Page 379 - Battery Reference Book

3W6 Lead-acid secondary batteries
7- are gradually being replaced by sealed valve regulated
designs for telecom and USP applications. They have
a life of 10 years, do not need topping up during this
- 20 time and give off little gas, Le. can be located in offices
9
.+ without special venting requirements. The energy den-
E sity of these sealed cell valve regulated designs is about
2 10
0 twice that of the Plant6 and flat plate designs. These
sealed battery plates are cast in antimony-free lead
alloys, usually lead-calcium or lead-calcium-tin, and
0 60 120 180 240 electrolyte immobilization may or may not be used.
Time (minl
(a) Load profile 1 32.1.3 Charging and operation of standby
600 power batteries
For completeness, this is included here. Battery char-
500 ging is discussed more fully in Part 5.
The essential feature of any standby power system
is immediate availability of the battery power if the
a
A
- 400 normal mains supply fails. The battery must, there-
fore, always be in a fully charged condition. Charging
equipment is essential to a reliable system, and the
recommendations of the battery manufacturer should
always be followed.
200 The basic charging equipment is a means of trans-
forming the normal alternating current (a.c.) mains
supply to a direct current (d.c.) matching the bat-
100 tery requirements. Two rates of charge are normally
0 60 120 180 240 provided, quick boost and trickle. The quick boost
Time (min) rate must be capable of an output between 2.0 and
(b) Load profile 2 2.70V/cell for recharging of the battery. The trickle
charge output should be capable of compensating for
Figure 324 Load example profiles for lead-acid standby power
battery type selection (Courtesy of Chloride Batteries) the internal losses of the battery, at a voltage level of
2.25 V/cell. The outputs required, in terms of current,
be needed in deciding the battery size. It is safest to will depend on the ampere hour capacity of the battery.
assume that the loads will occur at the end of the work During charging, ‘gassing’ will start when the cell
cycle so that sufficient battery capacity and voltage are voltage has reached approximately 2.30-2.40 V/cell.
available for the work. Otherwise the voltage may drop The cell voltage will continue to rise and the charge
so low that the battery fails to do its job and this can may be considered to be completed when the volt-
be costly, if not dangerous. age and relative density have remained the same for
Choosing the most suitable and economical battery approximately 3 h.
The voltage could be as high as 2.70 V and charging
can be bewildering, but most manufacturers now have should not be allowed to continue beyond this level.
computer programs to help select the most effective A recharge voltage table for a Plant6 cell is illus-
type and battery size. Their advice should always be trated in Table 32.3.
sought.
In any event specifying the correct size and type Trickle charging
of battery is of paramount importance. It is certainly
prudent to allow a margin of safety, as one day lives The essential requirement of present applications of
may depend on the correct battery standing behind the standby batteries is that the battery shall be ready
system. instantly to perform its duty. One method of meet-
Traditional alarm and emergency lighting standby ing this requirement is by trickle charging of the bat-
batteries have thick plates reflecting the need for dura- tery. After a battery has been discharged, it must be
bility (up to 20-year life), rather good energy and recharged at the recommended quick rate as soon as
power density. possible. Once restored to the fully charged condition,
This type of cell is now gradually being replaced trickle charging will then keep it fully charged until
by a sealed cell design, e.g. the Bell Telephone Sys- the next demand. No amount of trickle charging will
tem and Western Electric Company Round Cell which ever recharge a substantially discharged battery, but
contains saucer-shaped pure lead grids stacked hori- when it is part of the duty of a standby system to meet
zontally. These cells have a life of about 30 years. occasional brief demands the energy thus used can be
Plant6 type cells are still in use to some extent but gradually replaced by trickle charging at a suitable rate.