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Abstract

I want to develop a fast rechargeable battery for electric bicycles. The battery must be able to be charged in 20 minutes and, moreover, the battery must be as lightweight as possible.

Fast charging - slow discharging

Ideally, when using electric vehicles, you want to charge the battery in a few minutes and then drive all day long, just like when refueling gasoline. However, in practice, the situation is reversed: most lithium-ion batteries can be discharged fast and may only be charged slowly.

Charging batteries of electric bicycles normally takes several hours, this means that you can hardly recharge during a trip. To drive long distances with motor-support, a large battery is required which weighs more than 3kg (note that I made a 12.5kg ebike, so every gram counts). But if we had a battery that can be charged very fast on the go, it does not have to be that large. Therefore, I want to build a small and lightweight ebike battery that can completely recharge on the go in just 20 minutes.

Fast charging batteries for e-bikes
Fast charging batteries for e-bikes

This is the lightweight battery of the Maxun One solar bike:

Ultra fast charged lightweight ebike battery
Ultra fast charged lightweight ebike battery

The required capacity for a fast charging e-bike battery

With an electric car, the battery capacity must be larger than strictly necessary because you never want to end up with an empty battery. With an electric bicycle, this is not a big problem since you can always pedal. So, to save weight, we don’t take the battery larger than strictly necessary.

The most used electric bicycle batteries have a capacity of 400Wh with a weight of about 3kg. Much more capacity is needed for intensive driving in the mountains. But for a fast charging battery, the capacity can be smaller because we can fully re-charge several times during a trip. Suppose we re-charge every 2.5 hours and we want a quite strong motor support of 100W for driving hills, then the required battery capacity is about 250Wh. 

Lightweight 1000W battery charger

The battery charger that I'm developing, is placed in the battery housing. It should charge a 250Wh battery in 20 minutes, so the output power should be at least 750W. Because I want a weight below 500g, that will be quite a challenge. A weight of 500g is less than 1/3 of the normal weight of battery charger of 750W.

Evaluated batteries

Fast charging e-bike batteries NCR18650B ANR26650 Turnigy 1200mAh
Fast charging e-bike batteries NCR18650B ANR26650 Turnigy 1200mAh

Most lithium-ion batteries don’t tolerate fast charging, therefore only batteries that allow fast charging are examinated:

  1. A123 ANR26650 because its lithium iron phosphate chemistry (LFP) withstands high charge currents without any harm. The disadvantage is the low specific energy of 108Wh/kg.
  2. Panasonic NCR18650B because this is used in the Tesla S and X and are fast charged by Tesla. High specific energy of 250Wh/kg.
  3. Turnigy nano-tech because the maximum charge speed is 12 minutes (4C) and it it has a high specific energy of 180Wh/kg.

The following parameters are of interest:

  • Capacity in [Ah]
  • Energy capacity in [Wh]
  • Specific energy [Wh/kg]
  • The maximum allowed charging rate [C] *)
  • The internal resistance Ri [Ω]

*) The C-rate of a battery is the maximum discharge or charge current value compared to the capacity in ampere-hour of the battery. For example: a battery with a capacity of 1200mAh and a charge rate of 0.5C. The maximum charge current is 0.5 * 1200mA = 600mA and the minimum charge time is 1/C = 2 hours.

Fast charging loss compared

Fast charging increases the temperature of the batterie cells. If the temperature of a battery becomes too high, this can not only reduce the lifespan but also cause fire. 
I have simplified the power loss (Ploss) as being only caused by the internal resistance R and the charge current. Note that in practice, R is not constant but increases with SOC.

The battery configuration has been determined in such a way that the batterypack corresponds to 36V and 250Wh as much as possible.

Relative power loss (rpl)

To be able to compare the battery cells, I introduce a new term: relative power loss (rpl). This is the power loss relative to the battery energy capacity in Wh.

cr: charge rate
P: total battery pack power loss during charging
Ah: battery cell current capacity
Whc: battery cell energy capacity
Wht: total battery pack energy capacity
R: battery internal resistance
n: number of battery cells
rpl: relative power loss

n = Wht /  Whc
P = (cr * Ah)^2 * R * Wht / Whc
P = cr^2 * Wht * {Ah^2 * R  / Whc}
rpl = Ah^2 * R  / Whc
P = cr^2 * Wht * rpl

Panasonic NCR18650B

Discharge characteristics Panasonic NCR18650B
Discharge characteristics Panasonic NCR18650B

Notes

  • Used in the Tesla Model S and X
  • Chemistry: nickel cobalt aluminum (NCA) LiNiCoAlO2

Capacity: 3350mAh
Weight 47.5g
Energy capacity: 11.5Wh
Specific energy: 240Wh/kg
R: 35mΩ measured by myself (note: the required cooling will add extra weight)
Maximum charge current by Panasonic: 0,5C
rpl = Ah^2 * R / 11.5 = 0.034

Battery configuration: 10s2p
Battery pack energy: 10 * 2 * 11.5Wh = 230Wh
Battery pack weight: 1kg
Power loss at 3C charging per battery pack: 3^2 * 0.034 * 230 = 70W.

Note that the Specific energy of a complete Tesla model 3 long range battery pack (with cooling) is 167Wh/kg.

