Sizing of capacity of DM starting batteries

[mat]

Hi

I wrote this topic on one other forum and i didn't get the answer (I didn't know for this forum at that time).
I hope that isn't bad thing

Question:

My older collegues at firm have chosen starting batteries for starting diesel motor on the basis of their experience (without some analysis). Although I am beginner at this area, it seems to me that such batteries have been often excessive. So, I am looking for some analytic procedure (which includes some equation etc) for the same thing, i.e. procedure which will lead me step by step to the necessary capacity (which is not excessive) of starting batteries.

Manufacturer of baterries define few typical values of each battery: Ah, CCA, MCA, RC (+ voltage). Manufacturer of DC 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: http://www.prestolite.com/literature/stm/PP1205_MS7-400.pdf or (same motor)
http://www.prestolite.com/pgs_products/spe...esh=1173205754). And there is requirement of classification society: batteries have to provide 3 starting with min. 5 second period of each starting. So, I have to take into consideration these requirement from three sides.

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


Please, help.

Thanks

mat

 

[Dunaruna]

Re: Sizing of capacity of DM starting batteries

[colour=blue]I'm not quite sure what you are looking for, CCA is cold cranking amps - that equation is always supplied by the battery manufacturer, is that what you seek?

Maybe a bit of background info would help.

 

[mat]

Re: Sizing of capacity of DM starting batteries

There is concreter question:

What capacity of battery (at Ah) will you chose for starter motor of 9kW (link is in my first post) which have to start 3 time (e.g. 5 sec for each starting)? And, please describe how did you come to that value (do you look CCA at battery etc.).

Thanks

 

[Silvertip]

Re: Sizing of capacity of DM starting batteries

The battery needs to be sized to handle the current (amperage) draw of the starter when its installed on the engine. Since the horsepower of an engine determines how large the starter must be, and hence how much current the starter will draw at full load, that value (cold cranking amps or CCA) is the absolute minimum value required. A battery with a higher CCA does not make the starter turn faster, it merely provides longer cranking time should the engine not fire immediately. The 9000 KW rating you indicate is probably the power draw of the starter at its maximum load rating.
In that case 9000W/12.6V = 714 amperes. If this were my boat I would install an 850 - 1000 CCA battery to provide some reserve. The difference in cost is very small and well worth the peace of mind. On a boat, my theory is install the largest capacity battery you have room for. You cannot go too big -- only too small.

 

[mat]

Re: Sizing of capacity of DM starting batteries

Yes, 9kW is max. output power of starter motor. But current curve from data sheet of that starter motor (link is in my first post) shows that current for full load (9kW) is 900A (it isn't equal to 9000/24=375A; 24V is voltage of that motor).
You said that I should take battery with Icca more than 900A (I can chose it from catalogue of battery manufacturer)?
But, how can I be sure that this battery will be enough for e.g. 3 starting with min. 5 (10 or 20) second period of each starting? I think I have to know some analytic procedure for that. Because, it's a question how many starting cycle (e.g. 1 starting cycle is 5 sec) does that battery provide? Maybe 5, but maybe 100
So,
I think next expression is probably ok:

spent Ah for 1 starting=900A*(5/3600)h=1.25Ah.

I have three starting:

spent Ah for 3 starting=3*1.25=3.75Ah.

So, if I have battery with 135Ah (Icca=950A),
it's going to rest (if I have three starting):

135Ah - 3.75Ah=131.25Ah


What do you think about this solutin?

Thanks and regards

 

[Silvertip]

Re: Sizing of capacity of DM starting batteries

Your logic is valid but the concern is hardly worth the worry. Virtually any 900 - 1000 CCA battery on the planet will meet your starting criterea with no problems. The specifications for the battery (on the label or catalog) will specifiy what the "reserve capacity" is for that battery. The starter will not likely draw anywhere near that 900 amperes in normal service. If it did, there was a problem in starter capacity selection for that engine as it should not take 100% capacity to start the engine. Since 900A is the maximum specification for the starter I suspect in normal service it would draw perhaps 600A. To make the starter draw 900A you would have to fully load it which means the engine would be either very cold, seized or very close to it. Except when weight and physical size is an issue, bigger capacity is always better when dealing with batteries.

 

[mat]

Re: Sizing of capacity of DM starting batteries

Ok.

Short circuit current, e.g. first pick of starting current of this starter is cca 1800A. 900A is current of max. output power - you think that "usually" daily starting current will be less than 900A?I took 900A because it's practice to take the worst case in the calculation (900A).
In expressions of my last post I made mistake. I didn't take in calculation that CCA current means that battery will give e.g. above mentioned current of 950A by 30sec till full discharged state (at -18°C). So, baterry will give to starter her Icca current only 30s (at -18°C), so 30sec is the time that I have for (all number of) starting with that current (Icca).Actually, current of starter is (little) less than Icca of battery, so I will have (little) more time for startnig.

Is this thinking OK?

Regards and thanks.

 

[Silvertip]

Re: Sizing of capacity of DM starting batteries

In my view, worst case is "stalled armature" which should be the same or very close to the initial connect current. Just because 900A is the maximum power output for the motor does not mean the motor must exert maximum power to start the engine. Yes -- I feel the current draw during engine start will be much less than 900A. Back in the 1980's I owned several 5.7L diesel power cars. Those cars used 12V starters powered by two 12V batteries in parallel and I never had any starting issues, even in winter.

 

[mat]

Re: Sizing of capacity of DM starting batteries

Thanks, Silvertip and Dunaruna. If I find out something more about this theme, I'll write.