DC Electrics - Batteries — Page 60, Lesson 57

Let’s talk about the Nickel Cadmium battery — often called the NiCad or alkaline battery. This is the type that has largely replaced lead acid batteries in modern aircraft, though you’ll still find lead acid in some smaller aircraft. The NiCad battery uses different materials for its plates and its electrolyte. The positive plate is made of nickel oxide, the negative plate is cadmium, and the electrolyte is potassium hydroxide — that’s the alkaline part, which is why we call it an alkaline battery. The specific gravity, or SG, of that electrolyte is between 1.24 and 1.30. Now, the on-load voltage of one NiCad cell — that’s the voltage you measure when the cell is actually supplying current — is about 1.2 volts. And here’s a key difference from the lead acid battery: in a NiCad, the relative specific gravity of the electrolyte does not change as the battery discharges or charges. Also, the voltage variation from fully charged to fully discharged is very slight. That means you cannot simply measure the voltage or the specific gravity to tell how much charge is left. The only way to determine the state of charge is to carry out a measured discharge test — that’s also called a capacity test. Throughout most of the discharge, the terminal voltage stays substantially constant at 1.2 volts. Because the NiCad has a low internal resistance, it can supply a high current during discharge and also accept a low current during recharging without violent fluctuations in terminal voltage. But there is a serious hazard we have to watch for. NiCad batteries have a low thermal capacity — that means they don’t absorb and store heat well. Under certain conditions, heat is generated faster than it can dissipate, so the temperature rises rapidly. This rising temperature lowers the effective internal resistance of the battery. A lower internal resistance allows an ever-increasing charging current. If that current is not checked, it leads to total destruction of the battery. This condition is called a thermal runaway. It can generate so much heat that the battery may actually explode. For that reason, the charging of a NiCad battery must be closely monitored, and the battery includes some safety features. A built-in thermal switch monitors the temperature. When the temperature reaches a preset value, that switch operates and effectively isolates the battery from the charging source. The battery stays isolated until the temperature drops enough for the switch to revert back to its normal position. Associated with that temperature switch, there may be an indicator light on the flight deck to alert the pilot. Despite that risk, the NiCad battery is more robust than lead acid and can hold a constant terminal voltage much better during the discharge cycle. That’s why it is much preferred in large modern aircraft. In the event of a total failure of the aircraft generators, the NiCad battery will provide a much more stable voltage. Figure 4.8 in your materials shows a graphical comparison of the discharge voltage of a lead acid battery against a NiCad during discharge — you can see how much flatter and more stable the NiCad voltage stays.