will define that motor type. For instance, a motor with 2 brushes will be called a 2 pole motor, and a motor with 4 brushes is a 4 pole motor.

The down side to brushed motors is that they are louder, produce more drag, and the brushes wear out over time. The benefits of brushed motors lie in their simplicity. For the most part, replacement carbon brushes can be easily obtained and replaced. The torque present in a brushed motor is generally greater than in brush less motors. And, controllers are easily replaceable. To replace a generic brushed motor controller you will need to know the voltage and amperage that the motor operates at and the number of brushes present.

Brushed motor designs have come a long way. For their Rack Mounted Battery (RMB) electric bikes, Izip defeats traditional motor drag by incorporating a freewheeling sprocket on the left side of the rear wheel which is chain driven from the motor. The left side motor drive freewheel works in conjunction with the right side pedal drive multi-speed cassette freewheel to give virtually zero drag when either pedaling or using the motor. Traditional motor noise has been, recently, reduced to almost nothing through precision manufacturing and intelligent re-design of the Currie Tech motor.

Specifications for 12v 10ah batteries

• SLA (Sealed Lead Acid) 7.5 lbs. 1 x 300 charges $40
• Ni-Cd (Nickel Cadmium) Memory Effect - this battery is impractical and not used for electric bikes
• Ni-Mh (Nickel Metal Hydride) 3.6 lbs. 2 x SLA 600 charges $100
• Li-Ion (Lithium Ion) 1.9 lbs. 5 x SLA 1,000 charges $150
• LiFePO4(Lithium Iron Polymer) 3.4 lbs. 3 x SLA +1,500 charges $200
• Gasoline 1 gallon = 8 lbs 300 x SLA 20 miles @ 150mpg @ $2/gln = $0.26

SLA is heavy, easy to use, inexpensive, has a moderate self discharge rate, and a 95% recharge efficiency. This is the mainstay of car, motorcycle, and entry level electric bike batteries. This battery is very easy to use and takes a straight charge easily. You would choose SLA batteries in conjunction with solar panels.

Ni-Mh batteries have a low toxicity, are twice as good all around compared to SLA, require a smart charger which works on a Y-Delta, and has a 70% recharge efficiency.

Li-Ion are the future of batteries and have undergone several changes. The earliest Li-Ion batteries had a problem with their BMS (battery management system) which drew major attention from laptop computer fires. Li-Ion Polymer batteries were developed as a safer form of Li-Ion and the new design consisted of flat business card like cells. This is a benefit for manufacturers because of the way these batteries can be shaped into smaller and thinner configurations.

LiFePO4 is a sidestep for the Li-Ion production line. LiFePO4 batteries are made from stable earth materials. Their production is one of the least toxic. And, while they only have 3 times the energy density of SLA batteries, they have a lifespan which is greater than 1,500 charges. That means that if you were to get 20 miles per charge, and then multiply that by 1,500 charges, your LiFePO4 battery would last you 30,000 miles!

Click here to view the current selection of electric kits and parts that we offer.

Click here to view the current selection of electric bicycles that we offer.