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Old 05-20-2019, 07:18 PM   #46
Senojev
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City: Portage
Vessel Name: Sir Tugley Blue
Vessel Model: 2010 Nordic Tug - 2010
Join Date: Sep 2017
Posts: 78
QSB idle

I too have a Cummins QSB (5.9) that came with a 125A Delco alternator and it was not a great performer at our typical engine speed of 12 to 1400 rpm. Our start battery, a 4D Lifeline was just that, starting only. All other power for the engine, electronics, etc. came from a 440 Ahr house bank. Our typical output (net) from this alternator was always positive at idle and our load was around 40A with radar, radio, Furuno BBMFD, and a 24” Apple Cinema Display for our screen all powered up. We subsequently changed the alternator for a Delco 250 amp, externally regulated, alternator designed for bus application, i.e. low engine rpm and high loads at idle. We noted significant improvement in low speed and idle charging and have had no issues running at WOT with the right sensors and belt management settings.

To pull your battery voltage down from fully charged to 12.4V (approx 50% discharged) in a short time indicates to me that you may well have a battery issue or charging system issue. Assuming your worst case draw of 10A and a 200 Ahr battery you should be able to idle for many hours before reaching 12.4 V ... definitely more than 6 hrs. A simple load test should be able to pinpoint if the battery is the issue
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