Page 471 - Battery Reference Book

4714 Constant-current charging

12 - 60 - 1.55 -

10 - 1.50 -
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50
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30 - Pressure: charge at 0.3 C, A
2-

20 - I
L
0 50 100 150
Percentage capacity charged (% C, A h)
Figure 47.2 Effect of charge rate on pressure, temperature and voltage for two charge rates on sealed nickel-cadmium cells

47.1.1 On-charge voltage could be a suitable indicator for sensing the point of
tripping out the charge (the tripping level). However,
To achieve charge control by the external sensing of in practice, charge control based on pressure monitor-
the on-charge voltage of a nickel-cadmium cell or ing is not regarded favourably because of the technical
battery requires the use of auxiliary circuitry to shut off
the charging current in response to a voltage sensor.
The difsculty with the use of voltage sensing is that
the voltage of many types of cell tends to change with
negative cycling and may vary with temperature and
charge rate.
5 1.0 I I I I
47.1.2 Internal pressure (a) 0 1 Time (h) 2 3

The internal pressure is very low during the greater part 1
of the charge, and rises at the approach to full charge.
At this moment nearly all the energy applied is pro-
ducing oxygen. The pressure ceases to rise and reaches 30
equilibrium when the rates of production and con- 5 20
sumption of oxygen are equal. The pressure rises with
the charge rate, falls when the temperature increases,
and is different for each type of cell. The charge rate
(and the maximum period of charge) to be. considered (C) n :%=
for each type of cell corresponds to the equilibrium
pressure that may be mechanically withstood by the
cell. The equilibrium pressure must be lower than the a Cell pressure
pressure setting of the safety valve and must be less f 1 0 1 Time (h) 2 3
than that which will deform the cells, whether they are
mounted in batteries or not.
Figure 47.3 shows the related characteristics of volt-
age, temperature and pressure on a rolled sintered cell Figure 47.3 Related characteristics of voltage, temperature and
when charged at 10 x Zlo starting from a discharged pressure on charging of Varta sealed nickel-cadmium cells
state. It may appear from these curves that pressure (Courtesy of Varta)