electric bike batteries explained

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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.

Detailed technical information on Lithium-ion Batteries.

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.

Compare Prices on Popular E-Bike Batteries

Battery placement

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.

Cube aim pro mountain bike powered by a bafang BBS02 750w electric motor kit and 52 volt battery

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.

B TWIN Rock rider full suspension mountain bike powered by a bafang bbshd electric motor and 52 volt battery
This battery just fitted into the frame of this full suspension Mountain Bike

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.

Ladies Orbea mountain bike powered by a bafang bbs01 250w electric motor and 36 volt rack mounted battery
A Rack battery fitted on a Lady’s frame Mountain Bike

Battery Range

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 Manufacturerselectric bike batteries explained this photo is of samsung i n r 18650 lithium cells

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.

Battery Care

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.

Continuous current

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.

Further Reading

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    1. Ahoj,
      Ak máte na mysli batériu s kapacitou 1 000 Wh (watthodina), bolo by to v poriadku, pokiaľ je napätie batérie 36 V.

      S pozdravom,

  1. Bonjour,

    J’ai 2 batteries différente la 1ere fait : 24V-10Ah-250 Watt
    la 2eme fait : 48V 13Ah-750 Watt

    Ma question de type théorique, serait de savoir si j’ai un moteur(roue) par exemple de 48V-1000W + Contrôler : 48V-30A.
    Mes 2 batteries peuvent elle fonctionner dessus sans crée de quelconque problème.

    Merci de votre temps et vos réponses .

    1. Bonjour,

      Vous ne pouvez utiliser qu’une batterie 48v avec un contrôleur 48v. Si le contrôleur est évalué à 30 A en continu, vous devez vérifier le BMS sur la batterie pour vous assurer qu’il peut gérer ce courant. Certaines batteries 48v n’ont qu’un courant max de 20A. Vous ne pourrez utiliser la batterie 24v qu’avec une configuration contrôleur/moteur 24v.

      1. bonjour,

        merci pour votre reponse rapide,

        Désolé pour mon manque de science electronique, du coup ma batterie de 48V 13Ah-750 Watt et compatible avec mon moteur et contrôleur?

        merci encore

  2. Bonjour tony,
    j’ai acheté un super 73 zx avec un moteur de 48v et 12,8ah. Est-ce que je peux mettre une batterie de 52v a la place de celle d’origine (48v) ? Si, oui y a t’il d’autres choses a modifier ?
    Merci d’avance

    1. Vous aurez besoin d’un contrôleur avec un courant plus élevé (ampères), quelque chose comme un contrôleur KT 48v 30A avec une batterie 52v 20Ah. Vous aurez également besoin d’un écran KT-LCD3. Si votre vélo utilise un moteur de moyeu Bafang, la puissance de sortie la plus élevée que le moteur peut gérer en toute sécurité est de 1500w.

  3. Bonjour Tony,

    J’ai passé commande d’une batterie Green Cell 48v 13ah, sur la page de confirmation de commande il est indiqué que la batterie ne pourra pas fonctionner avec un moteur d’une puissance maximum de 250w (mon moteur fait 1000w). Je ne comprends pas pourquoi, est ce qu’il est possible que l’électronique bloque la puissance de sortie

    1. Bonjour,

      Une batterie de 48 V fonctionnerait avec un moteur de 250 W, à condition que le moteur et le contrôleur soient tous deux évalués à 48 V. La puissance en watt n’a aucune importance. Tant que la tension correspond, il ne devrait pas y avoir de problème.


  4. Hi Tony
    Great article and brilliant info.
    I’m new to e-bikes and have just bought a Halfords Carerra Crossfire MTB which has been professionally converted to an e bike. It has a mid mounted Bafang C961 motor and a Li ion 36v 11aH battery mounted on the frame.

    I have been very happy so far with the conversion, I took the bike for a 22km ride over mixed terrain, including a few hills, using a combination of 3 power assist levels and on burst of throttle only for approx 3-4 minutes. Upon returning I was please to find the battery level was still on 4 of 5 bars.

    I have a few questions:

    If I wished to replace the batter for one which would give me greater range, am I correct in thinking that all these type of batteries would simply need to bi fitted and wired to the motor.
    1. Would I need a different controller?
    2. If you are in the UK, what is the most powerful battery I can fit top provide the greatest range and still be legal?
    3. Would you suggest a particular brand or can you link to a company here I can purchase one?

