Page 391 - Battery Reference Book

3316 Nickel batteries
deliver very high rates of discharge for short periods. 15-module arrangement, connected in series, would
For example, the starting of light-weight motor cycles yield a 28.8 k W h (U3) total energy content battery
requires a breakaway current of 30-50A followed at a nominal 96V.
by a discharge for several seconds at 20-25A at a The cell design uses electrochemidly impregnated
nominal 12V. Recharge must be rapid in order to sintered nickel positive electrodes and rolled-bonded
be sufficient, since runs are short and uncontrolled. zinc oxide negative electrodes. The separator is a
All these requirements can be satisfactorily met by three-part system using proprietary Yardney separators.
using a battery of 0.5 Ah capacity, with the following Factors affecting cycle life and capacity maintenance
characteristics: include depth of discharge, operating temperatures,
vehicle design, battery mounting provisions, propul-
1. Voltage at end of starting at 20°C: 11 V.
2. Voltage at end of starting at -10°C: 9V. sion system design (that is, operating currents, pulse
3. Uncontrolled rapid charge rate 0.8 A, which permits magnitudes, etc.) and frequency of maintenance.
recharge after starting in less than 3 min. The nickel-zinc battery is lighter and smaller for a
given energy output and is capable of a higher specific
The same type of application is the starting of a power than either the lead-acid or nickel-iron systems
lawnmower, where the breakaway current may be as and is consequently a prime candidate for a future elec-
high as 75A. The power necessary is supplied by a tric vehicle power source. These features affect vehicle
1.2 Ah battery. performance beneficially by both extending range and
The nickel-cadmium battery is now the type used improving acceleration and operation at lower temper-
almost exclusively for engine starting in larger aircraft. atures. The major limitation of the nickel-zinc battery
Batteries with capacities up to 200Ah are now avail- system is its relatively short life. Extensive use of
able. Lead-acid batteries are used in the majority of nickel-zinc batteries, particularly as electric vehicle
small private aircraft and in some larger aircraft, e.g. power sources, will depend on significantly extending
the UK Harrier 'Jump Jet' and BAC Concorde. the deep discharge cycle life of full batteries beyond
the current level of 100-300 cycles. The major prob-
lem with the nickel-zinc battery arises from the high
33.2 Nickel-zinc secondary batteries solubility of zinc oxide, a discharge product of the
Nickel-zinc batteries may be the batteries of the future negative electrode, in the battery's alkaline electrolyte.
in applications such as utilities load levelling and This results in zinc dendrite formation under charging
electric vehicles, and indeed the nickel-zinc battery conditions, as well as shape change and densification
is now in commercial production in the USA. of the negative electrode on repeated charge/discharge
Nickel-zinc cells have a voltage of 1.85V at cycles. In addition, active material may be lost from the
25°C and a high energy density (75Wh/kg-', negative electrodes through deposition of zinc oxide in
150 W Wdm-'). the separators and negative electrodes of the battery.
Each of these factors both reduces cycle life and is
significantly influenced by the separator. Workers at
33.2.1 Electric vehicles W. R. Grace and Co., battery separator manufactur-
ers, at Maryland, USA, have studied this problem of
In an effort to broaden the field use of the nickel-zinc
system and to provide an early demonstration of the extending the deep discharge cycle life of nickel-zinc
state of the art of this system in vehicle propulsion, batteries and concluded that cycle life appears to be
Yardney are making available an unoptimized pro- related to the separator's mass transport properties.
totype battery module of 300 Ah nominal capacity, Batteries containing separators with the lowest elec-
which can be assembled into a vehicle battery of trolytic resistivity and highest water permeability give
the desired voltage; thus baseline performance can be the longest cycle life. The results favour the use of
established on which future performance can be pro- microporous separators with an average pore diameter
jected. A specially designed charger is available, which of about 300 x lo-'' cm which provide the necessary
maximizes the capacity maintenance and cycle life of mass transfer while retaining adequate dendrite penet-
this prototype battery. ration resistance.
The battery is 264 mm wide, 179 mm deep, 285 mm For best battery performance, the Yardney nickel-
high and weighs 2.9 kg. At the C13 discharge rate, the zinc battery charger should be used. This battery
nominal voltage is 6.4V and the nominal capacity is charger is basically a voltage-controlled current source
300 Ah. The estimated cycle life is 200-400 cycles at with preregulation to minimize internal power dis-
80% depth of discharge. sipation and maximize efficiency. During the initial
The module consists of four series-connected and final charge rate step, the unit is a constant-
300Ah cells providing 6.4V. The cells may be current charger. When charge is initiated, the battery
installed in any essentially upright orientation provided is charged at a high rate (60A) until the gas flow
that the 179mm x 285mm face is constrained by from a single pilot cell reaches a specific level, at
the battery box or by adjacent modules. A typical which point the charger automatically switches over to