Handbook for Sound Engineers

698 Chapter 19

prior to use to restore full capacity. Nickel cadmium
eventually fails due to permanent or reversible cell
failure. A reversible failure is usually due to shallow
charge and discharge cycles and the battery appears to
have lost capacity. This is often called the memory
effect. This problem can be removed by deep discharge
and a full recharge. A loss of capacity can also come
from extended overcharging. If this should occur, full
capacity can be restored by a discharge followed by a
full recharge.
The capacity of a nickel-cadmium cell is the total
amount of electrical energy that can be obtained from a
fully charged cell. The capacity of a cell is expressed in
ampere-hours (Ah) or milliampere-hours (mAh), which
are a current-time product. The capacity value is depen-
dent on the discharge current, the temperature of the cell
during discharge, the final cutoff voltage, and the cell’s
general history.
The nominal capacity of the nickel-cadmium cell is
that which will be obtained from a fully charged cell
discharged at 68°F (20°C) for 5 h to a 1.0 V cut off.
This is called the C/5 rate.
Discharges at the 20, 15, 10, and 1 h rates are called
C/20, C/15, C/10, and C, respectively. Higher rates are
designated as 2C, 3C, etc.
When three or more cells are series connected for
higher voltages, the possibility exists that during
discharge, one of the cells, which may be slightly lower
in capacity than the others, will be driven to a zero
potential and then into reverse. At discharge rates (C) in
the vicinity of C/10, cells can be driven into reverse
without permanently damaging the cell. Prolonged,
frequent, or deep reversals should be avoided since they
shorten cell life or cause it to vent. Cell voltage should
never be allowed to go below 0.2 V.
Nickel-cadmium batteries may be charged using
either a constant-current or constant-voltage charger.
There are four major factors that determine the charge
rates, which can be used on nickel-cadmium batteries.

They are charge acceptance, voltage, cell pressure, and

cell temperature.

No charge control is required for charge rates up to

C/3. This allows the use of the least expensive charger

design. When charging rates equal or exceed 1.0 C, the

charging current must be regulated to prevent over-

charge.

In Table 19-3, the notation that includes the letter C

is used to describe current rates in terms of a fraction of

the capacity rating of the battery. A comparison of cells

from different manufacturers requires rationalization to

a common standard for capacity rating at the same

discharge rate.

In general, discharge times will be shorter than those

for C rates greater than 1 and longer than those for C

rates less than 1. The charge input must always be more

than discharged output. For example, to ensure full

recharge of a completely discharged battery, the

constant-current charge time at the 10 h rate must be

longer than 10 hours due to charge acceptance

characteristics.

19.10.13 Alkaline-Manganese Batteries

The alkaline-manganese battery is gaining consider-

able importance in the electronic field since it is a pri-

mary battery and is rechargeable.

The polarity of this cell is reversed from the conven-

tional zinc-carbon cell, in which the can is negative.

Figure 19-31. Discharge characteristic for Sonotone type
20L420 nickel-cadmium battery, rated 25 Ah.

0 20 40 60 80 100

% Capacity

Cell voltage

0.6

0.8

1.0

1.2

1.4

1.6

4 A

50 A

150 A 100 A

20 A

Table 19-7. Charging Rates for a Nickel-Cadmium

Battery

Method of Charging Charge Rate

Name Nickname Current

Rate

Fraction Hour Rates

Standby Trickle 0.01C C/100 100 h

0.02C C/50 50 h

0.03C C/30 30 h

0.04C C/25 25 h

Slow Overnight 0.05C C/20 20 h

0.1C C/10 10 h

Quick Rapid 0.2C C/5 5 h

0.25C C/4 4 h

0.3C C/3 3 h

Fast C.0 C 1 h

2C.0 2C 30 min

3C.0 3C 20 min

4C.0 4C 15 min

10C.0 10C 6 min