Nickel-Cadmium batteries (NiCd or NiCad) have been used extensively on satellites. These batteries have a heritage based on high reliability that has been proven over time. When properly operated and maintained they have provided reliable reserve power well in excess of 15 years of on orbit operations on GEO satellites.

When monitoring cell voltages during eclipse or discharging operations, the voltage will drop by as much as 0.1 to 0.2 volts over the first few minutes then stabilize and decrease only 0.004 to 0.1 volts until it reaches less than 20 % of it’s capacity. Nearing the depletion of capacity the voltage will drop by as much as 0.1 to 0.2 volts over a few minutes resembling the initial rate at the start of discharge. It is critical to stop discharging when the voltage on any cell drops to 2/3 of the initial voltage. If the cell is rated at 1.5 volts the discharge termination voltage would be at 1.0 volt. To continue discharging below 1.0 volts could result cell reversal, cell failure or complete battery failure. During discharging of the battery the temperature will increase and stabilize.

After discharging the batteries the amount of energy removed must be calculated and 110 to 120% returned to reach the full state of charge. This is based on the internal resistance and inherent characteristics of the NiCad battery. Initially the battery will become endothermic and cool as it charges resulting in the battery heaters cycling to maintain temperature. Once the battery state of charge nears 80% the battery will become exothermic and the temperature will start to rise. Charging of NiCad batteries at higher temperatures can cause a chemical reaction that produces hydrogen or oxygen gasses in the battery. At this point the temperature rise rate should be monitored and if it exceeds a rate of 5 degrees per hour then charging should be terminated to minimize potential of gas buildup that could lead to cell failure or rupture. Monitoring the battery and cell voltages, the voltage will increase until they reach full charge and then slightly decrease before charging is complete.

Prolonged short discharge and recharge cycles can lead to a diminished capacity over time, this condition is known as memory discharge. In this condition the battery will discharge normally and then prematurely discharge rapidly to a secondary voltage level creating what appears as a step in the plotted voltage over time. By completing 2 full deep discharge and recharge cycles this condition can be minimized or eliminated. The process is called battery reconditioning it involves placing a large load on the battery and discharging it until the first cell reaches 1.0 volts (or 2/3 initial voltage) then reduce the load by one half. The voltage will slightly increase then decrease back to 1.0 volts again where the discharge is terminated. At this point the battery is left with no charge or load for a hour to allow the cells to stabilize. This open circuit stand is intended to allow cells to reach a chemical balance and independent cell voltages to equalize. This also minimizes the difference between the highest and lowest cell voltages, known as cell spreading. Charging current is applied and maintained until the battery reaches a full state of charge. This cycle is then repeated for a second time.

Self discharge will occur in NiCad batteries during storage periods due to the internal resistance of the battery. To overcome this affect a supplemental charge current at low level is applied to the battery known as a trickle charge.

Due to technological advancements in battery chemistry and design, driven by power storage density to weight ratio NiCad batteries have been replaced by more efficient Nickel hydrogen, and Lithium-ion batteries on GEO satellites.

Additional information about NiCad batteries is available on Wikipedia