Lightweight Electric/Hybrid Vehicle Design

8 Lightweight Electric/Hybrid Vehicle Design
than conventional designs and can be charged and discharged rapidly. However, there is a trick to
achieving this. Most batteries are made up of ‘rectangular’ arrays of cells so it is no wonder that
the temperature of the cells varies with position in the stack. To charge a battery quickly it is vital
to keep the cells at an even temperature. Consequently it is necessary to liquid cool the cells so as
to obtain best performance and long life. Other points worthy of note are that batteries work best
when hot; 40°C is ideal for lead–acid. The battery electrolyte is just the place to dump waste heat
from the motor/engine/fuel cell.
Nickel–cadmium batteries offer better performance than lead–acid but are double the cost per
Wh of storage at present and sealed versions are limited to 10 Ah but larger units are under
development. The best nickel–cadmium units available at present are the SAFT STM/STH series.
Sealed lead–acid and aqueous nickel–cadmium cells have peak power in W/kg of 90 and 180,
with Wh/kg values being 35 and 55 respectively.
In terms of safety, long series strings of aqueous batteries are not a good idea. The leakage
from tracking is high and they are very dangerous to work on. Consequently batteries should
be of sealed construction with no more than 110 V in a single string. Ideally, the maximum
voltage should be 220 V DC, that is +/− 110 V to ground arranged as two separate strings
with a centre tap, so that no more than 110 V appears on a connector, with respect to ground,
Fig. 1.5.
There is an opening in the market place for a low cost 2 pole, 220 V, 300 A remote-control
circuit breaker to act as battery isolator with 5 kA short-circuit capacity. However, there is a
problem with earthing the centre tap of the battery as one may need an isolating transformer
in the battery charger. Consequently, in many of the new schemes proposed in the USA, a
different route is implemented which is used in trolley buses – the all-insulated system. In
most of these schemes, large capacity batteries are used (15–30 kWh) at a typical nominal
voltage of 300 V. This will vary from 250 V fully discharged to 375 V at the end of charging.
The electrical system is fully insulated from earth. During charging, the mains supply can be
either centre tap ground or one-end ground. In the centre tap ground (typical USA situation,
6.6 kV
60 Hz
100 V
100 V
100 V
Method of
connecting
battery to
ensure string
does not
exceed 100 V
Fig. 1.5 Battery connections and earthing.
1
2
110 V
B
R
~ ~
Ground
European
system
0 V
110 V
Vehicle
power
converter
Battery
-180 V
American
system Balanced voltage to earth
~
+180 V
230 V
Y
Vehicle
power
converter
100% ripple
with respect to
vehicle chassis
SYSTEM INSULATED
AND SCREENED FROM
VEHICLE CHASSIS
SYSTEM
INSULATED
AND SCREENED
FROM
VEHICLE CHASSIS
Battery