So………what battery technology powers our electric bus?

Lincoln:Engineering Electric Bus prior to painting in the School Colours


ZEBRA Battery Pack

ZEBRA Battery pack installed in the vehicle

Our battery pack controller during charging, with battery temperature at 203 degrees C

Our bus is powered by a ZEBRA Sodium-Nickel battery pack……here’s some technical information………

The ZEBRA battery operates at 245 °C (473 °F) and utilizes molten sodium aluminumchloride (NaAlCl4), which has a melting point of 157 °C (315 °F), as the electrolyte. The negative electrode is molten sodium. The positive electrode is nickel in the discharged state and nickel chloride in the charged state. Because nickel and nickel chloride are nearly insoluble in neutral and basic melts, intimate contact is allowed, providing little resistance to charge transfer. Since both NaAlCl4 and Na are liquid at the operating temperature, a sodium-conducting β-alumina ceramic is used to separate the liquid sodium from the molten NaAlCl4. This battery was invented in 1985 by the Zeolite Battery Research Africa Project (ZEBRA) group led by Dr. Johan Coetzer at the Council for Scientific and Industrial Research (CSIR) in Pretoria, South Africa, hence the name ZEBRA battery. In 2009, the battery had been under development for more than 20 years. The technical name for the battery is Na-NiCl2 battery.

The ZEBRA battery has an attractive specific energy and power (90 Wh/kg and 150 W/kg). For comparison, LiFePO4 lithium iron phosphate batteries store 90–110 Wh/kg and the more common LiCoO2 lithium ion batteries store 150–200 Wh/kg. Nano Lithium-Titanate Batteries store energy and power of (116 Wh & 72 Wh/kg) and (1,250 W & 760 W/kg)[3]. The ZEBRA’s liquid electrolyte freezes at 157 °C (315 °F), and the normal operating temperature range is 270 °C (518 °F) to 350 °C (662 °F). The β-alumina solid electrolyte that has been developed for this system is very stable, both to sodium metal and the sodium aluminumchloride. The primary elements used in the manufacture of ZEBRA batteries, NaCl and Al have much higher worldwide reserves and annual production than the Li used in Li-ion batteries.[4] Lifetimes of over 1500 cycles and five years have been demonstrated with full-sized batteries, and over 3000 cycles and eight years with 10- and 20-cell modules. Vehicles powered by ZEBRA batteries have covered more than 2 million km. Modec Electric Van uses ZEBRA batteries for the 2007 model. The Th!nk City also uses ZEBRA batteries.[5]

When not in use, ZEBRA batteries are typically left under charge so that they will remain molten and be ready for use when needed. If shut down and allowed to solidify, a reheating process must be initiated that may require up to two days to restore the battery pack to the desired temperature and impart a full charge. This reheating time varies depending on the state-of-charge of the batteries at the time of their shut down, battery-pack temperature, and power available for reheating. After a full shut down of the battery pack, three to four days will usually elapse before a fully-charged battery pack loses enough energy to cool and solidify

About Paul Stewart

Professor of Control Engineering, and Pro Vice Chancellor Research at the University of Lincoln UK