Lithium Werks (A123) ANR26650m1B

LiFePO4 discharge characteristics
LiFePO4 discharge characteristics

Notes

  • LiFePO4 is intrinsically safer than other Lithium-ion batteries.
  • Very good cycle life > 4000 Cycles @ 1C/1C
  • Lithiumwerks

Capacity: 2500mAh
Weight 76g
Energy capacity: 8,25Wh
Specific energy: 108Wh/kg.
Maximum charge current: 4C
R: 18mΩ measured by myself (in datasheet: 6mΩ)

rpl = Ah^2 * R / 8.25=  0.014

With the ANR26650m1B cells we cannot build a 250Wh battery, but we can connect APR18650m1B 1200mAh cells in parallel to build a 240Wh battery.
Battery configuration:
ANR26650m1B 12s2p for 200Wh. Weight: 1.8kg
APR18650m1B 12s1p for 43Wh. Weight: 0,47kg
Total battery pack weight: 2,3kg

APR18650m1B
Capacity: 1200mAh
Weight 39,5g
Specific energy: 92Wh/kg
Energy capacity: 3,63Wh

Power loss at 3C charging for 200Wh: 3^2 * 0.014 * 200 = 25W.
Power loss for 240Wh would be approximately 240/200 * 25W = 30W.

Turnigy LiPo nano-tech 1200mAh

Discharge characteristics Turnigy LiPo nano tech 1200mAh
Discharge characteristics Turnigy LiPo nano tech 1200mAh

https://hobbyking.com/en_us/turnigy-nano-tech-1200mah-1s-15c-round-cell.html?___store=en_us
https://en.wikipedia.org/wiki/Lithium_polymer_battery
https://rogershobbycenter.com/lipoguide

Notes

  • No longer available
  • There are no datasheets available
  • LiPo has limited lifespan
  • A LiPo cell should not be discharged below 3.0V. For the longest battery life, LiPos should be stored at room temperature at 3.8V per cell.

Capacity: 1,2Ah
Weight 23g
Specific energy: 180Wh/kg
Energy capacity: 4.1Wh
Maximum charge current: 5C
R: 70mΩ measured by myself

rpl = Ah^2 * R =  0.025
Battery configuration: 10s6p. Weight = 1,4kg
Battery pack energy: 10 * 6 * 4.1 = 246Wh
Power loss at 3C charging per battery pack: 3^2 * 0,025 * 246 = 55W

Summary 

  • Panasonic NCR18650B
    A battery pack of 230Wh weighs 1kg. This is a big advantage.
    The power loss with fast charging is 70W.
    The disadvantage is that Panasonic doesn't recommend fast charging. As we will see later, Panasonic batteries are not a good solution.
  • Lithium Werks (A123) ANR26650m1B
    A battery pack of 240Wh weighs 2,3kg. The power loss with fast charging is 30W.
    The disadvantage is that the battery pack is with 2,3kg heavy. Still, this is the best solution as we will see later.
  • Turnigy LiPo nano-tech 1200mAh
    A battery pack of 250Wh weighs 1,4kg. The power loss with fast charging is 55W. 
    The disadvantage is that there is too much hassle with safety and the lifespan is too short. So this is not a good choice.

Measuring the capacity fading

EBC A40L battery tester

With my EBC A40L battery tester I can do cyclic charge - discharge tests automatically. Measuring 500 cycles of 20 minutes fast charging and 20 minutes discharging, will take 14 days, this is still manageable. In order to be able to compare the batteries fairly, the cells must be kept at the same temperature. 

Tesla Supercharger V3 charge profile

The Panasonic NCR18650B battery is used by Tesla Model S and X. In contrary to what's stated in the datasheet, it can apparently be fast charged as Tesla does. This makes this battery interesting for use in the fast-charging electric bicycle battery.

Tesla battery pack Panasonic batteries

On the forum of the Teslamotorsclub, you can find the charge profiles for the Tesla Model 3:

Tesla Model 3 Long Range on Supercharger V3
Tesla Model 3 Long Range on Supercharger V3

From this, I made a graph with the charge rate in C versus SOC: 

Tesla model3 long range charge profile
Tesla model3 long range charge profile

We can draw a number of conclusions from this graph:

  1. Tesla charges in 30 minutes from 0% SOC to 100% SOC (state of charge).
  2. The Panasonic batteries seems to be extremely overloaded by Tesla, because Panasonic recommend a charging current of just 0.5C. But Tesla use effective cooling, so this is apparently possible without causing damage to the batteries.
  3. To be able to charge in 30 minutes, the average charging current must be 2C. But because the charging current must decrease at the end of the charging cycle, Tesla increases the charging current at the start, so the average current is still 2C. The charging current of 3.3C is applied just for the first 6 minutes. Obviously this is harmless.
  4. Note that the chart is for the Tesla Model 3 which uses 21700 cells instead of the 18650 cells. But I think the charge profile also applies to the 18650 cells because both batteries have the same specific energy, so I suspect that the chemistry of both batteries is the same.
  5. Note that, to enhance the battery's longevity, Tesla advice that the battery level stays between 20% and 90%. But the graph above goes from 0% to 100% SOC. What does this mean for the battery cycle life with frequent fast charging from 0% to 100%?

Conclusion

On closer inspection, the fast charging solution from Tesla is unsuitable for my bicycle battery: I want to charge faster than Tesla does, namely with 3C instead of 2C. In addition, the charge current should be 1.5 times higher in the beginning with NCA batteries. That is a problem because then the battery charger has to deliver 1.5 times more power, (1100W vs 750W) which makes the charger heavier.

LFP has serious advantages:

  • Can be charged with 3C from 0% to 100% SOC.
  • Withstand frequent fast charging for a long time.
  • The voltage remains almost constant during discharge.
  • Much less incendiary.

I'm looking for fast charging (3C) LFP batteries with a specific energy above 140Wh/kg, who can help?

Do you have any comments about the website? Please let me know.