Page 473 - Battery Reference Book

47/6 Constant-current charging

47.2.1 Cell design 47.2.2 Charger design
This cell has been designed to exhibit sufficient tem- The Eveready fast-charge cell exhibits a relatively
perature rise to effect charge control without a sig- sharp rise in temperature during high-rate overcharge.
nificant change in operating pressures. The Eveready The particular type of thermal sensor to be used in
fast-charge cell series develop the desired temperature combination with the cell or battery and the charger
rise, and have the built-in ability to withstand short- system is not critical. Probably the least expensive
term overcharge at rates to 1 h values without physical overall cell or battery control unit is provided by use of
damage or loss in cell capacity. The cell construction a simple snap-action thermostatic switch, which com-
is specifically designed to withstand overcharge at the bines the temperature-sensing and circuit-switching
3 h rate without special control circuitry. Considerable functions in one small, inexpensive device that can
heat can be generated within the cell, however, if over- be easily attached to the cell or battery.
charge is extended beyond a reasonable period of time. A solid-state thermistor sensor may also be used.
To prevent this heat from causing gradual cell degra- The thermistor is also relatively inexpensive and is
dation, it is recommended that the cell temperature even more compact, although it performs only the
should not exceed 46°C during this extended over- function of a sensor. Auxiliary circuitry and a switch-
charge and that the cells be removed from the charger ing means are required to cut off the charging current
within 2 or 3 days of reaching full charge. in response to the thermistor input. Among the com-
Before the introduction of this construction by mercially available types of thermistor, the positive
Union Carbide, any cell overcharged at the 1 h rate temperature coefficient type is preferred because it
would be permanently damaged. The Eveready fast- changes resistance abruptly at a predetermined temper-
charge cell can withstand overcharge at these high rates ature. Auxiliary circuitry is therefore simplified with-
long enough for the temperature rise to be sensed by out loss of reliability.
simple control elements. This temperature rise is very In constructing individual cell or battery units, it is
pronounced, and provides a positive signal for charge not critical that the thermal sensor be placed or main-
control. As a result, the control element can be small, tained in actual physical contact with the cell proper,
lightweight and inexpensive. although this is preferred. Individual cell units may
Sealed secondary nickel-cadmium cells have been
manufactured for many years based on the so- be constructed with a small flat disc-type thermostatic
switch welded in contact with the bottom of the cell.
called ‘oxygen recombination’ principle. The charge- Similar battery units may be constructed with a small
accepting capacity of the negative electrode is made thermistor or bimetallic switch placed in the space
to exceed the charge-accepting capacity of the positive between adjoining cells. Any arrangement is satisfac-
electrode. On charging, the positive electrode reaches tory provided the thermal sensor is well exposed to the
a state of full charge before the negative electrode,
and oxygen is evolved at the positive electrode. heat generated by the individual cell or one or more
The oxygen gas reacts or combines with the active cells of the battery. The use of extensive heat sinks,
cadmium metal on the surfaces of the negative such as placing the entire battery in a water bath, is
electrode. Thus, recombination of oxygen prevents the not recommended since this can prevent heat build-
build-up of an excessive internal gas pressure. up, impede oxygen recombination within the cell, and
In charging nickel-cadmium cells, an overcharge, lead to cell venting before sufficient heat rise occurs.
that is, ampere hours input in excess of that previously The terminal leads from the thermal sensor may be
removed upon discharge, must be provided to ensure connected by additional external contacts or may be
that the cells have reached full charge. If overcharge brought out from the cell or battery unit and con-
is continued at too high a rate of charge current, the nected directly into the circuit. Where a sensor-switch
evolved oxygen gas may not fully recombine, and a device is used in a series-connected battery, it may
build-up of excessive internal gas pressure may result. be preferred to wire the switch internally between two
A safety resealable vent is provided to limit excessive series cells so that no additional external contacts are
build-up of pressure. In the Eveready design, proper required. The practicality of this connection depends
selection of the electrolyte volume controls oxygen on discharge current value and sensor current rating.
recombination pressure below the safety vent opening The advantage would be that the circuit would also
pressure. The safe charge rate for sealed secondary open on discharge in case the battery becomes over-
nickel-cadmium cells for extended charge periods has heated for any reason. The charger circuit required for
been established at the 10 h, or the C/10, rate; capacity charging the individual cell or battery is not unique.
(C) is the rated ampere hour capacity of the cell and A constant-current charger is recommended, and due
10 is the number of hours required at perfect charge regard should be paid to heat dissipation and wattage
efficiency to bring a completely discharged cell to ratings of all components.
full charge. At the 10h rate and lower currents, an Figure 47.4 shows typical voltage and temperature
equilibrium condition is maintained in the cell and characteristics of the Eveready CF1 fast charge cells
consequently there is no excessive build-up of internal when charged at the 1 h rate. Note that the temperature
gas pressure. remains relatively constant until the cell approaches