Handbook of Electrical Installation Practice - BeKnowledge

Fig. 6.11 End-cell tapping.
Fig. 6.12 Duplicate/split battery.
Standby Power Supplies 145
There are several ways of achieving this. The cheapest and simplest approach is
to use a technique known as end-cell tapping.
This involves arranging the system so that, during boost charging, the load is supplied
from less than the full number of cells, by automatically switching out a certain
number of cells using a voltage-sensing relay and contactor. Figure 6.11 shows this
arrangement. An alternative approach is to switch a diode into the load circuit so
as to introduce a voltage drop to compensate for the increased boost charge voltage.
It may be that by deliberately oversizing the battery, so that it achieves its desired
performance with only an 80% charge, the need for boost charging can be eliminated.
Whether or not it is acceptable to provide an extra 20% battery capacity that
will never be used depends on the particular application. Apart from considerations
of cost, the size and weight of the battery may be a problem – especially in restricted
locations such as offshore platforms.
Another alternative, which avoids the voltage fluctuations caused by boost charging,
is a split battery with a charger for each half. By ensuring that the boost charge
only takes place on the battery that is off-load, a very closely regulated supply
is assured. With this arrangement, shown in Fig. 6.12, it is necessary for each
battery/charger to have sufficient capacity to supply the load on its own.
Valve regulated lead-acid batteries
With valve regulated lead-acid batteries, end-cell tapping and similar techniques are
not required because:
(1) Maintenance boost charges are redundant, and
(2) Normal operating characteristics of valve regulated lead-acid batteries cover a
much narrower voltage band.
Figure 6.13 illustrates the performance of these batteries.