There are two basic types of hub motors in wide use today for electric bikes: direct drive and geared. The direct drive has no internal gears or other moving parts except the actual case which rotates around the axle on sealed bearings. The coils are wound around an assembly that is fastened to the axle and remains stationary. The outer ring of the case has a ring of magnets that rotate in close proximity to the electromagnets formed by the coils. As the coils are energized in a specific pattern by the motor controller, the magnets are attracted and repelled causing the wheel to rotate. The outer case directly drives the wheel. The geared motor has the same basic configuration, but does not directly drive the case. Instead, there is an intermediate gear assembly driven by the motor. This consists of a free-wheel and three planetary gears which transfer the rotation to the outer case and wheel. The gear ratio is typically 4:1.
The first geared motors were smaller and lighter. Due to the gear ratio, they have good torque, even at low speeds. Due to their small size and the physical strength of the gear assembly, there is a limit on the maximum power/voltage that they can handle. However, today’s bigger geared motors now offer all the advantages of the smaller planetary geared motor along with the ability to handle higher voltage and amperage levels. With a geared motor the internal free-wheel isolates the wheel from the motor so there is no resistance when turning the wheel.
Please note that we have discontinued the hub motor type known as a direct drive. Direct drives have no planetary gears, are bigger, heavier, less efficient, and have magnetic drag when just pedaling. Direct Drives were the first type of hub motors made and proved very durable; however, in our eight years of business we have found the planetary geared motors to be just as durable and offer significant advantages that the majority of our customers now come to expect.
Which Planetary Geared Motor System should you choose?
It comes down to how much power you need/want? At 36V you get a good strong assist and near 20 mph of speed. For most riders this is more than enough and you can still pedal faster if your bicycle is geared to do so. At 48V you will add 33% more power (torque and speed) but still have a light bike with the least amount of change in how it rides. This makes the geared motor a great choice over the old style direct drive. Geared motors are also at least 15% more efficient, which means you will get more distance out of the battery per charge.
Electric Bike Kit Motors
Motor power ratings are specified in watts. Watts(W) = Volts(V) x Current(A). For example, a motor running at 36V and (x) 22A is pulling 792W. The thing that not everybody realizes is that motors don’t have an exact value for watts. Consider the specified rating to be a general value as each motor will have a power range. Sometimes motors are purposely under-rated as in the case of electric bikes sold to Europe, which by law are limited to 250W. Ever wonder why there are so many motors rated for 250W? Taking one of these motors as an example, first calculate the current. 250W/36V = about 6.9A. That current is too low for 90% of ebikes so it doesn’t add up. The lowest E-Bike controller current is at least 10A, and many are 15A or more. So 36V x15A = 540W, twice what the specified rating is. Of course you can run at a lower voltage, and 24V is still common, especially for smaller bikes. In that case you have 24V x 15A = 360W.The point is that motors can be run at different power levels. They will be most efficient at a specific power level. Below this level, they will bog down and waste power for example when they are going slow under heavy load. At the higher point of the power range, the motor will start to get hot as it cannot dissipate the heat and will waste power.The standard way to rate a motor is test it on a sophisticated motor testing apparatus. It puts it under measured loads and plots the various parameters including efficiency and output power. The power rating corresponds to the point of peak efficiency. There’s actually a simpler method anyone can do. While not exact, it gets you in the ballpark. Simply increase the power level supplied to the motor while under load and measure the temperature of the motor. The power level at which point the motor is slightly warm when run continuously (under load) is the optimum operating level. You will see motors advertised as 1000W on sites like Ebay and you might think, wow what a great deal on a powerful motor! The problem is any power level can be slapped on a motor. Most of these are actually 500W motors. Both our geared hub motors operate very well at more than 1000W with a 48 volt battery; however, that is not where they operate most efficiently. They are most efficient cruising at 500W for the heavier duty geared motor and 350-400W for the smaller geared motor. So don’t get caught up on power ratings or fooled by exaggerated advertising claims. For more information on your power requirements, contact us at 707-439-3179.
