Lamar Technologies BETA D-50 - Manual (Page 11)

4. DETAILED OPERATING INSTRUCTIONS
4. DETAILED OPERATING INSTRUCTIONS
4.1 DISCHARGE CHARACTERISTICS
The lead-acid and the nickel-cadmium cells are generally assigned nominal open circuit
voltages of 2.10 volts and 1.35 volts respectively. Actual open circuit voltage at 75°F/ 25°C
for a fully charged battery cell depends on state-of-charge and time after charge.
During discharge, the voltage of the cell or battery immediately begins to decrease
because of the effective internal resistance of the cell. This includes the resistance of
the terminal posts, active material, plate lugs and grids, separators, and contact
resistance between the surface of the active material and the electrolyte. The internal
resistance increases during discharge, being greater toward the end of discharge, when
the terminal voltage is lower.
As voltage gradually becomes lower during discharge, the point of near exhaustion is
reached. At this point, the discharge voltage curve begins to drop very sharply to a value
which is of no further practical use. Usually this happens at 18-20 volts for a 24 volt lead-
acid battery and 1 volt per cell for a nickel-cadmium battery (see figure 4-1).
Battery manufacturers specify a cutoff voltage which the battery must exceed during a
discharge test to have acceptable capacity. The cutoff voltage varies with the rate of
discharge (the discharge current/time combination used to draw the same capacity). For
example, the minimum cutoff voltages for a 30 ampere-hour, 24-volt naval aircraft lead-
acid storage battery are generally specified as is shown in table 4-1.
Rate of Discharge Minimum
Discharge Current Cutoff Voltage
5 hours 6 amperes 21.0 volts
2 hours 15 amperes 19.2 volts
1 hour 30 amperes 18.0 volts
Table 4-1. Cutoff voltages at different discharge rates for a
30 ampere-hour, 24 volt lead-acid battery
The specified final minimum cutoff voltage represents the value of voltage at which the
rated ampere-hour capacity of the battery must have been delivered for the specified
discharge rate. Figure 4-1 is a typical discharge curve for a 40 ampere-hour sealed
lead-acid (SLAB) aircraft storage battery discharged at a 1-hour rate of 40 amperes.
From figure 4-1, it is observed that at the end of 1 hour of discharge time, the battery
voltage has reduced only to about 22 volts. Because the minimum required cutoff
voltage is 18 volts at the 1-hour discharge rate, the battery exceeds the minimum
requirements. With increasing hours of use or age, the battery capacity decreases.
Therefore, battery manufacturers usually recommend testing for a capacity equal to
80% of the original ampere-hour rating.
BETA D-50 TESTER / ANALYZER - OPERATING MANUAL V2.2 Page 9