Vented cell (wet cell, flooded cell) NiCd batteries are used when large capacities and high discharge rates are required.
Traditional NiCd batteries are of the sealed type, which means that charge gas is normally recombined and they release no gas unless severely overcharged or a fault develops.
Unlike typical NiCd cells, which are sealed, vented cells have a vent or low pressure release valve that releases any generated oxygen and hydrogen gases when overcharged or discharged rapidly.
Since the battery is not a pressure vessel, it is safer, weighs less, and has a simpler and more economical structure.
This also means the battery is not normally damaged by excessive rates of overcharge, discharge or even negative charge.

They are used in aviation, rail and mass transit, backup power for telecoms, engine starting for backup turbines etc.
Using vented cell NiCd batteries results in reduction in size, weight and maintenance requirements over other types of batteries.
Vented cell NiCd batteries have long lives (up to 20 years or more, depending on type) and operate at extreme temperatures (from −40 to 70 °C).

A steel battery box contains the cells connected in series to gain the desired voltage (1.2 V per cell nominal).
Cells are usually made of a light and durable polyamide (nylon), with multiple nickel-cadmium plates welded together for each electrode inside.
A separator or liner made of silicone rubber acts as an insulator and a gas barrier between the electrodes.
Cells are flooded with an electrolyte of 30% aqueous solution of potassium hydroxide (KOH).
The specific gravity of the electrolyte does not indicate if the battery is discharged or fully charged but changes mainly with evaporation of water.
The top of the cell contains a space for excess electrolyte and a pressure release vent.
Large nickel-plated copper studs and thick interconnecting links assure minimum equivalent series resistance for the battery.

The venting of gases means that the battery is either being discharged at a high rate or recharged at a higher than nominal rate.
This also means the electrolyte lost during venting must be periodically replaced through routine maintenance.
Depending on the charge–discharge cycles and type of battery this can mean a maintenance period of anything from a few months to a year.

Vented cell voltage rises rapidly at the end of charge allowing for very simple charger circuitry to be used.
Typically a battery is constant current charged at 1 CA rate until all the cells have reached at least 1.55 V. Another charge cycle follows at 0.1 CA rate, again until all cells have reached 1.55 V. The charge is finished with an equalizing or top-up charge, typically for not less than 4 hours at 0.1 CA rate.
The purpose of the over-charge is to expel as much (if not all) of the gases collected on the electrodes, hydrogen on the negative and oxygen on the positive, and some of these gases recombine to form water which in turn will raise the electrolyte level to its highest level after which it is safe to adjust the electrolyte levels.
During the over-charge or top-up charge, the cell voltages will go beyond 1.6 V and then slowly start to drop.
No cell should rise above 1.71 V (dry cell) or drop below 1.55 V (gas barrier broken).

In an aircraft installation with a floating battery electrical system the regulator voltage is set to charge the battery at constant potential charge (typically 14 or 28 V).
If this voltage is set too high it will result in rapid electrolyte loss.
A failed charge regulator may allow the charge voltage to rise well above this value, causing a massive overcharge with boiling over of the electrolyte.
Vented cell (wet cell, flooded cell) NiCd batteries are used when large capacities and high discharge rates are required. Traditional NiCd batteries are of the sealed type, which means that charge gas is normally recombined and they release no gas unless severely overcharged or a fault develops. Unlike typical NiCd cells, which are sealed, vented cells have a vent or low pressure release valve that releases any generated oxygen and hydrogen gases when overcharged or discharged rapidly. Since the battery is not a pressure vessel, it is safer, weighs less, and has a simpler and more economical structure. This also means the battery is not normally damaged by excessive rates of overcharge, discharge or even negative charge.
They are used in aviation, rail and mass transit, backup power for telecoms, engine starting for backup turbines etc. Using vented cell NiCd batteries results in reduction in size, weight and maintenance requirements over other types of batteries. Vented cell NiCd batteries have long lives (up to 20 years or more, depending on type) and operate at extreme temperatures (from −40 to 70 °C).
A steel battery box contains the cells connected in series to gain the desired voltage (1.2 V per cell nominal). Cells are usually made of a light and durable polyamide (nylon), with multiple nickel-cadmium plates welded together for each electrode inside. A separator or liner made of silicone rubber acts as an insulator and a gas barrier between the electrodes. Cells are flooded with an electrolyte of 30% aqueous solution of potassium hydroxide (KOH). The specific gravity of the electrolyte does not indicate if the battery is discharged or fully charged but changes mainly with evaporation of water. The top of the cell contains a space for excess electrolyte and a pressure release vent. Large nickel-plated copper studs and thick interconnecting links assure minimum equivalent series resistance for the battery.
The venting of gases means that the battery is either being discharged at a high rate or recharged at a higher than nominal rate. This also means the electrolyte lost during venting must be periodically replaced through routine maintenance. Depending on the charge–discharge cycles and type of battery this can mean a maintenance period of anything from a few months to a year.
Vented cell voltage rises rapidly at the end of charge allowing for very simple charger circuitry to be used. Typically a battery is constant current charged at 1 CA rate until all the cells have reached at least 1.55 V. Another charge cycle follows at 0.1 CA rate, again until all cells have reached 1.55 V. The charge is finished with an equalizing or top-up charge, typically for not less than 4 hours at 0.1 CA rate. The purpose of the over-charge is to expel as much (if not all) of the gases collected on the electrodes, hydrogen on the negative and oxygen on the positive, and some of these gases recombine to form water which in turn will raise the electrolyte level to its highest level after which it is safe to adjust the electrolyte levels. During the over-charge or top-up charge, the cell voltages will go beyond 1.6 V and then slowly start to drop. No cell should rise above 1.71 V (dry cell) or drop below 1.55 V (gas barrier broken).
In an aircraft installation with a floating battery electrical system the regulator voltage is set to charge the battery at constant potential charge (typically 14 or 28 V). If this voltage is set too high it will result in rapid electrolyte loss. A failed charge regulator may allow the charge voltage to rise well above this value, causing a massive overcharge with boiling over of the electrolyte.
Nickel–cadmium battery