Page 139 - Battery Reference Book

Silver-cadmium secondary batteries 515

and weight savings. Because it is capable of delivering The data below indicate, in brief, the performance
high watt hour capacities at discharge rates less than capabilities achieved with silver-zinc cells supplied by
30 min, the silver-zinc battery is used extensively, Yardney showing the requirements which can be met:
for example, in missile and torpedo applications. Its
high energy density makes it attractive in electronics 1. liibration: 30-40 sinusoidal vibration.
applications, satellite and portable equipment where 2. Random Gaussian vibration: 5-2000 Nz band,
low weight, high performance and low cost are prime with as high as 60g r.m.s. equivalent in both
considerations. It is highly efficient and mechanically non-operational and operational conditions; hard
rugged, operates over a wide temperature range, and mounted unit.
offers good shelf life; quick readiness for use and 3. Shock: tested up to 200g in all directions.
the ability to operate at temperatures down to -40°C 4. Acceleration: lOOg in all directions of sustained
without heating, and up to 73T, are two of the features acceleration, except in the direction of the vent for
of this battery. The corresponding battery storage vented cell systems.
temperatures are -53°C and 73°C. Eagle Picher claim 5. Thermal shock: from +70 to -50°C and repeated,
a dry shelf life of 2.5 years and a wet shelf life of up to with stabilization occurring at each temperature.
1 year at 21°C for their silver-zinc batteries. In excess 6. Altitude: successful activation, and operation at a
simulated altitude of 60km, for a period of 1 h
of 500 cycles are claimed with a charge retention of up without pressurization; with pressurization there is
to 1 year at 26°C. Either constant-current or constant- no limit.
potential (I .96- 1.98 V) charging methods can be used
and 16h is a typical recharge time. Table 5.3 gives the basic characteristics of the high-
The extremeliy low internal resistance of silver-zinc rate and low-rate versions of the Yardney Silvercel.
batteries permits discharges at rates as high as 30 times
the ampere hour capacity rating, and its flat voltage
characteristic enables highest operational efficiency 5.3 Silver-cadmium secondary
and dependability. Cells have been built with capacity batteries
ranging from 0.1 Ah to 20 000 A h.
The cells a:re leakproof and spillproof. The sil- The silver-cadmium battery has a voltage of
ver-zinc battery is available in high-rate and low-rate 1.4-1.1 Vkell (open circuit), 0.9-1.14V/ce!l (closed
cells, Le. it cam be subject to high or low rates of circuit) and combines the high energy and excellent
discharge. space and weight characteristics of the silver-zinc
Rechargeable silver-zinc batteries are available as battery with the long life, low-rate Characteristics
cells for discharges at high discharge rates (high-rate and resistance to overcharge of the nickel-cadmium
batteries) where the total energy must be delivered battery. Operating temperatures are between -43°C
within 1 h or as low-rate versions with discharge times and 75°C. The silver-cadmium battery also provides
of 2h or more. The latter type can be discharged high efficiency on extended shelf life in the charged or
in from 2h down to a few minutes. The high-rate the uncharged condition, level voltage and mechanical
ruggedness. Watt hour capacity per unit of weight
and low-rate systems differ principally in their elec- and volume is two to three times greater than
trode surface areas and the thickness of the separators that of a comparable nickel-cadmium battery, and
between the plates, both of which have an effect on the silver-cadmium battery has superior charge
the internal resistance of the cell. retention. The principal characteristics of silver-zinc
Silver-zinc batteries are also available in modular and silver-cadmium cells and batteries are compared
packs; one supplier (Yardney) supplies packs in nor- and their important differences shown in Table 5.4.
mal capacities between 3 and 80Ah. These batteries The silver-cadmium battery promises great weigh:
have voltages 'between 2.8 and 15.8 V and equivalent and space savings and superior life characteristics
energy densities of about 100 W Nkg, which exceeds to those of nickel-cadmium batteries cunently in
the energy of a. nickel-cadmium battery by four times use as storage batteries in most satellite programmes.
and a lead-acid battery by five times. They are, of Yardney have reported 6000 cycles with a 50% depth
course, more expensive, but in many applications their of discharge in satellite applications.
characteristics make them an attractive alternative. Prior to the introduction of silver-cadmium batter-
Untii recently, the fact that the rechargeable sil- ies most available types of battery fell into the cat-
ver-zinc battery has been more expensive and more egories of low-energy long-life types or high-energy
sensitive to overcharge and has a shorter cyclic life short-life types. Silver-cadmium batteries go a long
than conventional storage batteries has limited it to way to combining the best features of both types giv-
applications where space and weight are prime consid- ing two to three times more energy density per unit
erations. However, long-life silver-zinc batteries have of volume than the lead-acid, nickel-cadmium and
now been devdoped in which up to 400 cycles over a nickel-iron systems, and at the same time offering
period of 30 months can be achieved with ease. ruggedness and long life (Figure 5.2).