Lucent Technologies Lineage
®
2000 ECS Battery Plant J85500D-3
3 - 10 Engineering, Planning and Ordering
Issue 5 January 1999
is to minimize voltage drop. Multiple strings are balanced by
sizing cables for equal resistance (and therefore equal voltage
drop) between terminals and bus bars. If battery strings are
unbalanced, the string with the least voltage drop to the dc
distribution provides more than its share of current during each
discharge. A battery string that undergoes excessive discharges
may fail unexpectedly before its predicted end of life.
To both minimize and equalize voltage drops to parallel strings,
the largest practical wire size should be selected for the most
distant battery string. The cable sizes for the strings nearer to the
dc distribution are then selected so that the drop in each is
roughly the same as that for the most distant string.
Some using systems, such as electronic switching systems or
transmission systems, dictate maximum allowable voltage
drops. A common rule-of-thumb is a maximum drop of 0.25
volts in the leads from battery string terminals to the dc
distribution. Voltage drop calculation methods are described
below under Calculating voltage drop. For the calculation, use
the plant List 2 drain divided by the number of parallel battery
strings.
For extraordinarily long runs between batteries and dc
distribution, wire gauges may be called for that cannot be
conveniently terminated at the equipment at either end. In such
cases, the necessary larger cables may usually be tapped down to
smaller ones to make the actual connections to the bus bars and
battery terminals.
Battery size
versus voltage
drop
The critical requirement for a battery plant is that the input
voltage to the load equipment remain within the proper
operating range for the prescribed reserve time. Constants
imposed by the typical 48-volt battery system are the normal
battery float voltage and the minimum battery end voltage.
Note:
Engineering of plants with end cell or counter-emf cell
battery arrangements is not included in this discussion.
The variables that may be adjusted to ensure service for the
specified time period are battery capacity and voltage drop from
batteries to the load. If the system is engineered with a relatively
small voltage drop, large gauge cabling is required, but battery
capacity can be minimized. If a large voltage drop exists
