Electric bike battery: (almost) everything you need to know!

Electric bike battery: an essential and expensive part of the e-bike

The battery is still the most expensive component of an electric bike. For the sake of the initial cost, the electric rider may invest in a battery with limited capacity and then struggle with the range. You have to pedal, anticipate, brake as little as possible and not draw too heavily on the battery to be sure of reaching the end of the journey. Most of the time no watt-meter is fitted and the rider is blind to the information that would make it possible to manage the on-board energy.

Either way, the goal is to define a long-term investment rather than spending money on a system that is too limited: in the following lines you will find a few lines of thought, bearing in mind that these are generalities.

Rare earths: China is unavoidable

Today rare earth production – extraction and refining – is 90% Chinese (see more about it on Wikipedia). This activity is particularly harmful to the environment and for now China seems content with it: a market growing 20% a year worth 120 billion dollars is not to be turned down. With these rare earths, Chinese companies or joint ventures (Sanyo, Panasonic, etc.) produce cells used to power smartphones, laptops, wheelchairs, electric cars and motorbikes, not to mention electric-assist bikes (e-bikes). The market is so important strategically that it has become part of Chinese foreign policy: Japan has already been threatened with an embargo!

From rare earth to cells

A battery is made up of cells. By adding cells in series and parallel, you get the desired voltage. We will come back to the chemistry later; just remember that:

• with Li-Ion and LiPo technology each cell has a nominal voltage of 3.7V and the maximum voltage is 4.2V. A 48V battery is generally made up of 13 cells; once charged the maximum voltage is 13*4.2V i.e. 54.6V
• with LiFePO4 technology each cell has a nominal voltage of 3.2V and the maximum voltage is 3.65V. A 48V battery is generally made up of 16 cells and when the battery is charged, the maximum voltage is 16*3.65V i.e. 58.4V

The series connection of cells is identified by the letter S. On the 10Ah battery made up of 13 cells, we say it is a 13S1P, the “1P” meaning 1 in parallel. In the case of our 20Ah battery with two groups of 13 cells, we speak of 13S2P: two times thirteen cells.

Battery Management System (BMS)

The cells making up the battery are very fragile: certain rules must absolutely be respected: do not exceed the maximum voltage of 4.2V, do not go below the minimum voltage. To do this, an electronic board manages:

• the high and low cut-off,
• the discharge and charge current
• the cell balancing parameters
• sometimes the temperature and other parameters

The cells and the BMS determine the output capacity: a battery delivering 20Ah and of 20Ah capacity delivers 1C. If the battery is capable of delivering 40Ah, then it is of 2C capacity: current draws are higher, but the time during which the current is supplied is halved! The BMS is sized for the output capacity.

Regarding capacity, it is important to understand the match between the power requested (by the controller) and the battery capacity. If your controller is rated for 28Ah and your 15Ah battery can only deliver 20Ah, then your configuration is not correct.

The BMS together with the cells defines the discharge rate. For example:

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

The chemistry

A battery is made up of several identical cells. Lead batteries are no longer used for bikes (except for loss-leader products which we strongly advise against) and today three technologies are present, though 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 Europe we distinguish:
  • – Li-Ion for Li+ ions and a liquid electrolyte in a solid casing,
  • – Li-Po for a solid polymer allowing a soft casing that is easy to identify
  • – LiFePO4: Lithium Iron Phosphate

Warning: the first two technologies have cells with a 4.2V max and the third 3.65V: as a result, never use a charger meant for one technology with another technology. For example, at 48V Li-PO charges to 54.6V and LiFePO4 to 58.6V!

Number of cycles

Comparing chemistries

• The indicative price is for a battery bought in China, excluding shipping and import tax. For a price delivered to your door, use a factor of 1.5!

** The weight percentage varies quite a lot, from 7 to 8kg for a 20Ah, but depending on the battery you can reach 10 or 11kg for a 48V 20Ah in LiFePO4

*** A battery with a higher voltage will let you ride faster. Empirically 48V = 48km/h, but it depends on the motor winding!

A few comments:

• the initial price favours Li-Ion but the running cost favours LiFePO4. An empirical calculation (15000km for a 20Ah Li-Ion at €450 and 30000km for a LiFePO4 at €500) gives a running cost of €3 per 100km for the former and €1.67 for the latter!
• the energy density favours Li-Ion, which means the Li-Ion weighs less and this can be decisive depending on the application

Range and advice on capacity

Since the battery is expensive, people tend to calculate it too tightly. This is a mistake, because:

• a battery should never be discharged beyond 80% of its capacity, to preserve its number of cycles
• when you ride at 0° you consume 30% more (because internal resistance increases)
• when the wind blows, consumption explodes

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

As seen above, match your controller and your battery: a 20A controller will pair perfectly with a battery that delivers 20A or more continuously.

