Page 128 - Battery Reference Book

4/12 Nickel batteries
Dryfit batteries are thus seen to occupy approximately charging. From 1 to 40 x Zzo, the various types of sin-
the same Whkg range with the bandwidth of the tered nickel-cadmium cell perform better than the
range being in general defined by the individual Dryfit batteries, and similar or worse performance is
type variants. obtained up to 20 x 120 for the various button cells
The relationship between multiples of the 20 h nom- with mass electrodes.
inal discharge current and the available percentage of Looking at the high current end of the curves in
the nominal capacity for Dryfit and nickel-cadmium Figure 4.7, it is clear that definite limits are placed on
batteries is shown in Figure 4.7. The shaded area rep- nickel-cadmium batteries. These limits are imposed
resents the variation of capacity of Dryfit batteries from by the maximum allowable discharge current as shown
low discharge currents to very high currents. The lower in Figure 4.8. The upper curve emphasizes the excep-
edge of the band is for a new battery after recharging tionally high maximum load capacity of Dryfit batter-
while the upper edge represents the characteristic after ies, especially at the lower end of the capacity range
30-50 charging cycles or several months of trickle where it is a number of times greater than that of
nickel - cadmium batteries.
The self-discharge properties of sealed nickel-cad-
40 mium batteries differ greatly for mass plate and sinter
I
m
Y plate cells, but compared to the Dryfit battery they
2 35
z are substantially inferior. As Figure 4.9 indicates, mass
> 30 plate nickel-cadmium cells discharge at 20°C to half
r their original value after 13 months. The decrease in
25
U the first months is exceptionally steep and not as flat
>
P 20 as that for the sealed lead-acid battery (16 months
to lose half original capacity) - see Figure 4.10. The
5 15
1 5 10 15 20 26 construction of the mass plate cell, whether button cells
20 h capacity, I, with single or double electrodes, or rectangular cells,
makes practically no difference. The self-discharge of
Figure 4.6 Energy density comparisons: Dryfit and different sinter cells, as shown in Figure 4.11, is many times
types of sealed nickel-cadmium battery (6V). M + ML, prismatic greater. After 6 weeks at 20"C, half the capacity has
sintered cells in plastic cases; RS, sintered round cells (varta); VR,
sintered round cells (SAFT); SD, prismatic sintered cells (varta); already been lost. Compared to lead-acid batteries,
D, prismatic mass cells (varta); DK (Courtesy of Dryfit, SAFT and longer storage of a deeply discharged (due to self-
Varta) discharge) nickel-cadmium battery does not damage
it, whereas such treatment of discharged lead-acid
batteries results in an irreversible loss in capacity.
However, the nickel-cadmium battery cannot be used
for applications demanding immediate availability
without continuous recharging and therefore increased
maintenance costs.
Dryfit batteries may be charged and discharged over
an ambient temperature range of -20 to +50"C. If
continuous operation at one or more of the extreme
values is required, a temperature sensor should be used
to optimize charging. This can, however, be omitted
if the battery is operated infrequently at extremes of
temperature during the course of its life. The over-
charging at high temperatures and the charging, which
cannot be completed in its usual time at low tempera-
tures because of the lack of temperature compensation,
lo I affect the quality of the battery to only a minor degree.
Short-term temperatures up to 75°C can be tolerated.
01 I I I I 1 I 1 I No precautions need be taken against the lower temper-
1 2 4 6 10 20 40 70 140 ature of about -50"C, since the gelled electrolyte in the
Multiple of the 20 h discharge nominal current, I,,
charged state cannot freeze solid. The nickel-cadmium
Figure 4.7 Removable capacity dependence on charge current: battery also works at its best only if charging takes
Dryfit and nickel-cadmium batteries. Sinter round cells: RS 1-7, place at 20°C. Many types are severely limited by an
1 -7Ah (varta); VR (SAFT). Prismatic sinter round cells: SD 1-15, allowable temperature range of 0 to 45°C for charging.
1-15Ah (Varta). Prismatic mass cells: D (varta). Button cells
with mass electrodes: DK 1000-3000 (varta). Prismatic sinter cell Charging to a certain achievable percentage of full
batteries: ML, M (varta) (Courtesy of Dryfit) charge occurs in the freezing range and charging above