Sizing of capacity of diesel motor starting batteries

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Sizing of capacity of diesel motor starting batteries


How can I size capacity (at Ah) of starting batteries for diesel motor? I am looking for some analytic procedure (which includes some equation etc.) for the sizing of battery capacity, i.e. procedure which will lead me step by step to the necessary capacity (which is not to much excessive) of starting batteries.

My older collegues have chosen starting batteries for starting diesel motor on the basis of their experience, but without some analysis.

There are requirements from three sides that I have to take into consideration:

1.) Manufacturer of baterries define few typical values of each battery: Ah, CCA, MCA, RC (+ voltage).

2.) Manufacturer of starter motors define curves of correlation of starter torque, speed, current and terminal volts (there is a data sheet of starter that I have at diesel motor at latest project:
or (same motor)

3.) And there is requirement of classification society (our application are marine application): batteries have to provide 6 starting with min. 5 second (you can take 10 second) period of each starting.

Please, help.

If there is some web sites with this issues, please say. I googled, but didn't find nothing pragmatically.

I hope to your soon answer.


Edited by DavidC on 6 Sep 2007 - 09:30
Not registered

  Ah (Ampere-Hours) tells you how much power a battery will store.  The current multiplied by time in hours gives you the Ampere-Hours (Ah).  So a current of 1 (one) amp for 1 (one) hour would be one Ah (Amp-Hour). 

  Batteries that are utilized for Gasoline engines like SLI (Starting-Lighting-Ignition) batteries are not rated in AH but rather in the CCA (cold-cranking amps), due to the fact that they are designed only for use in starting the engine, once the engine has started they should not be used.  

  The reasoning is that deeply discharging this type of battery will Greatly reduce its life.  Also it will have statements such as 5 hour rate or 15 hour rate, this means that the battery will discharge perpetually over 5 or 15 hours, and the AH (amp-hour) capacity is a measurement of how much it puts out before reaching 100% DOD (Depth of Discharge).  The DOD is how much of the available charge is used compared to 100% (or 1.75 volts per cell, or 7.2 volts for 12 volt battery).

  The RC (Reserve Capacity) is generally used on deep cycle batteries.  The RC is the number of Minutes a battery can manage a useful voltage (1.75 volts per cell).  When using a system with heavy load requirements the Reserve Capacity (RC) is a more useful measurement.  AH equals RCx.60 (Minutes vs. Hours)

  MCA (marine cranking amps) basically is the same as CA (cranking amps) which is the measurment of how many amps the battery will put out for 30 seconds at 32 degrees F, before the voltage drops below 1.2 volts Per Cell.  The CCA (Cold Cranking Amps) of a battery is much more important at cold temperatures because the CCA is measured at Zero (0) degrees F, versus 32 degrees F for MCA and CA. 



Not registered

Did your question ever get answered?

I know what the abbreviations mean, I (like you) am looking for some hard numbers on how many CCA I need to provide my Perkins 6.354. My boat had an 8D rated for 620 CCA. It's an old and tired battery that should be replaced, but with what?

The local Perkins shop says to use a type 27. Thanks but I'm looking for some numbers? How many amps should I be able to provide? Am I throwing money away buying an 8D? Am I safe at low temperatures at anchor with a type 27 starter battery?

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