Posted on

Battery technologies and BMS

While Lithium Nickel Cobalt Aluminum (NCA) batteries are the main stream in e-bike batteries, other technologies may take over. Battery Management Systems (BMS) are often the Achilles heel of e-bike batteries.

Less suitable chemistries

Li-cobalt oxide is cheap and has a good specific energy but cannot supply the 14-18A peak currents that Pedelec require. NiMH was used but has <50% of NCA’s specific energy Li-Titanate is even worse and Li-manganese is only slightly better.

Current chemistries

Li-Ferro Phosphate (LFP) has a thermal runaway temperature of > 250°C, making this is a very safe battery. The lifetime is also good, but the low specific energy (less than half of NCA) makes them less popular for light Pedelecs.

At 250-290 Wh/kg, Lithium Nickel Cobalt Aluminum (NCA) is the chemistry with the best specific energy but scores low on cost and safety. The lifetime varies, typically 500-1000 cycles, roughly half of LFP. Tesla uses thousands of 18650 NCA cells and increases the lifetime by better testing (no weaker cells in the pack), avoiding full discharge and –charge. Tesla features 8 years/80%/no cycle limit warranty, like other EV’s. E-bike batteries are rarely fully tested, even when this increases the yield… Reducing the operating range is not done.

Future chemistries

Lithium Nickel Manganese Cobalt (NMC) is used by Renault and BMW EV’s. According to Samsung, the current specific energy is 130Wh/kg (about half of NCA, but by 2019/2020 this will be 250-300 Wh/kg. NMC’s cost is now 12-19% higher than NCA. As this will likely become the mainstream in EV’s prices will go down, while NCA is closer to the bottom. NMC’s lifetime is far better than NCA and safety is also superior. NMC is a strong contender for future e-bikes.

Oxis Energy claims that Li-Sulfur batteries will be cheaper, more sustainable and will have the highest specific energy. In July 2013 the CEO stated that Oxis and GP Batteries are “good to go”, but there are no signs of mass production. Currently, versions with 300 Wh/kg are at par with high end NCA but have a lower cycle life. Industry forums report that Tesla is willing to evaluate new chemistries once they can get a working prototype, but did not get one. Li-sulfur has merits, but needs more time.

Battery Management Systems

Good BMS’s have enough temperature sensors, recognize individual cell group voltages and weaker cells before the pack is closed. This 100% testing ensures better yield, higher capacities and reduces warranty claims. A good BMS also registers mechanical shocks, deep discharging dates & data, so unjust warranty claims can be avoided. The best BMS’s also recognize moist in the pack to reduce accidents.

The Better BMS Group (founded by the CALCE test lab of the University of Maryland, high tech electronics specialist Engineering Spirit and myself) presented the BBMS specifications, based upon the above, at Eurobike 2014. A major e-bike systems company requested Engineering Spirit (a BCT partner) to design this BMS. Uniquely, they do not charge R & D costs and allow 3-party production. More info: https://engineering-spirit.nl/en/battery-management-system/