Another newer type of motor out there is called a mid drive. If used properly it can be very efficient on hills and great for off road mountain climbing. It utilizes all the existing gears of your bicycle and the motor mounts low on the frame by the pedal crank. Here at Electric Bike Solutions we have been looking for a reliable and cost competitive model to offer. The challenge has been in its shortcomings listed here:
1. More expensive than a simple hub-motor.
2. Mid-drives are noisier than a hub motor.
3. Mid-drive kits are more complex to install than a simple hub.
4. Shifting a mid drive electric bike can be a real pain. With a hub motor bike, the only reason to use gears is to get the right pedal cadence which usually means just staying in high gear. Think manual transmission for the mid-drive and automatic transmission for the hub motor.
5. Mid drives are generally not as well suited for flat land street commuting as hub motor bikes.
6. Mid drives are not as stealthy as most hub motor bikes.
7. Mid drives have more moving parts and therefore tend to be less reliable than most hub motor kits (broken chains, gears, etc).
We will continue to evaluate suppliers, but at this point the cost to performance equation for our customers has not been realized. The geared motor is the optimum solution for cost conscious consumers while providing excellent reliability and performance.
This is a way over-hyped technology. Theoretically when applying the brake it will provide some power back into the battery for re-use. In practical application the amount is very small and the braking technology (think jake brake) can sometimes actually work against you. Another problem with it is that it can only be used with a direct drive motor. Since a planetary geared motor is already 15% more efficient than the direct drive it makes much more sense to just go with a geared motor. For all the technical information about it you can stand go here: http://www.ecospeed.com/regenbraking.pdf
. It was not written by us, but it conveys exactly what we know to be true.
Lithium Battery Break-in Process
Lithium battery packs have to go through a break-in period. The pack is made up of several cells connected in series, for example a 36V LiFePO4 pack has 12 cells. When new, the cells do not charge and discharge at the same rate. One reason may be a chemical inhibitor that is added to slow down self-discharge. At any rate, there is a chemical process going on with-in new cells that cause them to perform differently when new. Often the first thing a customer will do when buying a pack is to go out and really ride it hard to check the power and range. Unfortunately, that is not the best thing to do. It is recommended to perform 4 to 6 cycles of low discharges followed by full charges. A low discharge would be drawing 1-3 AH or riding 2-5 miles without heavy loads. Since the cells will discharge at different rates, the first cell to reach the minimum voltage will trigger the detection circuit in the BMS, which shuts off power. The range will be low and the customer thinks they have a bad pack. By doing short cycles the cells have a chance to equalize and not get far out of balance. The BMS has a balancing circuit, but it does not have the ability to bring up a cell that is much lower than the others. They have to be kept within a certain range of each other. In addition, leaving the pack on the charger over night when brand new gives the BMS time to equalize the cells. You may see the charge LED (green light) on the charger blink between red and green during this process. The pack is charged in “serial mode” where the charge current is passed through each cell to get to the next one. So each cell has to share part of the voltage going in, typically 42V for a 36v Li-ion battery. During the critical break-in period some cells will accept current more than others. Thus, it is critical to follow the break-in process to optimize the performance of the pack .Once properly broken in the pack will stay in balance if charged after each use and not left for long periods without charging. While rare, it is possible that a pack can become unbalanced even after being broken in. The solution is to charge up each cell separately until they are equal. This is not something the customer can do as working on the cells is dangerous. Not all suppliers have the technical ability to do this. At Electric Bike Solutions, however, we offer this service to our customers and can even replace individual cells in the unlikely event one goes bad. So the bottom line is when you receive a new pack, for the first 4-6 times you use it, ride the bike only a few miles followed by charging. For an excellent article on lithium-ion charging and discharging go here: http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries
Electric Bicycle Battery Technology
Batteries are the life blood of the electric bicycle and electric bike kit, and they come in many confusing forms. So as not to just present our own opinions, we will reference other sites as well to support what we know to be true. Electric Bike Solutions, LLC (GoCarLite.com) has used Lithium iron phosphate (LifePo4) and Lithium-ion batteries for many years. While the technologies are similar and sometimes referred to interchangeably, during the years we have found the lithium-ion to be a bit more stable and reliable. LifePo4 should theoretically provide more charge cycles, but if any cell in the battery breaks down then the whole battery is only as strong as the weakest link. Lithium-ion has proven to be more reliable over the years, which is why we now use it exclusively as of early 2013. We are also able to get more distance per charge in the same size battery case because of lithium-ion’s better power to weight ratio. They weigh less and go further on a charge, which is a winning formula. Still, each type of lithium battery offers give and takes, which is what we discuss here. This graph from electricbike.org.uk provides the best visual demonstration of various battery technologies. You get far more life out of this battery technology than any other.“Lifetime energy carried versus cost: On top of the bars it shows approximately how far the battery will carry you over its life, irrespective of cost. The bars themselves show the cost-adjusted lifetime energy capacity.”
