Electric bikes have two crucial components: the motor and the battery. Both of these components are useful with the other, so selecting the correct battery for your DIY conversion is critical. In this essay, I will explain electric bike batteries as simply as possible.
There are several aspects to consider when purchasing a battery, and the decision should be taken with seriousness since even lower standard batteries may be expensive.
I could write pages and pages on current batteries and how they operate. Still, I’d make this post as simple as possible, focusing on selecting the correct battery for your specific design.
Selecting the Best Battery
Before selecting a battery, there are various factors to consider. These are enumerated below, and each requires careful consideration:
For the sake of this paper, I shall concentrate on the most commonly used voltages today:
Most road-legal electric bike kits utilize a 36v battery, but more powerful motors may require a 48v or even a 52v battery. Voltages in the domains of really high-performance e-bikes can reach 72v and beyond.
For example, if you bought a Bafang 250w mid-drive, you’d need a 36v battery; if you bought a 1000w BBSHD, you’d need a 48v battery.
It would help if you determined where you want to fix the battery on your bike. The most frequent location is on the diagonal downtube, which looks nice and keeps the increased weight of the battery reasonably central and low, boosting stability.
Unfortunately, this is impossible on all bikes because it depends on frame size and geometry. Due to the rear suspension unit on many full-suspension mountain bikes, putting a decent-sized battery in the frame might be challenging. Thankfully, more compact battery packs are now available. However, they will have a lower output of roughly 36v10.4ah. Alternatively, you may install the battery on the bottom of the frame, but this will make it more exposed, and you will need to check for front tire clearance while the front suspension is fully compressed.
Rack batteries are ubiquitous for low-step framed bikes or bikes with short frames; this sort of battery typically comes with a specific ‘double-decker’ rack and will result in greater weight at the bike’s rear.
The Ah or ‘amp-hour’ rating is another crucial factor to consider. A one-amp-hour battery should be able to continuously give a current of one amp to a load for precisely one hour, or two amps for half an hour, or one-third amp for three hours, and so on, before getting entirely depleted. A medium-sized battery typically has a capacity of roughly 13Ah; multiply it by the voltage to get 36v x 13Ah = 468wh (watt-hours). A watt-hour is a unit of electrical energy equal to one watt of power usage for one hour. A 36v13Ah battery could provide 468 watts for one hour.
What does this mean in terms of a quantifiable range? Assuming you use cautious electricity, each mile will cost you around 20wh, giving you a range of 23.4 miles. This would be assuming that power use remained constant for the time. Of course, this is improbable in practice since there will be moments when you don’t need the electric support at all and other times when you need it. If you reside in an area with long, steep hills, you will consume more than 20wh each mile. Your usage may be lower if you reside in a somewhat flat location.
Most of my clients who utilize a Bafang 250w mid-drive electric motor in combination with a 36v13ah battery claim a range of 25-35 miles, in my experience.
If you wanted to cover more ground, a 36v17.5ah battery would give you a range of 31.5 miles at a steady 20wh each mile. However, in real-world circumstances, 50-60 miles have been observed with batteries of this size.
If you can only afford a smaller battery, several methods exist to improve the battery range on your e-bike without spending any additional money.
Manufacturers of Batteries
Most e-bike batteries use standard 18650 lithium cells manufactured by well-known manufacturers such as LG, Samsung, Panasonic, and Sanyo. Branded cell batteries have a longer lifespan and are more trustworthy than unbranded generic Chinese cells. I have delivered several battery packs utilizing Chinese cells without incident. It all comes down to money. I would pay extra for branded cells since buying cheaper batteries wastes money.
Lithium batteries must be handled with caution. There are constraints on how they are carried and for a good reason. The concern is that they burn at extremely high temperatures if they catch fire, causing severe burns or even death. They should never be stored in a place where they will be exposed to high temperatures.
These batteries do not tolerate temperature extremes at either end of the spectrum. When the temperature dips below zero, their performance suffers, and most manufacturers specify a minimum working temperature of -20c and a maximum operating temperature of 45c.
Putting a new battery through at least three complete charge and discharge cycles is usually suggested to ensure the cells are fully balanced. However, draining the battery by at least 50% is adequate during this period.
There is some evidence that completely charging a battery all of the time will lower cell longevity and that it is better for long-term battery health to charge to 80% most of the time and fully charge just once every couple of weeks.
This is a contentious issue; a well-known Lithium battery specialist assured me this is untrue. Indeed, I had a customer who followed the procedure above, and after a few months, the maximum charge voltage decreased dramatically, necessitating cell re-balancing.
It is also critical to ensure that your battery is at least 80% charged if not used for longer than a few months. When a battery is depleted and unused for a few months, the cell voltage might fall below its specified minimum, causing irreparable damage. At the same time, storing a battery at total capacity for more than a few days is not suggested since it harms the battery’s long-term health.
These battery packs employ a battery management system (BMS), which serves as the battery’s brain. This tiny piece of electrical circuitry prevents overcharging and over-discharging while also regulating overall amp output. A wholly charged 36v battery will have a voltage of roughly 42.2v, and the BMS will generally shut off the battery at 29v. A fully charged 48v battery will be 54.4v and typically shut down at 39v. This is significant because excessive discharge might irreversibly harm the cell’s chemistry.
This is based on various parameters but will be determined by cell quality, voltage, Ah rating, and BMS. Most 36v13ah batteries have a constant discharge rate of 15A-20A. However, they may be capable of temporarily greater output. This is also dependent on the type of motor controller utilized. For example, the controller on the 1000w Bafang BBSHD can handle 30 amps of continuous current.
Other Things to Think About
An excellent mid-drive electric motor, such as the Bafang or Tongsheng, leverages the gear ratios of the bike to transfer the work done by the motor to the rear wheel. As a result, efficiency improves, and battery energy consumption decreases. On the other hand, a vast, gearless hub motor cannot spin as rapidly, consuming more watt-hours each mile.
The rider’s weight is also a significant consideration. A person weighing 100kg pedaling a 250w e-bike at maximum power will use more energy than someone weighing 75kg.
If you plan on riding your bike for short distances of 20-30 miles, a 36v13ah battery will be more than enough. A 48v motor would be the same. However, if you want to travel or spend long days on the saddle, a battery with at least 36v17.5ah or even 20ah is recommended.