Page 368 - Battery Reference Book

Self-discharge characteristics and shelf life 31/17
100
Different types of batteries vary appreciably in their -
self-discharge rates; indeed, some types of batter- -
z
ies are designed to have inherently very low self- .-
2 75
discharge rates. The self-discharge characteristics of 0
various types of battery are discussed below.
50
C .-
C
31.13.8 Lead-acid batteries .-
E 25
W
Low-maintenance lead-acid batteries usually fea- CT
ture a special calcium-lead grid instead of the I I I I I I I
lead-antimony alloy used in conventional lead-acid 0 3 6 9 12 15 18
batteries such a8 the traction battery. The elimination Storage time (months)
of antimony and other metallic impurities from the Figure 31.46 Self-discharge characteristics of a Yuasa sealed
alloy increases the ability of the battery to retain its lead-acid battery (Courtesy of Yuasa)
charge when not in use and gives the battery a shelf
life of about file times as long as when the conven-
tional lead-antimony alloy is used. Even at elevated of their initial capacity during 12-16 months and an
temperatures the shelf life with calcium-lead alloys annual recharge is all that is necessary to reinstate
exceeds that ob1 ained with conventional alloy. the full charges (Figure 3 1.45). Another manufacturer
Figure 31.44 shows a curve of percentage of nom- of such batteries is more conservative, recommending
inal capacity at various storage times at four temper- storage at temperatures between 0 and 10°C with a
atures between 40 and 100°C for a low-maintenance 6-monthly recharge (Figure 3 1.46).
lead-acid batteiy. Most manufacturers of this type of At 8°C the storage time to reach half the ini-
battery claim thlat during storage at normal tempera- tial capacity exceeds 2 years, while a conventional
tures around 15-20°C these batteries lose about 5% lead-antimony alloy battery would reach this value
in about 3 months at 20°C.
Figure 31.47 shows the effect of time and tempera-
ture on the self-discharge rate of a fully recombining
lead-acid battery (the General Electric D cell). This
shows, for example, that at 25°C the available capa-
city is reduced to 60% of fully charged capacity after
approximately 300 days’ storage.

38°C 27°C 16°C
31.13.2 Nickel-cadmium batteries
W
These cells have an acceptably low rate of self-
discharge (Figure 3 1.48(a)). Self-discharge is very
much dependent on temperature and increases rapidly
lo l2 l4 l6 with rising temperature. Self-discharge is small at
Storage time (months) very low temperatures and is almost zero at -20°C
(Figure 3 1.48(b)). These cells and batteries can be
Figure 31.44 Typical self-discharge characteristics between 40
and 100°C of Eagle Picher Carefree rechargeable lead-acid stored in the charged Or discharged condition. When
batteries (Courtesy of Eagle Picher) charged they should be handled with care to avoid

I I 1 I I J
a 1 2 3 4 5 6 7 8 9 10 11 12
Storage time (months)
Figure 3’1.45 Self-discharge of Sonnenschein spillproof lead-acid standard and permanent batteries at 20°C (Courtesy of Dryfit)