Introduction
An electric bicycle consists of two essential components: the motor and the battery. Both are crucial, and it is important to choose the right battery for your DIY customization. In this article, I will do my best to explain e-bike batteries in a simplified manner.
There are many factors to consider before purchasing a battery, and it's important not to make hasty decisions, as even lower-spec batteries can be costly.
I could write pages upon pages about modern batteries and their working principles, but I want to keep this article as simple as possible, focusing on selecting the right battery for your specific build.
Choosing the Right Battery
Several variables need to be considered before buying a battery. I will list them below, and each one requires careful consideration:
For the purpose of this article, I will focus on the most popular voltages currently used:
Most road-legal e-bike kits use a 36V battery, while more powerful motors may require a 48V or even a 52V battery. When using high-performance electric bikes, the voltage can reach up to 72V or even higher.
For example, if you purchase a 250W mid-drive motor, you would need a 36V battery. If you opt for a 1000W BBSHD motor, you would need a 48V battery.
Battery Placement
You need to decide where you want to mount the battery on your bicycle. The most popular location is the downtube in a diagonal position because it not only looks neat but also keeps the additional weight of the battery relatively centered and low, improving stability.
The Cube Target Pro mountain bike is powered by a Bafang BBS02 750W electric motor kit and a 52V battery.
Unfortunately, this may not apply to all bicycles and depends on the frame size and geometry. For many full-suspension mountain bikes, it can be challenging to mount the battery on the frame due to the rear suspension. Fortunately, there are now more compact battery packs available, but these packs have smaller outputs, typically around 36V 10.4Ah. Alternatively, you can mount the battery on the underside of the frame, but this may make it more susceptible to damage, and you need to check the clearance with the front tire when the suspension is fully compressed.
Frame-integrated batteries are also popular options for step-through frame bicycles or small frame bicycles. These types of batteries typically come with specific "double-layer" frames, which inevitably add weight to the rear of the bicycle.
Battery Range
Another important variable to consider is the Ah or "ampere-hour" rating of the battery. A battery with a capacity of 1 ampere-hour should be able to continuously provide a current of 1 ampere to the load for exactly 1 hour, or 2 amperes for 1/2 hour, or 1/3 ampere for 3 hours, and so on, before being fully discharged.
The capacity of a medium-sized battery is typically around 13Ah. When multiplied by the voltage, such as 36V x 13Ah = 468Wh, it gives the capacity in "watt-hours" (Wh). Watt-hours is a unit of electrical energy, equivalent to the power consumption of one watt over one hour. Therefore, a 36V 13Ah battery can effectively sustain 468 watt-hours.
How does this translate into measurable range? Assuming you are conservative with the power usage, you would spend approximately 20 watt-hours per mile of travel, giving you a range of 23.4 miles. This is based on the assumption of maintaining power consumption at that exact level throughout the duration.
Of course, in the real world, this scenario is unlikely as there are times when you may not need electric assistance at all, while other times you may heavily rely on it. For example, if you live in an area with long and steep hills, your power consumption per mile may exceed 20 watt-hours. If you reside in a fairly flat area, your consumption may be lower.
If you desire to achieve longer mileage, a 36V 17.5Ah battery would allow you to travel 31.5 miles at a constant power consumption of 20 watt-hours per mile. However, in real-world scenarios, I have reported ranges of 50-60 miles with such capacity batteries.
If you can only purchase a smaller battery, there are many ways to increase the battery range of an electric bike without spending any extra money.
Most electric bike batteries use standard 18650 lithium-ion cells produced by well-known manufacturers such as LG, Samsung, Panasonic, and Sanyo. In my opinion, opting for branded batteries is always wise as they tend to have longer lifespans and greater reliability compared to unbranded generic Chinese batteries. That being said, I have provided many battery packs using Chinese batteries without any issues. It all comes down to the price. Personally, I would spend a little more to purchase branded batteries as buying cheaper batteries might be a false economy.
Battery Maintenance
Lithium batteries require careful handling. There are restrictions on their transportation, and it's not without good reason. The problem is that if they catch fire, they burn at very high temperatures and can cause severe burns or even death. Do not store them in areas exposed to extreme heat.
These batteries do not like extreme temperatures at either end. Their performance decreases once the temperature drops below zero, and most manufacturers set the minimum operating temperature at -20 degrees Celsius and the maximum at 45 degrees Celsius.
Charging
When the battery is new, it is generally recommended to run it through at least three full charge and discharge cycles to ensure the battery is fully balanced, although I have ample evidence to suggest that discharging the battery to at least 50% during normal use is sufficient for this period.
There is some evidence to suggest that always fully charging the battery can shorten its lifespan, and most of the time, charging it to 80% and only fully charging the battery once is more beneficial for long-term battery health over weeks.
This is a somewhat controversial topic, as a highly respected lithium battery expert told me that this is not the case. In fact, I had a customer who followed the above practice, and several months later, the maximum charging voltage dropped significantly, and the battery needed to be rebalanced.
If the battery is not used for several months, it is also essential to ensure that the battery has at least 80% charge. If the battery is left discharged for several months without use, the voltage in the battery may drop below the design minimum and could result in permanent damage. Additionally, it is not recommended to store the battery at its maximum capacity for more than a few days, as this is also detrimental to the long-term health of the battery.
All these battery packs use a Battery Management System (BMS), which is the brain of the battery. It is a small electronic circuit that prevents overcharging and over-discharging and regulates the overall amplifier output. A fully charged 36V battery has a voltage of approximately 42.2V, and the BMS typically shuts off the battery at around 29V. A fully charged 48V battery has a voltage of 54.4V and usually shuts off around 39V. This is crucial because over-discharging can permanently damage the battery's chemical composition.
Continuous Current
This depends on several factors but will be influenced by the battery quality, voltage, Ah rating, and BMS. Most 36V 13Ah batteries have a continuous discharge rate between 15A-20A but may be able to provide higher output momentarily. This also depends on the type of motor controller being used. For example, the controller on a 1000W Bafang BBSHD can handle a continuous current of 30 amps.
Other Factors to Consider
A good mid-drive motor, such as Bafang or Tongsheng, utilizes the bike's gear ratio to transfer the power generated by the motor to the rear wheel. This results in higher efficiency and less battery energy consumption. On the other hand, large direct-drive hub motors cannot spin as fast, so they will consume more watt-hours per mile.
The weight of the rider also plays a significant role. A person weighing 100 kg riding a 250W electric bike at full power will consume more energy compared to a person weighing 75 kg.
Conclusion
If you only plan on using the bike for short trips of up to 20-30 miles, then a 36V 13Ah battery should be sufficient. The same applies to a 48V motor. However, if you plan on traveling or spending long periods in the saddle, it would be worthwhile to purchase a battery with a capacity of at least 36V 17.5Ah or even 20Ah.
