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Electric bike batteries Explained

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Summary

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.

hailong case 52 volt electric bike battery

 

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.

You can find all the technical information on electric bike batteries here.

Choosing the right battery

There are a number of 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 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.

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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 in 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 Ladies 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 could sustain 468 watts for one hour.

How does this translate into 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 that 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 mileages, 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.

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

Most e-bike batteries uses standard 18650 lithium cell’s, 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 cell’s, and are also more reliable. Having said that, I have supplied many battery packs using Chinese cell’s without any issues. It all boils down to price. I would personally spend a little more and go for the branded cell’s, as buying cheaper batteries can be a false economy.

Battery Care

Lithium batteries definitely 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.


Charging

When a battery is new, it is usually recommended to run it through at least three full charge and discharge cycles, in order to ensure the cell’s 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 cell’s, and that it is better for long-term battery health to charge to 80% for most of the time and only fully charge maybe once every couple of weeks.

This is quite a controversial topic, and I been told by 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 cell’s 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 a period of 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, 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 a number of factors, but will be influenced by the quality of the cell’s, the voltage, Ah rating and the BMS. Most 36v13ah batteries will have a continuous discharge rate of between 15A-20A, but maybe capable of a 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 30Amps 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.

Conclusion

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 for 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.

If you are looking for an excellent battery supplier that ships globally, please visit this page where I have compiled a list of the best selling e-bike batteries currently on offer. My supplier has literally hundreds of different battery configurations available, they will even custom-build you a battery if required.

 

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28 thoughts on “Electric bike batteries Explained

  • May 2, 2020 at 8:43 am
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    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

    Reply
    • May 2, 2020 at 10:26 am
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      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,
      Tony

      Reply
  • March 14, 2020 at 9:22 pm
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    Hello, Planning to buy BBS02 750w and still not clear about batteries. Please help. So if motor is 25ah should battery continuous discharge also by 25ah or can it by higer like 30ah because motor will take how much it need,or will it make stress(heat,shorter life,another effects) to motor

    Reply
    • March 14, 2020 at 10:07 pm
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      Hi,
      Ah relates to amp hours, which is the energy capacity of the battery, I think you mean A (amps) as in the maximum current. A battery with a 30A BMS can be used quite safely on a BBS02B 750w, as the motor controller will only draw a maximum of 25A from the battery in full power mode.

      Regards,
      Tony

      Reply
  • December 17, 2019 at 1:22 am
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    Tony, thanks for the article. I’m looking to replace my rack battery. It’s a Samsung 48 VOLTS /750 WATTS / 10.4 AH.

    Any thoughts on where I can get a replacement? I’m having trouble finding exact specs. Can I increase AH?

    Reply
    • December 17, 2019 at 9:13 pm
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      Hi John,

      Yes, you can increase the AH rating without any problems. Here is a link to a more powerful 48v 18Ah rack battery on Amazon US, I have purchased over 100 batteries from this particular supplier in the past, and their batteries are excellent quality.

      All the best,
      Tony

      Reply
  • November 16, 2019 at 9:11 pm
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    Good afternoon Tony,
    I am fitting the tsdz2 250w 36v with the 10,4a battery stock set up on a cube touring lady geometry bike.
    I am planning to build a custom battery to the near future for an extra mile gaining.
    What would you advise me to build with this motor, more volts, amps or a combination of volts/amps.
    Which should you consider as the best solution for extra mileage and boost at the same time.
    Thank you very much in advance.

    Reply
    • November 16, 2019 at 10:18 pm
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      Hi Makis,

      I would personally go for a 36v 17ah using a high-quality cell like the Sanyo NC18650GA cell. Or maybe one of the latest generation Samsung cells. A 20A BMS will be more than adequate.

      The TSDZ2 is very efficient. I used to ride a hybrid fitted with a 36v 500w version and I regularly managed over 120km + range out of a 36v 13ah using a combination of ECO and Tour mode – I weight 110 kg and regularly ride 1000m + elevation gain over 30 miles.

      If you wanted to go down the route of custom-programming the motor using the open source firmware, it is possible to use up to a 52v battery and you can get the same power output with much less current (and higher pedalling cadence).

      If you have any more questions, please let me know.

      All the best,
      Tony

      Reply
  • July 5, 2019 at 3:28 pm
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    HI we are looking for my wife`s first e bike obviously we are looking at second hand rather than new, We have found a bike that is 5 years old and hardly been used. The problem we have is that the seller has said that the battery is a 5Ah. Looking over the past few weeks I have only come across 10 ah or higher. Is the 5ah battery no obsolete and would it be easy to fit a 10 ah battery if needed, As you can see we are new to this and need some advice. I dont know what the voltage is of the motor at present. Thanks

    Reply
    • July 5, 2019 at 8:46 pm
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      Hi, There are still 5ah batteries available, but it is a very low energy capacity that would limit the electric assist range. If you can email me a photograph of the battery, I should be able to point you in the direction of a suitable replacement. My email address is cycletek@outlook.com. It would also be helpful to know the voltage, I would imagine it is a 36v, but it could possibly be 24v.

