NiCd Batteries A NiCd battery consists of a metallic box, usually stainless  steel, plastic-coated steel, painted steel, or titanium  containing a number of individual cells.  [Figure 9-34]  These  cells are connected in series to obtain 12 volts or 24 volts.  The cells are connected by highly conductive nickel copper  links. Inside the battery box, the cells are held in place by  partitions, liners, spacers, and a cover assembly. The battery  has a ventilation system to allow the escape of the gases  produced during an overcharge condition and provide cooling  during normal operation. NiCd cells installed in an aircraft battery are typical of  the vented cell type. The vented cells have a vent or low  pressure release valve that releases any generated oxygen  and hydrogen gases when overcharged or discharged rapidly.  This also means the battery is not normally damaged by  excessive rates of overcharge, discharge, or even negative  charge. The cells are rechargeable and deliver a voltage of  1.2 volts during discharge. NiCd batteries are capable of performing to its rated capacity  when the ambient temperature of the battery is in the range  of approximately 60–90 °F. An increase or decrease in  temperature from this range results in reduced capacity. NiCd  batteries have a ventilation system to control the temperature  of the battery. A combination of high battery temperature (in  excess of 160 °F) and overcharging can lead to a condition  called thermal runaway.  [Figure 9-35]  The temperature of  the battery has to be constantly monitored to ensure safe  operation. Thermal runaway can result in a NiCd chemical  fire and/or explosion of the NiCd battery under recharge by a  constant-voltage source and is due to cyclical, ever-increasing  temperature and charging current. One or more shorted cells  or an existing high temperature and low charge can produce  the following cyclical sequence of events:  Aircraft that are outfitted with NiCd batteries typically  have a fault protection system that monitors the condition  of the battery. The battery charger is the unit that monitors  the condition of the battery and the following conditions  are monitored. This does not become a self-sustaining thermal-chemical  action if the constant-voltage charging source is removed  before the battery temperature is in excess of 160 °F. Capacity is measured quantitatively in ampere-hours  delivered at a specified discharge rate to a specified cut-off  voltage at room temperature. The cut-off voltage is 1.0 volt  per cell. Battery available capacity depends upon several  factors including such items as: If the battery charger finds a fault, it turns off and sends a fault  signal to the Electrical Load Management System (ELMS). 1.  Overheat condition 2.  Low temperature condition (below –40 °F) 3.  Cell imbalance 4.  Open circuit 5.  Shorted circuit 1.  Cell design (cell geometry, plate thickness, hardware,  and terminal design govern performance under  specific usage conditions of temperature, discharge  rate, etc.). Capacity 1.  Excessive current,  2.  Increased temperature,  3.  Decreased cell(s) resistance,  4.  Further increased current, and  5.  Further increased temperature.  Figure 9-35.  Thermal runaway damage. Figure 9-34.  NiCd battery installation.  9-22