Page 472 - Battery Reference Book

The Eveready fast-charge cell (nickel-cadmium batteries) 47/5

and economic problems involved in fixing a pressure- of oxygen recombination at the negative electrode.
sensitive device to a standard cell or in designing a Unfortunately, the ability of most commercially
special pressure-sensitive cell. available nickel-cadmium cells to recombine oxygen
gas is rather limited, as evidenced by the acceptance
47.1.3 Electrolyte temperature of the C/10 overcharge rate. The cells can withstand
higher rates, but in overcharge at high rate
As shown in Figures 47.2 and 47.3, temperature is a only a fraction of the evolved oxygen will be
suitable indicator of the state of charge of a battery. recombined, resulting in heat generation. This change
Temperature is practically constant during the greater in temperature is gradual and difficult to detect.
part of the charge. When the fully charged state and The excess of unrecombined oxygen will result in
overcharge are reached, the heat generated by the internal pressure build-up and cell venting. Excessive
recombination reaction causes a rise of temperature. venting of cells may reduce useful capacity, and cause
This stabilizes at a level that depends on the rate dehydration and electrode imbalance.
of charge and the thermal exchange characteristics of TO accomplish charge termination safely and reli-
the cell or battery. The maximum rate of charge for ably through temperature sensing requires fast-acting,
the cell or battery corresponds to that at which the precise and expensive equipment at the lower charge
stabilized temperature is still acceptable; in general, rates. Because of the size, cost and complexity of
this temperature should not exceed 60°C. such a system, the thermal sensing approach to over-
Cell voltage rises at the moment the charging ends; charge control has previously been impractical for the
then the rise of temperature on overcharge has the consumer-oriented nickel-cadmium battery-powered
effect of reducing the voltage. The increase of voltage portable appliances and devices. To overcome these
at the end of charge is a function of the rate of charge problems, Union Carbide have developed a special
and the tempcrature. This characteristic is developed fast-charge sealed nickel-cadmium battery design (the
in the system of controlled rapid charge. Eveready cell) and a temperature-sensing high-rate bat-
One of the dangers of using electrolyte temperature tery charger design.
as a means of controlling battery charging is that, if The Eveready fast-charge cell has been specially
the battery is subjected to even short-term overcharge, designed to withstand high-rate overcharge (up to
the distinct temperature increase can cause physical the 1 h rate) and thus to overcome these drawbacks.
damage to the cell and/or loss of cell capacity. High Nevertheless, Eveready cells and batteries should not
cell pressures are also a possible source of danger or be continually overcharged at high rates. Undesirable
damage. cell degradation or heat build-up damage will occur if
Cell temperature monitoring has commonly been batteries are left on high-rate charge.
used as an indication of battery overcharge, the rise Eveready sealed secondary nickel-cadmium cells
in temperature being fairly slow and concurrent with and batteries are now widely used as a rechargeable
the rise in pressure. To achieve accurate temperature power source in many different types of portable
sensing it is necessary to have good thermal insula- or cordless electrical appliance. Charging at the safe
tion between cells and normal ambient temperature recommended C/10 rate has proved satisfactory for
during discharge. Without accurate control, cells may recharging the cells or batteries used in many of
vent before the charge is terminated, with consequent these appliances, such as toothbrushes, shavers, etc.,
dehydration and electrode imbalance. where relatively long rest periods between uses are
In the sealed nickel-cadmium system most of the possible. However, there is now a demand for the
oxygen evolved is reduced at the cadmium-metal use of sealed nickel-cadmium cells and batteries in
surface, with the consequent oxidation of the cadmium other appliances, such as chain saws, electronic Rashes,
to cadmium hydroxide. This statement certainly portable drills and professional hair clippers, where the
applies for normal rates of charge. However, the rate rest periods between uses of the appliances are much
of charge and temperature affect the recombination shorter; consequently shorter recharging times, from
of oxygen. Provided the charge rate is sufficiently about 3 h to about 1 h, that is, C13 to C/1 rates, are
high, the oxygen evolved for a given temperature required.
will not combine with the cadmium and the pressure The charging of sealed nickel-cadmium batteries
will build up within the cell. While the pressure is has been discussed further in a book published by Gate
rising, a pronounced rise in cell voltage will occur Energy Products (see Bibliography).
(approximately half the magnitude of that occurring in
open cells). This voltage may be detected and used to
terminate the charge. 47.2 The Eveready fast-charge cell
More recently, however, cell temperature monitor- (nickel-cadmium batteries)
ing has been developed as a means of charge control
as discussed below. As the design of the cell and the charger are, in this
Sealed nickeI-cadmium cells normally tend to case, intimately linked a description of the cell design
warm up om overcharge in proportion to the rate will be given first.