Batteries are at the heart of a lot of what we do in our lives. As the industry has matured and the number of e-bikes has increased, the development of these batteries has increased, and it’s all down to the rise of the Lithium-ion battery. Lithium-ion batteries last much longer than lead-acid batteries. Typically a lead-acid battery needs to be replaced after 200-300 charges, whilst a Lithium-ion battery is likely to last between 500-2000 charges, depending on battery quality. Charging is also a lot more efficient, with a fast charger able to fill up an e-bike battery in just a few hours. The only major downside to Lithium-ion batteries is that the charging process needs to be more carefully regulated to avoid damaging the cells, and that adds a little bit to the overall cost. Normally the electronics needed to control the charge are added to the battery unit.
Most e-bike kits are rated for a specific voltage range. A typical 36V e-bike needs a battery that delivers power between 30V and 42V. A 36V pack at 42V is 100% charged and holds as much energy as it can; at 30V it should shut down and stop giving power before causing permanent damage. More on the BMS (Battery Monitoring System) in a bit. The common number ’36V’ is the average operating voltage, sometimes called nominal voltage. Voltage is measured in volts (V) which is used to describe how fast electrons move. More voltage = more speed!
Capacity is the amount of energy that can be stored in the battery. It is the main characteristic of any battery. The unit of measurement is the ampere-hour (Ah). This value is a measure of the fixed number of amps a battery can sustain for 1 hour (C rate). Double the amps for half an hour. Or cut the amps in half for two hours, etc.
A battery has a certain number of discharge-charge cycles, after which the capacity starts to fall proportionately. The actual battery life is determined by the number of cycles and it depends on the operating conditions of the battery. The longer the battery is discharged, the fewer possible cycles can be expected from it.
Amp is a measure of Force. Typically, higher Voltage electric bikes go faster, while higher Amps require thicker gauge wires and/or more powerful and larger battery packs. How wide the road is, more lanes, more cars can pass at the same time side by side.
The combination of Volts and Amps equals Watts ( Volts X Amps = Watts ).
Should always be listed, typically 10 to 20 Amp Hours ( abbreviated ” Ah ” ) a measure of how many fixed numbers of Amps a battery can sustain for 1 hour ( C rate ). Or, double the amps for half the time. Or half the amps for two hours, etc.
This is a far more accurate way to know how much usable energy is in a given battery pack ( abbreviated Wh ) when available, this is the number to look for! Also, you can translate it into how many watts, continuous, for 1 hour! A 500wh battery can deliver 500 watts for 1 hour or 1000w for 30 minutes， or 250w for 2 hours， etc. Most e-bikes do not use power at an exact level, continuously, so this does not directly translate into ride time, but you can quickly see how a larger battery with more energy (capacity) can deliver lower power levels for longer periods of time, and go further on a charge.
Ah vs Wh
This can get confusing, but it is very important to understand the difference. Amp Hours (Ah) means nothing unless you factor in the voltage. Watt-Hours (Wh) is far more important because it factors in the Voltage and the Amp Hours together and determines how far you might go on a full charge. Not all packs are labeled and/or constructed the same, so be careful and pay close attention. Wh Example: 36v 10ah = 360wh and 48v 10ah = 480wh.
BMS and Chargers
Lithium batteries are light and durable but they must be used within their specified limits. In order to ensure a long life, battery packs should contain a BMS, aka Battery Monitoring System. It stands between the actual battery and the power wires, monitors all the voltages of cells within, and also typically watches how many Amps are flowing. If any limits are reached the BMS should intervene by cutting off the power safely. In a perfect world, the BMS will sit there and do nothing but if you do ride until you use up all the energy available, it will shut you down and prevent any damage to the battery cells. Similar situation with the charger, the BMS will allow the charger to do its thing as long as all the cells are in harmony and within their limits, a quality charger is very important and often completely overlooked, some customers have witnessed great battery packs destroyed by horrible chargers.
Size, weight and shape
It has to fit and not be too heavy and should look good once installed on your bike. You will likely not find all the ideal factors in a pack but some are vital, while others are not as much. The most common mistake that many ebikers make is to want a huge battery, this is all fine and great until you have to figure out how to mount it on your ebike, and then it gets worse when you need to carry it up a flight of stairs or put it on a bike rack! Be realistic and you can save yourself a lot of headaches and money, also end up with the best ride possible for your needs!