Handbook for Sound Engineers

Power Supplies 697

Longevity. All batteries die. The number of cycles it
takes to kill them is a function of the type and quality of
the battery. When cycled between 25% and 50% depth
of discharge (recommended deep cycle use), AGM bat-
teries will normally outlast the other two types.

Durability. Some battery designs are simply more dura-
ble than others are. They are more forgiving in abusive
conditions —i.e., they are less susceptible to vibration
and shock damage, over-charging, and deeper dis-
charge damage. Gel acid batteries are the most likely to
suffer irreversible damage from overcharging. Flooded
acid batteries are the most likely to suffer from internal
shorting and vibration damage. AGM batteries are usu-
ally more durable and can withstand severe vibration,
shocks, and fast charging.

Efficiency. Internal resistance of a battery denotes its
overall charge/discharge efficiency and its ability to
deliver high cranking currents without significant drops
in voltage and is a measure of how well it has been
designed and manufactured. Internal resistance in
NiCad batteries is approximately 40%—i.e., you need
to charge a NiCad battery 140% of its rated capacity to
have it fully charged. The flooded wet battery’s internal
resistance can be as high as 26%, which is the charging
current lost to gassing, or breaking up of water. Gel acid
batteries are better at approximately 16% internal resis-
tance and require roughly 116% of rated capacity to be
fully charged. AGM batteries have an internal resistance
of 2%, allowing them to be charged faster and deliver
higher power.

19.10.11 LeClanche (Zinc-Carbon) Batteries

LeClanche batteries consist of a carbon anode, zinc
cathode, and electrolyte solution of ammonium chlo-
ride, zinc chloride, and mercury chloride in water
(called a mix). The nominal voltage is 1.5 V. This type
of cell is quite inefficient at heavy loads, and its capac-
ity depends considerably on the duty cycle. Less power
is available when it is used without a rest period. Maxi-
mum power is produced when it is given frequent rest
periods, since the voltage drops continuously under
load. Shelf life is limited by the drying out of the elec-
trolyte. A typical discharge curve is given in Fig. 19-30.
Zinc-carbon cells may be recharged for a limited
number of cycles. The following information is
extracted from the National Bureau of Standards
Circular 965:
The cell voltage for recharge must not be less
than 1 V and should be recharged within a short
time after removing from service. The ampere

hours of charge should be within 120–180% of

the discharge rate. The charging rate is to be low

enough to distribute the recharge over 12–16 h.

Cells must be put into service soon after

recharging as the shelf life is poor.

19.10.12 Nickel-Cadmium Batteries

For optimum performance, many battery-operated items

require a relative constant voltage supply. In most appli-

cations, nickel-cadmium cells hold an almost constant

voltage throughout most of the discharge period, and the

voltage level varies only slightly with different dis-

charge rates. Nominal discharge voltage is 1.25 V at

room temperature.

Nickel-cadmium cells are especially suited to high

discharge or pulse currents because of their low internal

resistance and maintenance of discharge voltage. They

are also capable of recharge at high rates under

controlled conditions. Many cells can be rapidly

charged in 3–5 h without special controls, and all can be

recharged at a 14 h rate.

Nickel-cadmium cells are designed to operate with a

wide temperature range and can be discharged from 

40°F to +140°F (40°C to +60°C).

These cells can be continuously overcharged at

recommended rates and temperature. This will not

noticeably affect life unless the charge rate exceeds

design limitations of the cell.

The cell construction eliminates the need to add

water or electrolyte, and, under certain conditions, the

cell will operate on overcharge for an indefinite period.

A typical discharge curve for a cell, rated at 25 Ah and

weighing approximately 2 lb, is given in Fig. 19-31.

The charge retention varies from 75% for 1 month to

as low as 15% for 5 months. Storage at high tempera-

tures will reduce high retention. Cells should be charged

Figure 19-30. Typical discharge curves for three different

types of penlight cells discharged continuously into a 50:

load.

1.6

1.4

1.2

1.0

0.8

0.6

0 20 40 60 80 100

Time–hours

Volts

Zinc Carbon

Mercury

Manganese Alkaline

50 Ω continuous load

at room temperature