Lightweight Electric/Hybrid Vehicle Design

2
Viable energy storage systems 29
Viable energy storage systems
2.1 Electronic battery
Electric vehicles are at a historical turning point – the point where technology permits the
performance of electric vehicles to exceed the performance of thermal engines 1 . Currently quality
battery technology is expensive and heavy. This favours hybrids with small peaking batteries –
typically 10 Ah at 300 V, with a capability of 70 kW for 2 minutes. New battery geometries have
been developed for this application in the form of high performance D cells with 6 mm thick end
caps and M6 terminals. A single string of cells handles 100 amps for 10 seconds. The heat is
transferred to the end caps then removed by forced air cooling. It is vital to maintain an even cell
state of charge in long strings. This solution has two drawbacks at present: (1) Cost – a 300 V 6.5
Ah stack costs more than $10 000 in January 98; (2) reliability – with only one string a single high
resistance cell disables the battery.
Using the best available nickel–cadmium cells, it is possible to build a reliable peak power stack
and use electronic control to maintain equal currents in strings. A later section will consider the
next generation, an all electric hybrid with aluminium battery and alkaline fuel cell; this last
important energy storage system is also discussed in depth in Chapter 4. A review of different
battery types and performances is given in Chapter 5.
2.2 Battery performance: existing systems
All battery technologies can offer some solution to the peak power problem but there is only one
parameter which ultimately matters, and this is the internal resistance of the cell. This is much
more related to cell geometry than cell chemistry, as we shall discover. When AA, C, D and F cells
were originally designed, nobody was thinking of discharging them at hundreds of amps, so it is
hardly surprising that they are not ideal for the purpose. This problem will become even more
extreme as power density improves:
D cell characteristics
Lead Acid NiCad NiMh Lithium Aluminium
Amp Hr (20hr) 2.5 4.0 6.5 7.5 50
Cell voltage 2.0 1.2 1.2 3.6 3.0
Max C rate 40.0 25.0 11.0 40.0 Unknown
Ah/25°C 0.5 3.0 5.0 6.0 Unknown