Handbook for Sound Engineers

694 Chapter 19

charge. Flooded batteries must bubble (gas) to insure a
full charge, and to mix the electrolyte. Float voltage for
flooded batteries should be 2.15–2.23 V/cell, or
12.9–13.4 V for a 12 volt battery. At higher tempera-
tures, over 85°F, charge voltage should be reduced to
2.10 V/cell. Float and charging voltages for gelled
batteries are usually about 0.2 V less than for flooded
batteries.

19.10.7.1 Equalizing

The equalize cycle puts a controlled overcharge to
remove lead sulfate from the plates that is not removed
during the normal charging of the battery. Flooded bat-
tery life can be extended if an equalizing charge is
applied every 10 to 40 days. This is a charge that is
about 10% higher than the normal full charge voltage,
and is applied for 2–16 h to be sure that all cells are
equally charged and the gas bubbles mix the electrolyte.
If the liquid in the cells is not mixed, the electrolyte
becomes stratified, creating a strong solution at the top
and weak solution at the bottom of the cell. AGM and
gelled batteries should be equalized a maximum of two
to four times a year.

19.10.7.2 Charging Voltage versus Temperature

Battery charging is sensitive to temperature. As the
ambient temperature decreases, the charging voltage
must be increased, Table 19-4.

19.10.7.3 State of Charge

Table 19-5 shows no-load typical voltages versus state of
charge for a 12 V battery. These voltages are for batteries

that have been at rest for 3 hours or more. Note the large

voltage drop in the last 10%.

19.10.7.4 Internal Resistance

All batteries have internal resistance that causes the bat-

tery voltage to fluctuate with the load. To calculate the

internal resistance of a single cell or battery, the open-

circuit voltage V 1 is measured using a voltmeter with an

internal resistance of at least 1000:/V. The battery or

cell is then loaded with resistor R 1 , and the voltage V 2

across the resistor is measured. R 1 should be at least 10

times the battery resistance. The current through the

resistor R 1 is

. (19-24)

The internal resistance, Ri, of the battery may now be

calculated using

. (19-25)

19.10.8 Lead-Acid Batteries

The lead-acid storage battery was invented by Gaston

Planté in 1860 and is one of the most widely used forms

of battery power. The principal drawback to this type of

battery has been the liquid electrolyte and the fumes

given off when charging and discharging. Today the

sealed lead-acid battery may take its place with other

rechargeable batteries such as the nickel- cadmium bat-

tery. Since small amounts of gas may be generated in

any battery during the charge or discharge cycle, lead-

Table 19-4. Temperature Compensation for Various
Types of Batteries

Temp Liquid Gel—Std. Gel—Fast AGM

oF oCAccept Float Accept Float Accept Float Accept Float

120 49 12.5 12.5 13.0 13.0 13.0 13.0 12.9 12.9

110 43 13.6 12.7 13.5 13.0 14.0 13.4 13.9 12.9

100 38 13.8 12.9 13.7 13.2 14,1 13.5 14.0 13.0

90 32 14.0 13.1 13.8 13.3 14.2 13.6 14.1 13.1

80 27 14.2 13.3 14.0 13.5 14.3 13.7 14.2 13.2

70 21 14.4 13.5 14.1 13.6 14.4 13.8 14.3 13.3

60 16 14.6 13.7 14.3 13.8 14.5 13.9 14.4 13.4

50 10 14.8 13.9 14.4 13.9 14.6 14.0 14.5 13.5

40 5 15.0 14.1 14.6 14.1 14.7 14.1 14.6 13.6

30 -1 15.2 14.3 14.7 14.2 14.8 14.2 14.7 13.7

Table 19-5. No Load Voltage versus State of Charge

for a 12 V Battery

State of Charge 12 Volt battery Volts per Cell

100% 12.7 2.1

90% 12.5 2.1

80% 12.4 2.1

70% 12.3 2.1

60% 12.2 2.0

50% 12.1 2.0

40% 11.9 2.0

30% 11.8 2.0

20% 11.6 1.9

10% 11.3 1.9

0% 10.5 1.8

I

V 2

R 1

----- -=

Ri

V 1

I

----- -–= R 1