Negative electrode:
The electrode from which electrons flow when the battery is discharging into an
external circuit. Reactants are electrochemically oxidized at the negative electrode. In the lead-acid
cell, the negative electrode contains spongy lead and lead sulfate (PbSO4) as the active materials.
In the nickel-cadmium cell, the negative electrode contains cadmium and cadmium hydroxide
(Cd(OH)2) as the active materials.
Nominal voltage:
The characteristic operating voltage of a cell or battery. The nominal voltage is 2.0 V
for lead-acid cells and 1.2 V for nickel-cadmium cells. These voltage levels represent the approxi-
mate cell voltage during discharge at the C-rate under room-temperature conditions. The actual
discharge voltage depends on the state-of-charge, state-of-health, discharge time, rate, and tem-
perature.
Positive electrode:
The electrode to which electrons flow when the battery is discharging into an external
circuit. Reactants are electrochemically reduced at the positive electrode. In the lead-acid cell, the
positive electrode contains lead dioxide (PbO2) and lead sulfate (PbSO4) as the active materials.
In the nickel-cadmium cell, the positive electrode contains nickel oxyhydroxide (NiOOH) and
nickel hydroxide (Ni(OH)2) as the active materials.
Separator:
An electrically insulating material that is used to prevent metallic contact between the positive
and negative plates in a cell, but permits the flow of ions between the plates. In flooded cells, the
separator includes a gas barrier to prevent gas diffusion and recombination of oxygen. In sealed
cells, the separator is intended to allow gas diffusion to promote high recombination efficiency.
State-of-charge:
The available capacity of a battery divided by the capacity available when fully charged,
normally expressed on a percentage basis. Sometimes referred to as “true state-of-charge.’’
State-of-health:
The available capacity of a fully charged battery divided by the rated capacity of the
battery, normally expressed on a percentage basis. Sometimes referred to as “apparent state-of-
charge.” Can also be used in a more qualitative sense to indicate the general condition of the battery.
Ampere-hour capacity:
The quantity of stored electrical energy, measured in ampere-hours, that the
battery can deliver from its completely charged state to its discharged state. The dischargeable capacity
depends on the rate at which the battery is discharged; at higher discharge rates the available
capacity is reduced.
C-rate:
The discharge rate, in amperes, at which a battery can deliver 1 h of capacity to a fixed voltage
endpoint (typically 18 or 20 V for a 24-V battery). Fractions or multiples of the C-rate also are
used. C/2 refers to the rate at which a battery will discharge its capacity in 2 h; 2C is twice the C-
rate or that rate at which the battery will discharge its capacity in 0.5 h. This rating system helps
to compare the performance of different sizes of cells.
CCA:
The numerical value of the current, in amperes, that a fully charged lead-acid battery can deliver
at 18°C (0°F) for 30 s to a voltage of 1.2 V per cell (i.e., 14.4 V for a 24-V battery). In some cases,
60 s is used instead of 30 s. CCA stands for cold cranking amperes.
Electrolyte:
An ionically conductive, liquid medium that allows ions to flow between the positive and
negative plates of a cell. In lead-acid cells, the electrolyte is a mixture of sulfuric acid (H2SO4) and
deionized water. In nickel-cadmium cells, the electrolyte is a mixture of potassium hydroxide
(KOH) dissolved in deionized water.
Imp:
The numerical value of the current, in amperes, delivered after 15 s during a constant voltage
discharge of 0.6 V per cell (i.e., at 12 V for a 24-V battery). The Imp rating normally is based on
a battery temperature of 23°C (75°F), but manufacturers generally can supply Imp data at lower
temperatures as well.
Monobloc:
A group of two or more cells connected in series and housed in a one-piece enclosure with
suitable dividing walls between cell compartments. Typical monoblocs come in 6-V, 12-V, or 24-v
configurations. Monoblocs are commonly used in lead-acid batteries, but rarely used in nickel-
cadmium aircraft batteries.
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