Page 145 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CH AP TER 6 .1 Battery/fuel-cell EV design packages

heat loss not removed by the cooling system which is and more than 2000 charge/discharge cycles can be
incorporated into the battery is stored in the heated-up sustained.
cells – and covers losses up to 30 hours. Additional heat The negative electrode is a hydrogen energy-storage
must be supplied for longer standstill periods either from alloy while nickel hydroxide is the positive electrode.
the electric mains or from the battery itself. Effective, An optimum design would have weight around 300 kg,
vacuum-type, thermal insulation maintains the power and capacity of 15 kWh, with life of 2000 discharge
loss at just 80 W so that when fully charged it can cycles. For buses Varta have devised a mobile charging
maintain its temperature for 16 days. In order to main- station, in cooperation with Neoplan, which will allow
tain the battery in a state of readiness, the battery must round-the-clock operation of fleets. This removes the
be held above a minimum temperature and it takes about need for fixed sites and allows battery charging and
4 – 10 hours to heat up the battery from cold – but a limit changing to be carried out by the bus driver in a few
of 30 freeze–thaw cycles is prescribed. Life expectancy minutes. The mobile station is based on a demountable
of the battery otherwise is 10 years and 1000 full dis- container which can be unloaded by a conventional
charge cycles, corresponding to an EV road distance of truck. Trials have shown that a bus covering a daily total
200 000 km. distance of 75 miles on a three-mile-long route needs to
stop at the station after eight journeys. Discharged
6.1.2.3 Nickel–metal hydride batteries are changed semiautomatically on roller-belt
arms, by a hand-held console.
As recently specified as an option on GM’s EV1, the
nickel–metal hydride alkaline battery, Fig. 6.1-3, was 6.1.2.4 Sodium chloride/nickel
seen as a mid-term solution by the US Advanced Bat-
tery Consortium of companies set up to progress battery Sodium chloride (common salt) and nickel in combina-
development. According to the German Varta company, tion with a ceramic electrolyte are used in the ZEBRA
it shares with nickel–cadmium cells the robustness battery, Fig. 6.1-4, under development by Beta Research
necessary for EV operation; it can charge up quickly and (AEG and AAC) and Siemens. During charging the salt is
has high cycle stability. The nickel–metal hydride how- decomposed to sodium and nickel chloride while
ever, is superior, in its specifications relative to vehicle during discharge salt is reformed. Its energy density of
use, with specific energy and power some 20% higher 90 Wh/kg exceeded the target set by the USA Advanced
and in volumetric terms 40% higher. Unpressurized Battery Consortium (80 Wh/kg energy density, to achieve
hydrogen is taken up by a metallic alloy and its energy 100 miles range under any conditions and 150 W/kg peak
then discharged by electrochemical oxidation. The raw power density to achieve adequate acceleration) and can
material costs are still signalling a relatively high cost but achieve 1200 cycles in EV operation, equivalent to an
its superiority to lead–acid is likely to ensure its place as 8 year life, and has a recharge time of less than 6 hours.
its associated control system costs are lower than those The USABC power to energy ratio target of 1.5 was
of sodium sulphur. Specific energy is 50–60 Wh/kg, chosen to avoid disappointing short-range high power
energy density 150–210 Wh/litre, maximum power discharge of a ZEV battery and for a hybrid vehicle a dif-
more than 300 W/kg; 80% charge time is 15 minutes ferent ratio would be chosen.

Fig. 6.1-3 Nickel–metal hydride battery.

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