Sizing of capacity of DM starting baterries 1

[zeco]

Hi, all

My older collegues have chosen starting baterries 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 baterries 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 baterries.

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

http://www.prestolite.com/literature/stm/PP1205_MS7-400.pdf

or (same motor)

http://www.prestolite.com/pgs_produ...d=4383&item_series_id=86&refresh=1173205754).

And there is requirement of classification society (our applications are marine applications): baterries 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.

Please, help.

Thanks

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

 

[itsmoked]

Wow.. Engine starting is just so complex. I suspect the cost of analyzing the huge sea of data far surpasses the cost of "too much battery".

Other factors are:
Starter motors can take huge overloads.
Oil temperature causes large starting effort variation.
Bigger battery capacity increases battery life - significantly.
Cabling has a large impact.
...
...
...

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

I agree with itsmoked and then some. If you size your batteries to provide 3 x 5 second starting cycles, you will probably have batteries small enough to explode from boiling electrolyte the first time the engine doesn't fire on the second turn. (How do you spell "Eveready"?)
I see a lot of diesel generators, both shore based and marine.
Bigger is better for shore based diesels. When you consider the implications of inability to start an engine at sea, marine batteries tend to be larger than shore based batteries for the same starting load.
Consider this scenario: It doesn't happen often but it happens. You have a lot of water in the main fuel tank. One day it happens that you get a few inches of water above the fuel pickup in one of the day tanks. The engine quits. Check the oil and cooling and try to restart. Check the filters, change the filters and try to restart. Realise that the filters are full of water again and check the day tank, hopefully the first time. Drain the water from the day tank, operate the engine mounted fuel pump manually to purge the engine mounted fuel lines and change the filters again. Try to restart. The engine fires a few times but doesn't start. Must be water carryover and/or air in the injector lines. Crack the injector lines and crank the engine for 20 or 30 seconds 2 or 3 times. Tighten the injector lines, manually pump and bleed the filters and try again. Now it starts and falters for a half a minute and settles down to run. You put the load to it and all goes well for almost 3 minutes and then the residual water in the long, large fuel lines from the day tank, carried along by the velocity of the fuel with the pump running at full engine speed makes it's way to the filters and the engine starts to falter again. Change the filters again and possibly bleed the injector lines again. By this time, you are at least a few thousand percent over your textbook 3 x 5 second starting duty. Oh and by the way, these batteries are quite possibly a few years old and are still able to handle the job at hand even though their age may have reduced their capacity considerably.
After an oil change, many engineers like to disable the fuel solenoid and crank until the oil pressure comes up before allowing the engine to fire.
Yes, I am sure that your old timers are oversizing the batteries. They are also considering the performance levels of the local crews, the frequency and gravity of dumb mistakes, and a few other factors you won't find in books.
"Have you ever been to sea, Billy?"
"No Captain Highliner, but I've been washed ashore a few times".
Every couple of years we get a ship on the beach here due to engine or auxilliary failure in bad weather. They don't usually come off again. One about 5 or 6 years ago lost its auxiliary (and hydraulic steering) as it was trying to come in across a nasty bar. Water in the fuel tank. The crew tried to control with the propulsion engines but they were getting erratic from wet fuel. No injuries but vessel and cargo were a complete loss.
respectfully

 

[davidbeach]

To paraphrase waross' well written response - "ain't no such thing as excessive capacity in starting batteries."

 

[itsmoked]

If you look at the price of an engine and its installation and the price of batteries just doesn't even come up to your toe nail.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[zeco]

Thanks all for comments.

What you say is true, but you've missed the point of my question. There isn't problem at excess of battery capacity. That's normal and good practice. But, problem is that they don't have any analytic procedure to get approximate value of capacity (Ah) of battery for concrete starter motor. They ordinarily use their experince from previous ships. When I have some correct „basic“ number which I got from some procedure (e.g. 100Ah), then it's easy to take reserve of e.g. 50% (150Ah) or 100% (200Ah). But, I don't know get that basic number.

Waross,
5 second is minimum. I planed to take 15-20 sec for each starting in analysis (+ general excess of battery).

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.).

