Handbook for Sound Engineers

Power Supplies 695

acid batteries are vented so that the gas but not the elec-

trolyte escapes.

Lead-acid cells are normally 2.1 V and are easily

connected in series to produce 6 V and 12 V automotive

types, 24 V aircraft types and 36 V types for golf carts,

etc. Lead-acid batteries, because of their availability,

high Ah ratings, and ability to be connected in series,

work well powering sound systems in the field.

The type and amount of charge determine the condi-

tion of the cell. If a lead-acid battery is overcharged,

excessive water consumption and hydrogen evolution

result, while constant undercharging results in a battery

with less and less capacity.

The recharge factor (RF) is defined as the charge Ah

divided by the previous discharge Ah. The RF must

always be greater than 1 to bring the battery back to

capacity. The actual RF is between 1.04 and 1.20, with

the sealed lead acid batteries requiring less than the

standard vented type. Fig. 19-29A shows the state of

charge (SOC) achieved versus the RF for a lead-acid

battery. Fig. 19-29B shows the SOC versus the RF after

a number of cycles. Note that the battery rapidly loses

its capacity if it is not overcharged—i.e., more is put in

than is taken out.

The use of a trickle charger with a storage battery

shortens the life of the battery because of overcharging.

Trickle chargers should only be used when it is imprac-

tical to charge a battery by other means. A practical

approach to the problem is to adjust the charging

voltage to a value between 2.15 V and 2.17 V per cell.

A better, but more elaborate, method is to check the

specific gravity of the cells over a period of several

months and adjust the charging voltage to a value where

the specific gravity is maintained at 1.250. Compensa-

tion must be made for temperature changes when

reading the specific gravity. Four gravity points are

added to the reading for every 10°F (5°C) the electro-

lyte is above a temperature of 80°F (27°C).

The freezing point of a battery electrolyte depends

on the specific gravity of the electrolyte, Table 19-6.

Lead acid batteries freeze when in a discharged state, so

it is imperative that they be kept fully charged when in

subfreezing temperatures. If a storage battery is left in a

discharged condition for any length of time, the plates

may be damaged due to sulfation.

19.10.9 Lead-Dioxide Batteries

A lead-dioxide battery is a gelled electrolyte, mainte-

nance-free type that exhibits high capacity and long life

when properly applied and charged. To prevent electro-

lyte movement in the battery, the electrolyte in sealed

batteries is immobilized by the use of a gelling agent

that stores the electrolyte in highly porous separators.

With this construction, the loss of water is minimized.

The terminal voltage of each cell is approximately

2.12 V. The cell voltage is higher for a battery that has

Table 19-6. Effect of Specific Gravity on Freezing
Point of a Battery

Specific

Gravity

Freezing

Point

Specific

Gravity

Freezing

Point

1.275 85°F 1.175 +4°F

1.250 62°F 1.150 +5°F

1.225 35°F 1.125 +13°F

1.220 16°F 1.100 +19°F

Figure 19-29. Lead acid battery.

RF = 1.16, 1.20

RF = 1.12

RF = 1.08

RF = 1.04

RF=1.00

Change Ah

RF = discharge change Ah

25% DOD

50% DOD

100% DOD

100

98

96

94

92

90

0.96 1.00 1.04 1.08 1.12

102

86

90

94

98

0 1 2 3 4

Cycles

Recharge factor (RF)

State of charge (

SOC

)–%

State of charge (

SOC

)–%

A typical state of charge for lead-acid

battery versus the recharge factor.

State of charge versus recharge factor after three

discharge cycles of a lead-acid battery.

A.

B.