Starter Motor Current Demand

[LORDNELSON]

How much current does a starter motor take/demand at its peak output of power when starting an engine? For example supposing ther starter is marked 2.5 kilowatt is its peak demand 2500watts divided by 12 volts = 208 amps? or is it possible that it demands more current at the stage when it commences turning the engine over and has to overcome the inertia of the flywheel, friction etc ?

 

[Guest]

The starter motor is basically a series field DC motor. It is designed to draw maximum current whilst its rotor is stationary. ie. at the moment of switch-on. This is what gives it the grunt to turn the engine.
If it is rated at 2.5kW max, then it tries to draw the 208 amps you mention from the battery. It is close to a short circuit on the battery at that instant. As the starter motor rotor starts to turn it begins to build up a back emf which has the effect of reducing the current drawn. At cranking speed the drawn current is significantly lower than maximum.
Starter batteries are normally rated in amp/seconds. This rating describes the current that can be drawn from the battery in the first second after switch-on.
In car batteries, 320 amp/sec is typical. This means that, provided that big fat battery leads are employed and the solenoid contacts are in good condition, the battery is rated to deliver 320 amps for the first second after switch on.
The current actually delivered will then be determined by the resistance of the static rotor + field coil resistance. About 0.006 ohms. In this case approx 200 amps, well within the capability of a 320 amp/sec. battery.
Hope this helps, Rod.
e-mail me if more help needed.

 

[vyv_cox]

That's a good, informative post. I roughly knew the numbers (one of those strange Trivia memories, I suppose) but not the thought behind them. Thanks for that.

 

[LORDNELSON]

Many thanks indeed for a very clear explanation. Barry

 

[ean_p]

My perkins 4-108 has a max starter current of 900amps ......according to the workshop manual.....

 

[johnsomerhausen]

LordNelson, you wrote "batteries are rated in amps/second". I always thought they were rated in "amps/hour" (i.e. 3600 times more...)Otherwise, how could a 100 amps battery delivers the 200 amps needed to start a diesel ?
john

 

[tyger]

Re: Battery Rating

Manufacturers give batteries two classes of ratings.

Ampere hours measures the capacity of the battery to handle a steady small current draw (amps or tens of amps). This is the value used when calculating battery capacity for domestic use.

The other rating is CCA (Cold Cranking Amps). This is a measure of the battery's capacity to supply the short burst of high current to the starter motor.

EG MK's 8G24 12v gel battery has the following specifications.

CCA at 0 degrees F is 410 amps

Ampere hour capacity (20 hour rate) 40 ampere hours.

Note that both capacities are quoted for specific conditions: CCA is at a specified temperature (0), ampere hours are quoted at a specified discharge rate (20 hours).

 

[LORDNELSON]

Just to keep the record straight I did not write about amps/second. I posted the original query and amps/second were mentioned by border-reiver in his excellent reply!