[Dave Young]

My 2004 3500 Open with 370 Cummins had, and still has, 5 group 31's.  Three for the house and two for the engine bank.  She wouldn't crank over without using the combiner solenoid either.  

By using a battery tester, I found that even though the starting batteries showed a voltage reading of 13.2, their cranking amp capacity was only 30% of their new rating.  They were 6 years old.  So, I replaced them all and went with two AGM 31's for the engine bank and three Oasis FireFly 31's for the house.  I couldn't be happier!  I installed two new alternators, left the Charles regulator in place to control the port side and put in a new Balmar external regulator for the house along with Balmar's SG200 battery monitor down in the DC panel.   She wings over beautifully and those house batteries let us anchor out all we want.