If you are considering purchasing or building your battery powered trike then you need to educate yourself a bit on different technical terms so you can make an informed decision when purchasing batteries for your bike.
One term that may come up is watt-hours or Wh for short. A Watt-hour is an important measurement when determining how much battery capacity you require to travel a specified distance. It represents the total energy available in a given battery pack. It is calculated by multiplying the amp-hour or Ah of a battery pack by the total voltage of the pack. A 36-volt 20Ah pack and a 72-volt 10Ah pack both contain 720 watt-hrs. therefore, they should both be able to get you the same distance provided you ride at the same speed. You can accelerate faster with the 72v pack and have a much higher top speed than the 36v pack. If you take advantage of this (it’s really hard not to use all that extra power) and ride at a higher speed you will encounter more wind resistance using up more power.
This will result in your distance traveled before the batteries running out being less than that of the 36v pack traveling at a lower speed but you will reach your destination faster. With a Crystallite 5304 hub motor in a 24-inch rim and a motor controller set to 90 amps you will average around 35 to 45 watt-hrs. per km (58-75 watt-hrs. per mile). This is at a cruising speed of around 65KPH (40MPH) and no pedaling. How fast you accelerate and your top speed will influence your power consumption. It will also vary depending on how flat or hilly the terrain is and how much weight you are carrying. You can use less than 25 watt-hrs. per KM if you ride slowly (using the same setup mentioned above) on flat terrain or use over 65 watt-hrs. per KM riding full throttle up hills.
I hope this gives you some useful information on what a watt-hour is and why you need to know. It makes it much easier to determine what size of battery pack you need to travel a specified distance.
Electric Bike Batteries – Why You Shouldn’t Completely Discharge Them
If you are thinking about buying or building your best bike for arthritic knees, you should know some things to help maximize the life of your expensive battery pack. DOD or depth of discharge is one of these things. When calculating how much battery capacity you need for the distance you want to travel on your bike you should only plan on discharging the pack to 80 percent DOD. Most battery cycle life (how many times it can be recharged) specifications are based on 80% DOD. If you discharge more than this each time the cycle life can be reduced. The batteries will also slowly lose capacity over time so the extra 20% capacity gives you an added buffer zone before you need to replace them. Don’t forget to add any electrical loads from accessories powered by your battery packs such as lights and a heated jacket if you intend to use one. Something else to consider is your battery capacity will decrease as the temperature gets colder.
You will have approximately a 15% decrease in capacity when riding in temperatures close to or below freezing so you may need a larger capacity pack if you intend on riding in cold weather. On an average daily commute, it’s best to only use about 70 to 75 percent of your battery pack with all accessories running so you have a little extra capacity if you decide to take a detour and run some errands on your way home. If you need to use close to 100 percent of your battery pack to reach your destination you will be constantly worried about using too much power accelerating or fighting a headwind that could leave you stranded with a dead battery. That’s why you need a good buffer zone of power so you can ride stress-free. If you try to push the limits of your battery pack every time you ride by completely draining it you will overstress it and not get the specified cycle life from it. It pays to treat your battery pack with care.
To prevent you from over-draining your battery you can add or buy a battery pack that has an LVC or low voltage cutoff. A wire connects from the battery pack to the motor controller and if the voltage of any one of the battery cells drops below a preset voltage, the LVC will trip and cut off the power to the controller stopping the bike. Once the voltage rises the power is restored and the bike will move again. An LVC is a great safety device and will guarantee you never accidentally over-discharge your battery pack.
I hope this helps you understand why you should never completely drain your battery pack.