Description of BB-5421U

TM 11-6140-224-13&P

2.3. Description of BB-5421U

The BB-5421U is a rechargeable 24-volt, 2 ampere hour, sealed nickel-cadmium battery containing 20 cells, charge control

and internal heating elements (fig. 2-1) with a capability of being charged in approximately 20 to 60 minutes under all

temperature conditions. The BB-5421U is designed to mate directly with the AN/PRC-70 radio by mounting under the radio.

A connector (MS3112E:14-5S) is provided on the side of the battery for cable connection with a G-76/G handcranked

generator or other power sources, so that the battery car be recharged and/or a AN/PRC-70 radio directly powered. The

BB-542/U contains controls designed to prevent overcharge, overdischarge, or rapid charging at extreme low temperature

conditions internal to the battery. Figure 2-2 is a schematic detailing the various control elements.

Cell Construction. Each of the 20 cells consists of a sealed cylindrical can containing the battery plates. The cells

are nonspillable and can operate in any position. Two of the 20 cells are equipped with pressure switches for charge

termination.

Electrolyte. The electrolyte which is a solution of potassium hydroxide (KOH) is completely contained within the

cells and requires no additions or changes throughout the life of the battery

Battery Box. The battery box provides a waterproof container for the cells, when used with the transmit cover or

mounted under the using radios. Two latches are provided for mounting to the catches on the radio. A manual vent relief

valve is located in the end of the battery box to release any vacuum induced by air transport.

Heater. A thermostatically controlled heating element of 10 ohm resistance, which provides 130 watts of heat at a

36 volt input, is located in two parts, one under and one on top of the battery cells. Heat transfer paste is employed to

rapidly transfer heat to the cells.

Control Circuits. Overcharge is prevented by stopping the charge when either one of two pressure switch equipped

cells measures an over-pressure condition, which signals the start of overcharge. Overdischarge is prevented by

terminating discharge when the battery voltage falls to approximately 20 volts. In order to accomplish this function without

excessive drain currents, the discharge cutoff uses a magnetic latch relay and a microampere sensing circuit which does

not drain the battery on standby. A fuse is provided in the event of short circuiting to protect the control elements. Low

temperature protection is provided by a thermostatically controlled charge limiter and a heater circuit which warms the

battery prior to the start of rapid charge.

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