Page 478 - Battery Reference Book

Types of constant-current charging 47/1 d

end of charge. The on-charge voltage characteristics This system of control does not require the use of
vary with the temperature and the charge rate, and special batteries and may be applied to all standard
show significant differences between sizes of cylin- batteries including cylindrical types with capacities up
drical cells. For each cell, for a given rate of charge, it to 7 Ah charged at the Cs A rate. The time-switch ter-
is possible to draw a theoretical cut-off voltage curve minates the charge in all cases after 1 h. The capacity
as a function of cell temperature. An example is shown recharged is close to the rated capacity between 20 and
in Figure 47.9(a) for a battery of ten SAFT VR 1.2RR 40°C and is less below this lower temperature.
(1.2 Ah) cylindrical cells (Table 5 1.1) charged at rates In control by voltage-temperature, the termination
of Cs and 2Cj A. At each temperature the selected cut- of rapid charge is obtained from values of voltage that
off voltages are defined to coincide approximately with depend on the temperature of the battery. For eco-
the fully charged condition and to be clearly above the nomic reasons, it is not possible to follow precisely
first voltage plateau and below the maximum end-of- the theoretical characteristics of cut-off voltage as a
charge voltage. function of temperature (Figure 47.9(a)). The charac-
teristics may be linearized while retaining sufficient
Depending on the operating temperature range,
security for cut-off. Figure 47.9(b) shows the charac-
two methods of control (voltage-time and volt- teristics used in practice for a battery pack of ten SAFT
age-temperature) can be used. Control by volt- lOVR 1.2RR 1.2Ah (i.e. 12Ah) cells (Table 51.11,
age-time is used in applications where the operating charged at Gs A and 2C5 A.
temperature is of the order of 20°C (from 10 to 40°C). The capacity obtained with this system remains
Chargers designed for this method of operation ter- satisfactory in the temperature range of -20 to +40"C.
minate the charge when the battery voltage reaches a Most rapid-charge systems with voltage-temperature
fixed preset value. This value corresponds to the cut- control leave the batteries on low-rate charge after cut-
off voltage at 4YC (the lowest cut-off voltage in the off of the rapid charge. The object of this is to give
temperature range under consideration). A time-switch 100% of capacity charged. To provide maximum se-
ovenide is used to allow for the case where the battery curity during charging, batteries can be equipped with
temperature might exceed 40°C or the end-of-charge thermostatic protection which interrupts this charge in
voltage might be abnormal. case of irregularity in the battery or the charge system.
This protection operates when the battery temperature
reaches the abnormal value of 5.5-60°C. Batteries for
'181 rapid charge with voltage-temperature control must be
adapted to this system because they must be equipped
with a temperature sensor for voltage control and a
thermostat for protection against overheating.
47.3.4 Ultra-rapid constant-current charging
By limiting the capacity charged to a certain proportion
of the rated capacity of the cell or battery, it is pos-
sible to charge cylindrical and button nickel-cadmium
11 4 I i I I I 1 I sealed cells and batteries at very high rates and thus
-20 -10 0 10 20 30 40 50 obtain a short-duration charge of the order of a few
Battery temperature ("C)
minutes. The amount of the capacity that may be
charged is determined by the increase of internal pres-
I sure, which must always remain below the operating
pressure of the valve for cylindrical cells and batter-
ies and the deformation pressure for button cells and
batteries. Ultra-rapid charge requires a prior discharge
of the cells, which may be done in periods as short
as 30 s.
Figure 47.10 shows the durations of ultra-rapid
charge that are possible for a SAFT VR 0.5AA
(0.5Ah) cylindrical cell (Table 51.11, as well as the
security limit corresponding to opening of the valves
--20 -10 0 10 20 30 40 50 at 20°C. A method of charge derived from the ultra-
Battery temperature ("C)
rapid is the one-shot charge for use in an application
Figure 47.9 (a) End-of-charge cut-off voltage versus temperature: that requires only one single cycle, such as a mil-
SAFT IOVR 1.2RR battery pack (ten 1.2Ah cells, 12Ah battery); itary application where the battery is charged only
fast charge at C5A rate (1.2Ah) then 2CsA rate (2.4Ah). once and is subsequently destroyed with the appara-
(b) Charger cut-off voltage versus temperature: SAFT 1 OVR 1.2 RR tus it supplies. For operational reasons, the battery
battery pack (ten 1.2 A h cells; 12 Ah battery); fast charge at CS A
rate (I .2 A h) then 2C5 A rate (2.4A h) (Courtesy of SAFT) must be charged in a few seconds, which 3s possible