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Electric bikes have two key components: The motor and battery, neither of these parts are any good without the other, so choosing the right battery for your DIY conversion is very important. In this article, I will do my best to get electric bike batteries explained in a simplified way.
There are lots of factors to consider before buying a battery, and ultimately the decision cannot be taken lightly, as even the lower specification batteries can still cost a lot.
I could write pages and pages of information on modern batteries, and how they work, but I would like to keep this article as simple as possible, focusing on choosing the right battery for your particular build.
Choosing the right battery
There are several variables to consider before purchasing a battery. I have listed these below, and they all require careful consideration:
For the benefit of this article, I will concentrate on the most popular voltages currently in use:
Most road-legal electric bike kits use a 36v battery, the more powerful motors may use a 48v or even a 52v battery. When you are in the realms of very high-performance e-bikes, voltages can go to 72v and beyond.
For example, if you purchased a Bafang 250w mid-drive, then you would require a 36v battery, if you purchased a 1000w BBSHD then you would need a 48v battery.
You will need to decide, whereabouts on your bike you would like to fix the battery. The most popular place is on the diagonal downtube, as this not only looks neat but also keeps the added weight of the battery fairly central and quite low, thus improving stability.
Unfortunately, this is not possible on all bikes, and depends on the frame size, and geometry. With a lot of full-suspension mountain bikes, it can be very difficult to fit a decent-sized battery in the frame due to the rear suspension unit. Thankfully there are now more compact battery packs available, but these will have a smaller output of around 36v10.4ah. Alternatively, you may be able to fit the battery on the underside of the frame, but this can make it more vulnerable and you will need to check for front tyre clearance when the front suspension is fully compressed.
Rack batteries are also a popular choice for low-step framed bikes or bikes with small frames – this type of battery usually comes supplied with a specific ‘double-decker’ rack and will inevitably lead to more weight at the rear of the bike.
The other important variable to consider is the Ah or ‘amp-hour’ rating. A battery with a capacity of 1 amp-hour should be able to continuously supply a current of 1 amp to a load for exactly 1 hour, or 2 amps for 1/2 hour, or 1/3 amp for 3 hours, etc., before becoming completely discharged. A medium-range battery will usually have a capacity of around 13Ah, multiply that with the voltage and that will give you 36v x 13Ah = 468wh or ‘watt hours’. A watt-hour is a measure of electrical energy equivalent to the power consumption of one watt for one hour. So a 36v13Ah battery could in practice sustain 468 watts for one hour.
How does this translate into a measurable range? Well assuming you are conservative with your power, each mile you travel will cost you roughly 20wh giving you a range of 23.4 miles. This would be on the assumption that power usage stayed at that exact level for the duration. Of course in a real-world situation, this would be unlikely, as there would be times when you didn’t need the electric assist at all, and other times you may need it a lot. If for example, you have very long steep hills where you live then you would likely be consuming more than 20wh per mile. If you live in quite a flat area then your consumption may be less.
In my experience, most of my customers who use a Bafang 250w mid-drive electric motor in conjunction with a 36v13ah battery usually report a range of between 25-35 miles.
Let’s say for example you were looking to cover greater mileage, a 36v17.5ah battery would give you a range of 31.5 miles at a constant 20wh per mile. But again, in real-world scenarios, I have had mileages of 50-60 miles reported with batteries of this capacity.
If you can only afford a smaller battery, there are lots of ways you can increase the battery range on your e-bike without having to spend any extra money.
Battery Cell Manufacturers
Most e-bike batteries use standard 18650 lithium cells, made by well-known manufacturers such as LG, Samsung, Panasonic and Sanyo. In my opinion, it is always wise to go for the branded cell batteries, as they tend to have better longevity than unbranded generic Chinese cells, and are also more reliable. Having said that, I have supplied many battery packs using Chinese cells without any issues. It all boils down to price. I would personally spend a little more and go for the branded cells, as buying cheaper batteries can be a false economy.
Lithium batteries need to be handled with care. There are restrictions on how they are transported, and that is not without good cause. The problem is, that if they do catch fire, they burn at a very high heat and can cause severe burns or even death. They should never be stored in an area where they are exposed to extreme heat.
These batteries do not like extremes of temperature at either end of the scale. Their performance can be reduced once the temperature drops below zero, and most manufacturers set a minimum operating temperature of -20c and a maximum of 45c.
When a battery is new, it is usually recommended to run it through at least three full charge and discharge cycles, to ensure the cells become fully balanced, although I have it on good authority that discharging the battery by at least 50% is sufficient during this period.
There is some evidence to suggest that fully charging a battery all the time can reduce the lifespan of the cells, and that it is better for long-term battery health to charge to 80% most of the time and only fully charge maybe once every couple of weeks.
This is quite a controversial topic, and I have been told by a very well-respected Lithium battery expert that this is not the case. Indeed, I had a customer who followed the above practice and after a couple of months the maximum charge voltage dropped significantly and the cells needed to be re-balanced.
It is also very important to make sure your battery is at least 80% charged if it is not going to be used for more than a couple of months. If a battery is discharged and then unused for a few months, the voltage in the cells can drop below their designed minimum and may result in permanent damage. At the same time, it is not recommended to store a battery for more than a few days at its maximum capacity as this will also be 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. This is a small piece of electronic circuitry that prevents overcharging, and over-discharging and regulates the total amp output. A fully charged 36v battery will be at around 42.2v and the BMS will usually shut the battery down at 29v. A 48v battery will be 54.4v fully charged and usually shut down at 39v. This is important as over-discharging can permanently damage the cell’s chemistry.
This is dependent on several factors but will be influenced by the quality of the cells, the voltage, the Ah rating and the BMS. Most 36v13ah batteries will have a continuous discharge rate of between 15A-20A but may be capable of briefly higher output. This also depends on the kind of motor controller being used. For example, the controller on the 1000w Bafang BBSHD can handle 30 amps of continuous current.
Other Factors to Consider
A good mid-drive electric motor, like the Bafang or Tongsheng uses the bike’s gear ratios to transfer the work the motor is doing to the rear wheel. This results in greater efficiency and less battery energy used. On the other hand, a large, gearless hub motor is not able to spin as fast and as a result would consume more watt hours per mile.
The weight of the rider also plays an important part. Someone who weighs 100kg riding a 250w e-bike on full power will consume more energy than someone who weighs 75kg.
If you are just going to be using your bike for shorter journeys of no more than 20-30 miles, then a 36v13ah battery will be more than ample. The same would apply to a 48v motor. If, however, you were planning on touring or spending long days in the saddle, then it would be worth getting a battery of at least 36v17.5ah or even 20ah.