      Regards,
      Tony

      Reply
      • August 23, 2019 at 4:15 pm
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        Dear Tony,

        this is Ninh again. After the first 3 times of fully charging the battery, should I use/discharge it to below 50% and recharge it to 100% again?

        Kind regards,

        Ninh.

        Reply
        • August 23, 2019 at 11:31 pm
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          Hi Nihn,

          Yes, that charging / discharging process is correct.

          Regards,
          Tony

          Reply
          • August 24, 2019 at 2:22 am
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            Very much appreciated, Tony. Kind regards. Ninh.

          • August 24, 2019 at 9:26 am
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            No worries, anytime you need advice, let me know.

            Cheers,
            Tony

      • May 9, 2020 at 4:43 pm
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        What do you think of the small triangle frame battery bags? I really want to keep at least one water bottle. Is it possible to use a hailong type battery and still have a water bottle? Does having the weight higher in a battery frame make the bike handle funny compared to the hailong type? Are the battery bags as durable as the hailong? Thanks for the help!

        Reply
        • May 9, 2020 at 9:01 pm
          Permalink

          Hi,

          I personally quite like the smaller triangle bags available from UPP. I have fitted a couple in the past and I didn’t really notice too much difference having the extra weight higher up.

          If you have a large frame it is possible to mount a small Haliong battery (365 x 90 x 90mm) vertically on the seat tube, which would leave space for a bottle cage on the downtube.

          Another option would be to get seat post mounted bottle cages.

          All the best,
          Tony

          Reply
  • June 12, 2019 at 2:37 am
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    Very good Tony. I should have known you before making the order. But, it is not too late. Many thanks again for your patience in answering each of my questions and also for the values of the answers. Kind regards from Hanoi, Vietnam. Ninh.

    Reply
    • June 12, 2019 at 5:55 am
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      Hi Ninh,

      Glad to have been of assistance. If you ever need any help or advice in the future, please let me know.

      All the best,
      Tony

      Reply
  • June 11, 2019 at 1:16 pm
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    Dear Tony,

    many thanks for your advice. I was planning to stop asking you questions, but… still have one more. In this case, should I switch to a 250W motor while keeping the same battery or keep the 350W but get a 15A/20A battery?

    Kind regards,

    Ninh.

    Reply
    • June 11, 2019 at 2:31 pm
      Permalink

      Hi Ninh,

      Your 36v 8.8ah battery should work just fine with the 350w motor.

      As your battery uses Samsung cells (probably INR18650 29E) you can safely assume these lithium cells can sustain a higher discharge current 15-20A.

      The BBS01B 350w pulls a maximum of 18A in full power mode, but that current would taper off, when your motor rpm reached a certain speed.

      I have used similar batteries before with the 350w Bafang with no problems.

      I hope this answers you questions.

      All the best,
      Tony

      Reply
  • June 11, 2019 at 8:55 am
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    Dear Tony,

    I am not a native English speaker. I am not sure if I understand you correctly. In your second paragraph of the “Charging” section, do you mean that after 3 consecitive first full charges and discharges one should (or should not?) charge to 80%?

    And when the battery is 36V * 8.8mA = 316.8W, then it is below the BBS01 350W. Should it the battery be “below” or “above” the motor wattage in this sense?

    Many thanks.

    Ninh.

    Reply
    • June 11, 2019 at 11:01 am
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      Hi Ninh,

      With the first 3 consecutive charges it is advisable to run the battery down by at least 50% before recharging. You should also fully charge the battery in order to maintain cell balance. If you were to only charge to 80% the cells may go out of balance and your battery would not be running at its full potential.

      I have a friend who is an expert on Lithium batteries and he is of the opinion, charging batteries regularly to only 80% of their full capacity it detrimental rather than beneficial to the battery performance.

      The 36v x 8.8mAh = 316.8 watt hours – this relates to the total energy capacity of the battery. For example if you were using your motor at a constant 316.8 watts, it would deplete the battery in 1 hour. This would have no effect on motor compatibility. The continuous discharge rate of the battery in amps would be more relevant. Ideally this would need to be 15A minimum for your motor, but ideally around 20A.

      If you need any clarification, please let me know.

      All the best,
      Tony

      Reply
  • December 30, 2018 at 5:17 pm
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    Fantastic information. I’ve been looking for an unbiased report for a long time. Clear, informative and unbiased. ??

    Reply
    • December 30, 2018 at 10:11 pm
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      Hi Chris,

      Your comment is greatly appreciated.

      If you ever have any further questions regarding this topic, please feel free to contact me.

      All the best,
      Tony

      Reply
  • December 30, 2018 at 5:13 pm
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    Very interesting read, great write up

    Reply
    • December 30, 2018 at 10:08 pm
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      Hi Louise,

      Thank you for your comment. I’m glad you enjoyed the article.

      If I can offer you any further help or advice regarding electric bikes, please let me know.

      All the best,
      Tony

      Reply
  • December 30, 2018 at 5:13 pm
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    What a very helpful and informative article. I’m sure I will be in touch for further advice.

    Reply
    • December 30, 2018 at 10:06 pm
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      Hi Jon,

      Thank you for your comment. If I can offer you any help or advice in future, please let me know.

      All the best,
      Tony

      Reply

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