Regards

 

[waross]

Hi zeco;
That's a pretty big performance curve you've shown us. No indication where on the curve the starter will operate.
That depends on the characteristics of the engine, the size of the engine, the temperature of the engine, the ambient temperature of the air and the capacity of the batteries.
You may want to try sonething;
Look at an existing installation. Figure enough amp hours for at least 5 minutes of cranking when the batteries are older and down to about 60% or 75% of original capacity.
Look over to the right of the graph. Use about 1000 or 1500 amps to determine amp hours. Don't forget that at 24 volts you will have two twelve volt batteries in series.
Compare this to the batteries that are in service. Do this with several installations and see how your battery choices match the choices of experience. Expect a lot of variation between engines. There is a wider range of sizes for engines than there is for batteries.

Speaking of the graph, it may have been developed on a dyno.
On the engine, the current will be dependent on the circuit resistance, (starter resistance and back EMF, cables, and battery internal resistance).
The Back EMF will depend on the speed of the starter, which will depend on the ease with which the engine is cranked.
A graph of the starter current, speed and HP when turning a given engine will be a completely different graph.
respectfully

 

[zeco]

But, how can I figure amp hours for at least 5 minutes?
That is question (from the beginning) How?
Because, the next expression (if I take I=1000A for this starter of 9kW which has I=900A for max.output power):

Ah=1000A*(5/60)h=83.3Ah

is not good. At -18°C (the worst case for me) Icca=1000A
has got battery of cca 150Ah (from catalogue). Besides, CCA current means that battery will give e.g. above mentioned current of 1000A by 30sec till full discharged state (at -18°C). So, battery 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).
So, could you tell how can i figure that? And how can i involve temperature?

Regards and thanks

 

[waross]

A couple of points: You seem to be confusing amp-hours and cold-cranking amps in your last post.
Involve temperature by sliding over on the graph to about 1500 amps.
I use about six battery sizes:
for 12 volts.
Medium sized truck battery.
Large sized truck battery.
8D battery.
and for 24 volts:
Two x Medium sized truck batteries.
Two x Large sized truck batteries.
Two x 8D batteries.
That covers the range of about 12 KW residential generators to Vee-16 Cats pushing 2 Mega Watts.
I look at the set and the existing battery. I judge whether the existing battery is oversized or undersized.
Based on observation, history, and experience, I then see what's available on the local shelves and select the best compromise.
My method is not very precise, but after several decades of success in selecting batteries, and seeing all the various starting problems and failures that develop in the field, i wouldn't trust a scientific method if it was presented.
Any method of analysis will have so many parameters that are almost impossible to determine accurately that it would end up as a guess taken to 3 decimal places.
A couple of more points.
The starter doesn't put out 10 Hp. It develops enough horse power to turn the engine.
Cold cranking amps is a valuable value to compare batteries, but as soon as the load is applied the battery and the starter both start to heat up. This changes the draw of the starter and the cranking amps the battery may develop.
Low battery volts (partial discharge) and cool ambient temperatures will shift the curve into higher current regions.
I understand and appreciate your desire for a formula for battery sizing and I encourage you to continue in your attempts to develop a formula. Such an effort is, to my mind, one of the most powerful learning methods there is.
I have embarked on similar attempted rigourous solutions of problems. You will in all probability gain a full understanding of the subject, and then go on to size batteries by the experience you have gained and not repeat the rigours.
By the way, you may consider a few phone calls to engine manufacturers and ask for their recommendations for battery sizes for particular engine models.
respectfully

 

[itsmoked]

An excellent synopsis.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Thanks for the flattery David and Keith. I have a lot of respect for you both and that makes your comments all the more valuable.
respectfully

 

[zeco]

Thanks all, specially to you Waross. If I find out something more about this theme, I'll write.

 

[ruggedscot]

1.6 MW diesel Vee-16

Resiliance - data centre support. If we need the diesels then we need the diesels, believe me these have to start.

Data centres tend to carry a lot of processes that need to be supported and the idea of an outage just does not bear thinking about. Multiple gen-sets and each engine is fitted with double starters. The batteries fitted to these will ensure that the diesel will have a pretty strong attempt to start.

Honestly I really dont see the point of cutting or skimping on starter batteries. If those fail then you have probably comprimised a whole backup for the sake of a few dollars.

It dont make sense.