    4. I’ve read that there is a risk of burning out the motor when using throttle only. What you you recommend? i.e, only to use throttle on level terrain? only use for short bursts of a few minutes etc?

    If the motor fails or is in need of repair, are their many bike shops that would offer a repair service?
    I’m based in Barnet, Herts.

    I currently have 9 gears with a single cog fitted to the peddle and motor unit. Would it be possible to fit a dual cog in order to provide greater choice of gears? if so would it be worthwhile or would I potentially encounter problems with additional load on the motor?

    Sorry for all the questions, I guess many newbies like me would wish to know the answers and having visited many ebike sites, I believe your knowledge and advice is by far the best.

    Many thanks

    1. Hi Keith,

      For maximum range you’d need something like a 36v 20Ah battery although these are quite large and heavy. Yose Power sell a good one on eBay for £225. Here is the link.

      If you’d prefer a battery similar to the one you have the Green Cell 36v 15.6Ah battery is good and will be an improvement over what you currently have. Here is the link.

      For the ultimate battery then I would speak to Jimmy at ebikebatteries.co.uk – they custom build e-bike batteries using the highest quality assembly process and Panasonic 18650GA cells. I’ve used Jimmy a few times and his batteries are still going strong after 4 years of use.

      You will not need to change the controller – as long as you stick to 36v. The energy capacity (Ah) rating doesn’t affect the power output so there’s no legal worries.

      Regarding the throttle, I’m not keen on them as they put the motor under a lot of load. If you’re on the flat and in the right gear then it shouldn’t be a problem for short periods. You can reprogram the throttle start up current using a USB programming lead and free software – I usually lower it to 5% (set to 20% as standard) this puts less load on the motor when using it.

      You won’t be able to use a double chainring with the Bafang due to the already outboard chainline. What you can do to increase gear range is fit an 11-40 9 speed rear cassette. Depending on what rear derailluer you have if it’s a medium cage, you may need to get something like a Shimano Alivio long-cage rear. You’ll also need to get a longer chain (KMC X9).

      The other more expensive option is to change to a 10-speed system, but you will need to change the derailleur and shifter, but you can go up to an 11-46 cassette for even greater range.

      I hope this info helps, if you have any more questions, let me know.


  5. Hi Tony, thanks for all the information and links. My question relates to the Battery controller – If I use the Tongsheng Torque Sensor kit, does the controller come with/inside it?.. Thought I had my head around this until I started digging into the battery construction side of it. Sorry if it sounds like a daft question Btw. – Cheers Mike

    1. Hi Mike,

      Yes the controller is inside the Tongsheng, it’s like the brains of the motor and is connected to the display via a wiring loom. It also regulates the current coming from the battery.

      All the best,

  6. hi tony.looking at 36v batteries for my 250watt motor but most state that they are rated for 500w motors does this make any difference or cause any problems if i fitted on e of these. thanks

    1. Hi,

      As long as the battery is 36v then there won’t be a problem. The reason they state ‘rated to 500w’ means the battery can handle a higher current (amps) – most 500w e-bike controllers draw about 22amps from a battery whereas a 250w motor will only draw around 15amps. Basically it means the battery will be under-stressed when used with a 250w motor and produce greater mileage.

      I hope this helps, if you have any more questions, please let me know.

      All the best,

  7. Hi Tony
    I really enjoy reading all your articles and reviews. They really helped me a lot.
    Now question about batteries: I’m new user of Tongsheng TSDZ2 36V/350W motor and Hailong 17,5 Ah battery. Does motor recharges the battery when you are pedalling with assistance set to “off” or zero?
    It is of course a little bit harder, but it was mostly a flat ride. I was riding 44 km yesterday (400 vertical meters up the hill in 22 km) and battery still shows all 4 lights (so 75 – 100%). It was my first ride with this motor and battery.
    Thanks for your answer and keep spread your experience to the community
    Many thanks, Matjaz

    1. Hi Matjaz,
      Thank you for your comments, glad you have found the articles useful. The TSDZ2 motor does not currently support a regenerative function so it will not put energy back into the battery. I would say that your battery usage is about right. The Tongsheng is very efficient, especially when used at lower power levels. I used to regularly get 120 km out of a 36v 13Ah with a TSDZ2 (with 2000m elevation gain).
      All the best,

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