T&T: Cummins QSM11 question

[Steve Bedford via Trawlers-and-Trawlering]

I have a single Cummins QSM11 660 hp diesel on my new boat. Looking to hangs battery bank abs configuration. Currently, the 12 volt AGM start battery bank serves the QSM11, Northern Lights generator, and bow thruster. Want a have a dedicated AGM just for the QSM11. Hard to get consistent information on the max amps required for starting the QSM11. An 8D AGM is reported to have 1800 Marine Cranking Amps (MCA) at 32 degrees and 1450 Cold Cranking Amps (CCA) at 0 degrees. The placement of the battery has a very typical run with very large 4/0 to the starter. Obviously I would prefer to have one 8D rather than two in parallel (3600 MCA or 2900 CCA).

Does anyone out there have this engine and what battery size do you use to start it (and have there been any issues with the arrangement)?

Thanks
Steve
----------------
Steve Bedford
maxmarineproducts.com
Home of the Super MAX Anchor
M/V No Regrets II, Legacy 42
Burgess, VA

Sent from my iPad. Please excuse misused words due to the "auto correct" function.
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[TW Collins via Trawlers-and-Trawlering]

snip:

I have a single Cummins QSM11 660 hp diesel on my new boat. Looking to hangs battery bank abs configuration. Currently, the 12 volt AGM start battery bank serves the QSM11, Northern Lights generator, and bow thruster. Want a have a dedicated AGM just for the QSM11. Hard to get consistent information on the max amps required for starting the QSM11. An 8D AGM is reported to have 1800 Marine Cranking Amps (MCA) at 32 degrees and 1450 Cold Cranking Amps (CCA) at 0 degrees. endsnip

Steve,

The Cummins spec on the QSM11 (http://www.sbmar.com/docs/QSM-11%20General%20data%20sheet.pdf) states the minimum recommended battery capacity for CCA as 1800 and the MCA at 2250 amps for a 12 volt system.

Obviously the specs you cited on the 8D you are considering don't meet those minimums. Choices are to use more/bigger batteries or consider the actual conditions you will be experiencing. If you don't expect to be starting the engine at those low temperatures, then likely the 8D will suffice.

Another alternative to consider is to use one large battery bank for starting and house with a separate generator starting battery for backup. This would give you more than enough starting reserve and put all your batteries to work which will ease the stress on all of them. I won't go into detail as this alternative has been discussed on this list previously.

Tom Collins - Misty Sea
presently at anchor, Zihuatanejo Bay, Mexico

[Jim Healy via Trawlers-and-Trawlering]

Steve,

Check the electrical specs section of the QSB11 owner's manual. It will give you a spec for Reserve Capacity, which is a duration measure of a batteries intended for "start" service (RC = minutes at 25 amps and 75 degrees F until the battery reaches 10.7 volts). For your engine, I'd bet a RC of 1000 or more would be right. If you really care about the amps the motor will draw, you need to look at the ratings published for the starter motor. In your case, "locked rotor" would be approximately 750 amps, and full load would be approximately 300 amps, and no load (which you don't care about, and is of laboratory interest only) would taper off to be approximately 150A, but NO LOAD would also result in a series-wound starter motor over-revving and flying apart.

By the way, a good quality Group 31 battery should do you fine. Furthermore, be advised, if all you're doing is starting the engines, you should go with "Start Service" batteries, not "traction" batteries. The AGM hybrid "Commercial" batteries are a reasonable compromise. If you go with hybrid or traction batteries, they will need to be a physically larger form factor to meet the Reserve Capacity rating.

Many deep cycle batteries are only rated in amp hours, and equally, many start service batteries are rated only in CA/CCA and RC. For start service batteries, RC is the more demanding measure on the battery manufacturer. If your starter motor draws 700A for 5 seconds, you have used 300A*(5/3600 hrs)=2.7 amp hours. If you run a thruster drawing 150A for two minutes, you've used 150A*(120/3600)= 5 amp/hours. So having a large deep cycle battery bank for motor starting applications IS NOT NECESSARY. BUT AMPS ARE! LOTS OF AMPS FOR JUST A SHORT PERIOD OF TIME. ERGO SIR KNIGHT, START SERVICE BATTERIES for big currents in short bursts.

Jim

Peg and Jim Healy aboard Sanctuary, currently at Charlotte Harbor, Punta Gorda, FL
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