Peformance of Akasol's OEM NMC Lithium Ion Battery Packs for the BEV and Hybrid Electric Bus Market

A German company called Akasol is manufacturing lithium in battery packs for hybrid and electric bus markets.They use batteries with the nickel-manganese-cobalt (NMC) cathode chemistry which is widely used in electric vehicle applications. This is the cathode chemistry used in the batteries produced in Tesla's lithium ion Gigafactory. Akasol uses two versions of this cathode, a standard version and so-called nano-version. Characteristics of battery packs produced using these cathodes (taken from the data sheet on Akasol's website) are listed below:

Standard: Mass=353kg, Volume=250liters, Capacity=35.3kWh, cycle life>3100 at 80% DOD

Nano: Mass=372, Volume=250liters, Capacity=30.6kWh, cycle life>6800 at 80% DOD

One can use these number to calculate the effective energy density of the whole battery pack:

Standard: gravimetric density = 100Wh/kg, volumetric density = 141Wh/liter

Nano: gravimetric density = 82Wh/kg, volumetric density = 122.4Wh/liter

These effective battery pack energy densities are much smaller than the typically quoted energy density for individual NMC batteries (i.e.>200 Wh/kg). Of course the real battery back mass includes the cooling system, battery management system, and the enclosure

The most impressive performance parameter of these batteries is the cycle life of 3100 for the standard cathode and 6800 for the nano-cathode. What lifetime mileage range can be obtained with these packs depends on the performance of the electric vehicle and the number of packs employed. A NREL study of a fleet of twelve 35 foot long, 35 seat BEV Proterra buses run for an average 33 thousand miles each found thate the average milage was 2.15miles/kwh using 88kWh battery packs. Obviously such buses required frequent charging which was provided by fast charging stations along the bus route. The Proterra buses which took part in this study used lithium titanate cathodes which actually have significantly lower energy density than NMC cathode batteries. Three packs of the Nano-cathode NMC battery would give roughly the same capacity as in the NREL study. If we assume that because of higher energy density we could use four packs with the same weight as the lithium titanate cathodes then the total life time milage range (assuming 2.15miles/kWh) would be:

Life Time Range = 0.8×30.6×4×6800/2.15 = 0.8×30.6×4×6800/2.15 = 308,700 miles.

Therefore, in spite of the low effective energy density, the life time range seems very respectable. Aksol's short term market for these batteries is primarily electric buses, and it is building a new facility with capacity to manufacture 300MWh/year of battery packs

Nov 13, 2017

rogerkb at energystoragenews dot com