Electric Bike Battery (almost) know!

Electric Bike Battery (almost) know!

Electric Bicycle Battery: essential and expensive job of VAE

The battery is still the single most expensive electric bicycle. To a question of the initial cost, the electric bicycle may invest in a limited capacity battery to discuss then with autonomy. We need to pedal, anticipate, brake as little as possible and do not overtax the battery to be sure to get to the end of the journey. Most of the time there is no watt-meter installed and the driver is blind to tell us how to manage onboard energy.

Nevertheless, the objective is to define a long-term investment rather than an expense for too limited a system in the following lines we can find some lines of thought, knowing that these are generalities.

Rare earth: the inescapable China

Today production - extraction and refining - rare earth is 90% Chinese (in know more sur wikipédia). Cette activité est particulièrement nocive pour l’environnement et pour l’instant la Chine semble s’en accommoder : un marché avec 20% de croissance annuelle et de 120 milliards de dollar, ça ne se refuse pas. Avec ces terres rares des entreprises chinoises ou des joint-ventures (Sanyo, Panasonic, etc…) produisent des cellules permettant d’alimenter les smartphones, les ordinateurs portables, les fauteuils roulants, les autos et les motos électriques, sans oublier les vélos à assistance électrique (VAE). Le marché est si important d’un point de vue stratégique que c’est devenu un élément de la politique étrangère chinoise : le Japon a déjà été threatened to embargo !

Rare earth cells

A battery is made up of cells. By adding the cells in series and parallel, the desired tension is obtained. We will come back later on chemistry, just know that:

  • Li-ion technology and Lipo each cell has a nominal voltage of 3.7V and the maximum voltage was 4.2V. A 48V battery is composed of usually 13 cells, once the maximum charge voltage is 4.2V or 54.6V 13 *
  • LiFePO4 technology in each cell has a nominal voltage of 3.2V and the maximum voltage is 3.65V. A 48V battery is generally composed of cells 16 and when the battery is charged, the maximum voltage is 16 * 3.65V is 58.4V
Below is a diagram showing a battery 13 composed of nominal voltage 3.7V cells. Assuming that the capacity of each cell is of 10Ah, for a 48V 20Ah battery and it takes 13 * 2 or 26 cells (as well as BMS, see below):
BMS and Battery
The series connection of the cells is identified by the letter S. In 10Ah battery comprises 13 cells, it is said that it is a 13S1P, the "1P" indicates 1 parallel. In the case of our 20Ah battery with two groups of 13 cells, it is called 13S2P: twice thirteen cells.

Battery Management System (BMS)

The battery component cells are very fragile: it is essential to follow certain rules: do not exceed the maximum voltage of 4.2V, do not go below the minimum voltage. To do an electronic card manages:

  • la coupure haute et basse,
  • l & rsquo; discharge current and load
  • parameters d & rsquo; balancing cells
  • sometimes the temperature and d & rsquo; other parameters

BMS cells and determine the output capacity: a battery supplying capacity 20Ah 20Ah and delivers 1C. If the battery is capable of outputting 40Ah, so she's capacity 2C: Current calls are louder, but the time during which power is supplied is halved! The BMS is sized to the output capacitor.

About the capacity, it is important to understand the correspondence between the power required (by the controller) and the battery capacity. If your controller is given to 28Ah and 15Ah battery that you can come out only 20Ah then your configuration is not correct.

BMS defined associated with the rate of discharge cells. For example :

  • Maximum Continuous Discharge C-Rate: 30Amps / means that the battery can deliver 30 amps continuously
  • Maximum Discharge Current: 60Amps / means that the battery can deliver 60 Amps for a few seconds


A battery is composed of several identical cells. Lead batteries are no longer used for bicycles (except loss leader to formally advise against) and today three technologies are present, but not always easy to identify: Europe and China do not use the same terminology!