So why would anybody choose another technology? A number of things can be factored in that include up front cost mostly, but as the previous graph demonstrates, LifePO4 technology is the most cost effective in the long run if the cells remain stable. That being said, lithium polymer (li-po) does present the most viable option because of its power to weight ratio. While the LifePO4 has a better ratio than the others, in essence the polymer will provide as much power with approximately 6 lbs of battery compared to 10 lbs for the LifePO4.
All “Lithium” batteries are not all the same… yet they all seem to fall under the title “Lithium”. In fact over twenty different Lithium chemistries exist. Most of these chemistries fall under the grouping of Primary Cells like button cells and are not generally rechargeable. Under a separate category there are Lithium Ion, rechargeable, batteries. These Lithium Ion batteries are considered the best type for automotive propulsion. These are a type of rechargeable battery in which the anode (positive Electrode) contains lithium in some form while the cathode (negative electrode) is made of a type of porous carbon. In this category there are three general groups. These groups are made up of a wide variety of materials. These materials are:
- Layered Oxide: These batteries usually contain lithium cobalt oxide. Lithium cobalt batteries are considered a hazardous material and must be disposed of under strict guidelines. This type of battery is prone to explode if overheated during a rapid discharge or if charged to and excessively high voltage. (The authors opinion is to forget them as a candidate for your project).
- Spinel: This group of lithium batteries contain minerals like manganese or cobalt. These minerals lower the cost of the battery while sacrificing the battery life and depth of charge which will affect the vehicles driving range. Batteries that contain these minerals typically have about 1200 charge cycles before they reach 80% depth of discharge (DOD)
- Polyanion: This lithium battery group contains lithium iron phosphate, also known as LiFePO4. Lithium iron phosphate batteries are not considered to be a hazardous material (not necessarily true for shipping purposes). LiFePO4 batteries are better than other cathode materials in terms of safety and are available in larger energy density capacities than the other groups of batteries. The choice of materials and construction can dramatically affect the voltage, capacity, life, safety and cost of a lithium battery. All Lithium Ion batteries are sensitive to overcharge or if discharged below a certain voltage. To reduce this risk they all must have a circuit which shuts down the battery when it is discharged below 3 volts and charged above 4.2 volts. This circuit is referred to as a Battery Management System or BMS.
Advantages of Li-ion Technology
Lithium Ion batteries are lighter than other energy equivalent batteries – often much lighter. A key advantage of using lithium ion chemistry is the high open circuit voltage that can be obtained in comparison to aqueous batteries such as lead acid, nickel-metal hydride and nickel cadmium.
Lithium ion batteries do not suffer from memory effect. They also have a slow self discharge rate of approximately 5 – 10% per month compared to over 30% in common nickel metal hydride batteries and 10% per month in nickel cadmium batteries.
Contact Doug, the owner of Electric Bike Solutions, with your questions at the numbers and email below.
Office: (707) 439-3179
Cell: (707) 290-9764