Calculation of Battery CCA based on Peak Current and Average Current

[APPENG]

Something I have never done before as our vendors usually specify how many and of what type to use.

I have a 24 volt system (heavy truck) Using an MTU 16V 4000 Engine. The manufacturer specifies that the peak starting current is 1900 Amperes. Then once the Engine reaches 150 RPM the current is 850 Amperes.
I know that this this is basically an inductive load and has wave form on a time scale that peaks on a time constant, then falls off to the average current. Ignoring the rise and fall in current related to each compression stroke. How would I calculate the CCA Rating of the battery? There must be a rule of thumb for this right? Also this would be two or more 12V batteries in series parallel to get the 24V. Anyone have a way to do this?

Your help would be greatly appreciated.

Regards,
Jon

 

[waross]

The drop in current has less to do with induction than it does with rising EMF as the starter speed increases. If induction was a noticeable effect, the current would rise with time rather than fall.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[JohnMcNutt]

In the absence of more learned responses, I offer this from the school of hard knocks:

There are many variables in vehicle battery selection other than the draw of the starter. How cold does it get? What are the parasitic draws? How long does it have to sit between uses? What happens when the batteries start to lose capacity with age?

A 14 liter truck engine typically requires two 8D batteries, although one will sometimes do. If we used a straight ratio of number of batteries to displacement, a 65 liter engine would require 9 of them. (Of course, connected in series, they would need to be in multiples of two.)

My gut tells me that 6 or 8 would be the right number, but I would look around and see if you can find a similar engine in a similar truck and see if what they have been doing for batteries is satisfactory.

There are sizes larger than 8D available but I am not familiar with them in real life.

 

[itsmoked]

You've got it. You need batteries in parallel that total up to something a little more than 1900 CCA. Then the same exact batteries in series to get you up to 24V.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[iop995]

In most case battery capacity must increase with engine capacity. If start current is about 1900A, you need to assure enough voltage for needed time, at starter input, to develop such torque to begin move engine and speed-up (that lead to decrease current). So, main parameter for battery group is internal resistance. It's true that internal resistance decrease with increasing battery capacity and also with state of charge. Working with CCA, I think you need minim 30$ (time performance derating) CCA battery group over needed "flat" current (850A), so about 1100A. Must check also if group with CCA 1100A is able to delivery 1900A for about half second. Simply stated, higher CCA mean better but must take care about size, cost and charging current.