For real-life figures, see the “In practice” section below.

Discharges, recharges, maintenance, storage

The points below are tips for looking after your battery properly:

• In the early days of mobile phones and their NiCd batteries, because of the memory effect of that chemistry, the advice was to discharge completely and recharge “fully” to break the memory effect. That idea stuck in the collective memory and some (ignorant) people want to apply it to the Li-Ion or LiFePO4 chemistry of today’s bike batteries: this is a terrible and destructive mistake, you must avoid deep discharges
• If your battery is Li-Ion type, consider that each cell must not drop below 3.2V: for a battery made up of 13 cells, do not go below 41.6V, even if your BMS or the controller have lower cut-offs
• In addition to the previous point, the BMS is an electronic circuit that uses the battery’s energy to operate, and the cells tend to self-discharge: recharge your battery as often as possible
• there is no way to “service” your battery (you cannot “change the oil filter” or “redo the pressure”) other than by respecting the cells: high and low voltages, no deep discharges. That said, since the cells tend to self-discharge, you can recharge the battery at least once a month
• and finally about storage: the self-discharge tendency is lower at low temperatures: as much as possible, store it in the cold!

In practice

• a 48V 20Ah LiIon battery
• Nine Continent 9×7 motor
• about 70km per working day over a year, about 400 recharge cycles (every 35km). Capacity loss after 400 cycles: about 20% (16Ah left)
• the bike is used “in scooter mode”, i.e. without ever pedalling

Below, the monthly kilometres over a year (GPS measurement, logged in RunKeeper):

On the journey:

• average speed by car = 44km/h
• average speed by e-bike = 42km/h

A few remarks:

• cold increases the battery’s internal resistance: in winter with temperatures around 0° consumption is 30% higher than at 25°
• range 55km with a 48V 20Ah battery and the RH205 9×7 at 25° with no wind, 24Ah controller, flat route and no pedalling at an average above 40km/h
• range 41km with a 48V 15Ah and the RH205 9×7 motor at 25° with no wind, 24Ah controller, flat route and no pedalling at an average above 40km/h
• reducing the speed greatly reduces consumption. With the 20Ah battery at 30km/h you can count on 80km of range

A few real journey figures (RH205 9×7 motor, 48V 20Ah battery, 24A controller):

• 38 km at a 41km/h average, 10km/h headwind, max speed 48km/h, consumption 12A
• Max speed 55.4km/h with a 30km/h tailwind
• 36km at a 43.7km/h average
• 72km on one charge, 35km in “scooter mode” at top speed, the rest pedalling lightly

Hobby (RC) batteries?

On the face of it, hobby batteries can look like an attractive alternative: the cost is limited and you may think you can do the maintenance yourself by playing with the packs. For example you can assemble two 24V 8Ah LiPos to make a low-cost 48V pack.

In reality, the only advantage is supposedly the cost, but reality is a mirage:

• assembling a battery pack has a first drawback: the quality of the units received. Indeed, it often happens that:
  • in a batch of batteries you want to assemble, one of the packs is bad (one defective primary cell) and the whole pack is bad (for example Hobby King 24V 8000mAh…)
  • the advertised capacity falls far short (WinForce example, you need six 5000mAh ones to get the same performance as two 8000mAh Hobby King packs)
• the pack you assemble yourself has no BMS. So you can easily discharge the pack beyond the limit. This is impossible with a battery incorporating a BMS. You can solve the problem by building or buying a BMS…
• monitoring each cell is done through a LiPo screamer… Or not! So, more wiring to do, and above all do not forget to unplug the screamer, which will discharge the battery
• charging is a pain: you can either charge each battery one by one (ideal: get up at night at the end of one battery to charge the other) or buy a mess of cables and charge it all in parallel. Avoid charging all that at the office, it will show!
• You need to buy a specific, good-quality charger, which is not cheap
• In addition to the charger you need to buy a power supply for the charger

In conclusion, building your own batteries is possible, but in terms of cost it is not necessarily a win. Moreover, you will need to plan time, patience and trials before reaching a durable solution. Not to mention that all the cabling and boxes (charger, power supply, board for parallel charging, batteries) hardly go unnoticed: it is very difficult to charge discreetly at the office.