Lamar Technologies CA 1550 - Manual (Page 15)

4. OPERATING GUIDE - DISCHARGING
4 OPERATING GUIDE - DISCHARGING
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.
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
CA-1550 CHARGER / ANALYZER - OPERATING MANUAL V1.2 Page 13