Page 359 - Battery Reference Book

3118 Secondary batteries

60 min rate
30 min rate
I l l l l l l l l l l
20 18 16 14 12 10 8 6 4 2 0
I I I Discharge time (h)
-40 -30 -20 -10 0 10 20 30
Temperature ("C) (a) VPT 6.9/8 (6 V, 9 A h)
Figure 31.15 Effect of discharge temperature on plateau voltage
for high-rate and low-rate rechargeable silver-cadmium batteries

at various rates of discharge against battery temper-
ature at stipulated rates of discharge. Figure 31.15
shows such curves for high-rate and low-rate sil-
ver-cadmium batteries. :j/ , I I I , I , ,
A small change in plateau voltage occurs for the sil-
ver-zinc system when battery temperature is lowered.
4
31.4 Capacity returned (discharged 20 18 16 14 12 10 8 6 4 2 0
capacity)-discharge rate curves Discharge time (h)

Plots of the variation of the returned capacity of battery (b) VPT 6.15/30 (6 V. 28 A h)
versus the duration of the discharge in hours at a
stipulated discharge rate and battery temperature are Figure 31 .I6 Relationship between capacity returned and dis-
charge rate at 20°C at 20 h rate for Varley 6V non-spill lead-acid
a useful means of assessing battery performance. The batteries (Courtesy of Varley)
slower the discharge the greater the capacity returned.
Figure 31.16 illustrates such curves for a 6V non-spill
lead-acid battery (9 and 28 Ah) discharged at 20°C percentage withdrawable capacity) against battery tem-
at the 20 h rate. The relationship between capacity perature for various discharge currents down to the
returned and discharge rate is discussed in more detail end-point voltage.
in Section 31.9. Such data are discussed further below for several
types of battery.
31.5 Capacity returned (discharged
capacity)-discharge temperature 31.5.1 Lead-acid batteries
curves and percentage withdrawable The curves in Figure 3 1.17 give information regarding
capacity returned-temperature curves the variation of available capacity of low-maintenance
sealed lead-acid batteries, as a percentage of nomi-
As mentioned earlier, the nominal capacity of a battery nal capacity (withdrawable capacity), as a function of
delivered at the nominal 20 h discharge rate, when dis- temperature over the range -30 to +50T for three
charged at the nominal Z20 discharge current, depends different loads with continuous discharge down to the
on the battery temperature. Generally speaking the relevant discharge cut-off voltage. For the values of the
higher the temperature the higher the capacity returned upper edges of the curves, the batteries were charged
and vice versa. However, for many types of batteries, at an ambient temperature of 20°C voltage limited
the capacity starts to decrease above a certain bat- to 2.3Vkell. For the lower edge of the curves, the
tery temperature. batteries were charged at the indicated low ambient
Such data are invaluable for predicting the perfor- temperature and hence under somewhat less favourable
mance of batteries at extremes of service temperature conditions. The curves show the behaviour of the bat-
conditions from arctic to tropical. A refinement of this teries after a few charging cycles. When assessing both
method of presentation is to plot the available capa- the upper and lower edges of the curves for the higher
city obtained from the battery at the test temperature discharge currents, the dependence on the load must
as a percentage of the nominal capacity at 20°C (i.e. also be taken into account. This is already included