  • In China LiMnO4 and Li-NiCoMnO2 are called "Li-ion".
  • In which distinguishes Europe:
    • – Li-Ion pour des ions Li+ and une électrolyte liquide dans une enveloppe solide,
    • - Li-Po for a solid polymer to an easily identifiable flexible envelope
    • – LiFePO4 : Lithium Fer Phosphate

Warning: the first two technologies have cell 4.2V max and third of 3.65V: accordingly, never use a charger for a technology to another technology. For example, 48V Li-PO is charged to 54.6V and 58.6V LiFePO4!

Number of cycles

A very important information is the number of cycles. That is the number of times that we will be able to recharge its battery. In general the figures provided are optimistic, and measurement protocols are variable (not least in relation to use in real life).
Indeed, often the number of cycle is a discharge 1C *, on a landfill to 50% or 80%, at a temperature of 20 or 25 ° C! The reality is different, the 25 ° are present in the summer, but for the rest of time ... Be careful, the number of cycles is strongly determined by chemistry, as can be read below.
* C to estimate the time of charging and discharging. When empty a battery in one hour, then the vacuum in 1C. If using a battery of 10Ah and are consumed 20A continuous (regardless of duration), then one is being emptied in 2C. This also works for the load. A battery of 10Ah which is charged with a charger of 5Ah is charged at 0.5C. You will find Further explanation of C here.
Practical example: study of specifications for Zippy home Hobby King:
Max Charge Rate (C)2
On the above table have seen that the battery can be discharged at 30C. As she 5.8Ah, this is 174A! And for the load to 2C, we can send 5.8 * 2 = 11.6A.
Impact of the number of discharge C in the calculation of life: in the diagram below (source Battery University) It is seen that more intensely pulling the Li-Ion cell and measured the number of cycles is reduced:
Excuse me dear reader, but the article you're reading includes many general and as many shortcuts, the subject is inexhaustible and must achieve synthesize the main ideas in a nutshell.
To give an example of "truths" to put badly, here striking information provided by A123:
  • two A123 cells were discharged at 100% at 23 ° to 1C / discharge rate of 1C and load
  • after 20,000 cycles the cells still had 65% of their original capacity
Below the graph summarizing the test:

Comparison of chemistry

Li-Ion LiFePO4
Indicative * 48V 15Ah 299$ 409$ (+37%)
Number of cycles 800 2000 (+150%)
Weight 4.3Kg 6Kg (+40%)**
Protection VMax en volts 54.6 58.4 (+7%)***
Protection VMin en volts 35.75 32
  • The indicative price is for a battery purchased in China, without transport or import tax. For prices to your door, use a coefficient 1.5!

** The weight% is quite variable, from 7 to 8kg for a 20Ah, but according to the batteries can get to 10 or 11kg for a 48V 20Ah for the LiFePO4

*** A battery whose voltage is higher will drive faster. Empirically 48V = 48km / h, but it depends on the motor winding!

Some comments :

  • the original price is favorable Li-Ion but the operating cost is in favor of LiFePO4. An empirical calculation (15000km for a 20Ah Li-Ion 450 € and 30000km for LiFePO4 € 500 gives an operating cost of € 3 100km for the first and 1.67 for the second!
  • the energy density is favorable to the Li-Ion, this means that the Li-Ion weighs less and can be critical depending on the application

Autonomy and advice on capacity

Since the battery is expensive, we tend to calculate the fair. This is a mistake because:

  • a battery should never be emptied more than 80% of its capacity to preserve its number of cycles
  • when driving at 0 ° is consumed 30% more (as the internal resistance increases)
  • when the wind blows consumption explodes

This means that if you think a 10Ah battery is sufficient for your journey, it is advisable to take a 15Ah battery.

As seen above, adjust your controller and battery: a 20A controller perfectly with a battery that delivers 20A and continuously.

For numbers in actual use, see "In Practice" below.

