UFC 3 -520-01
June 10, 2002
BATTERY SIZING FOR ENGINE STARTING APPLICATIONS.
Depending on the design, a diesel engine might be started by an air system
or an electric motor (starting motor). Electric starting is the most convenient to use, is
usually the least expensive, and is the most adaptable method for remote control and
automation. The ambient temperature and lubricating oil viscosity affect the starting
ability of a diesel engine. The diesel relies on the heat generated by compression to
ignite the fuel. When first starting, this compression and heat is created by the diesel
cranking (starting) process, which is a function of the cranking speed and cranking time.
When the engine is cold, longer crank ing periods are required to develop ignition
temperatures. The battery powers an electric starting motor to accomplish this cranking
process. Lubricating oil imposes the greatest load on the cranking motor; oil viscosity
varies with oil type and temperature. For example, Society of Automotive Engineers
(SAE) 30 oil viscosity approaches that of grease below 0 C (32 F).
Either lead-acid or nickel-cadmium batteries can be used for engine starting.
The nickel-cadmium type is often used so that the battery can be located very near the
engine, which is usually a higher ambient temperature environment. Also, nickel-
cadmium batteries are capable of very high-rate discharges for the few seconds needed
for an engine cranking application.
In many cases, the associated battery's only purpose is to provide cranking
power to start the diesel engine. In these cases, battery sizing is performed differently
than described in the previous sections. The primary considerations for sizing a diesel
engine battery are:
The lowest temperature at which the engine might be cranked. Oil viscosity
increases with decreasing temperature and affects how long the starter motor must
turn before fuel ignition temperature is reached. Note also that lower temperatures
affect the battery's capacity. At lower temperatures, the battery's capacity requires
adjustment for both oil viscosity and decreased battery capacity. For very cold
applications, consider engine heaters or glow plugs to minimize the battery size
How many start attempts will be allowed. Select a battery that can provide at least
four 30 second cranking periods (total of 2 minutes of cranking). Engines are often
rated for up to 30 seconds of cranking before the starter motors begin to overheat.
Confirm the starter motor limitations with the manufacturer.
EGSA 100B provides guidance for sizing a diesel engine starting battery.
This performance standard should be used for battery sizing; it recommends providing
the following informati on to the battery manufacturer as part of a battery sizing
Nominal volts needed for the starter motor.