"Diesel engine-driven fire pump assemblies are commonly installed for backup systems to electric-drive fire pumps, or where electrical power may be unreliable. The required power source for electrically starting diesel engine-driven fire pumps is storage batteries. The batteries may be lead-acid, or they may be nickel cadmium if the code official approves. According to NFPA [National Fire Protection Association] 20, 'Standard for the Installation of Stationary Pumps for Fire Protection', each diesel engine driving a fire pump must have at least two storage battery units for starting. At 40 °F (4.5 °C), each battery unit must have twice the power capacity to sustain the cranking speed recommended by the diesel engine manufacturer through a 3-minute 'attempt-to-start' cycle. This works out to be six consecutive cycles of 15 seconds of 'cranking' and 15 seconds of 'rest'. Storage batteries must be on a rack above the floor and secured so they cannot be displaced. They should be located where they will not be subject to excessive temperature, vibration, mechanical injury, or flooding with water."
Coffee Break Training - Fire Protection Series No. 2008-3
United States Fire Administration: http://www.usfa.fema.gov/