Dumps, refills, maintenance, storage

The points below are tips to manage the battery:

  • Initially cell phones and their batteries NiCd, due to l & rsquo; memory effect of this chemistryThe advice was to completely drain and recharge "thoroughly" to break the memory effect. This idea there remained in the collective memory and some (ignorant) want to apply it to the Li-Ion chemistry or LiFePO4 battery current bike: it is a terrible and destructive error must avoid deep discharges
  • If your battery is Li-Ion type consider that each cell must not drop below 3.2V : For a battery composed of 13 cells, does not go below 41.6V, even if your BMS controller or have low lower cut
  • In addition to the preceding paragraph, the BMS is an electronic circuit that uses the battery power to operate, the cells tend to self-discharge; recharge as often as possible battery
  • there is no way to maintain its battery (you can not "change the oil filter" or "redo pressure") if not respecting the cells: high voltage low, no deep discharges . That said, as the cells tend to self discharge you may charge the battery at least once a month
  • et pour finir avec le stockage : la tendance à l’auto-décharge est moindre avec les basses températures : autant que faire se peut, cold store !

In practice

  • a 48V 20Ah battery LiIon
  • engine Nine Continent 9×7
  • about 70km a day of work in a year, about 400 recharge cycles (every 35km). Loss of capacity after 400 cycles: approximately 20% (it remains 16Ah)
  • the bike is used "scooter" mode, ie without pedaling

Below the monthly kilometers a year (GPS measurement, recording in RunKeeper) :

On the road :

  • average speed auto = 44km / h
  • average speed VAE = 42km / h
Below is a statement of time per kilometer (measured by GPS):

A few comments :

  • cold increases the internal resistance of the battery: in winter with temperatures around 0 ° consumption is 30% greater than 25 °
  • 55km autonomy with a 48V 20Ah battery and RH205 9×7 at 25 ° windless controller 24Ah flat course and without pedaling + 40km / h average
  • 41km autonomy with 48V 15Ah and motor RH205 9×7 at 25 ° windless controller 24Ah flat course and without pedaling + 40km / h average
  • decreasing the speed of consumption decreases sharply. With the 20Ah battery 30km / h can be counted on 80km of autonomy

Some real golf digits (Motor RH205 9×7, 48V 20Ah battery, controller 24A):

  • 38 km 41km / h average wind against 10km / h, max speed 48km / h, consumption 12A
  • Max speed 55.4km / h tailwind 30km / h
  • 36km to 43.7km / h average
  • 72km with a load, 35km in "scooter style" maximum speed, pedaling remains slightly

hobby batteries?

A priori modélismes batteries may represent an attractive alternative: the cost is limited and self-maintenance can make think playing with the packages. For example you can connect two LiPo 24V 8 Ah to be a 48V pack at low cost.

In fact, the only benefit is the cost a priori, but the reality looks like smoke and mirrors:

  • assembling a battery pack has a first disadvantage: the quality of received units. Indeed, it often happens that:
    • a batch of batteries is desired to be assembled, one of the packs is bad (defective primary cell) and it is the entire package that is bad (e.g., Hobby King 8000mAh 24V ...)
    • the announced capacity is off the mark (WinForce example, it takes six to 5000mAh to have the same performance of two Hobby King 8000mAh)
  • the self-assembled package has no BMS. So one can easily unload beyond the limit pack. This is not possible with a battery including BMS. We can solve the problem by making or purchasing a BMS ...
  • monitoring of each cell goes through a piping LiPo ... Or not! So, yet the connection to make, and do not forget to disconnect the piping will drain the battery
  • the load is painful, it can be charging each battery one by one (ideally, get up at night at the end of a battery to charge another) either buy the tow and load all in //. Avoid charging all this in the office, it will be!
  • You have to buy a specific charger and good quality, which is not cheap
  • In addition to the charger must buy a power supply to the charger

Finally assemble its own batteries is possible, but in terms of this cost is not necessarily the winner. Moreover, it will provide time, patience and testing before reaching a lasting solution. Besides all tow and boxes (charger, power supply, wafer to load // batteries) do not go unnoticed: it is very difficult to load discreetly in the office.




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