Marinette Aluminum Boat Maintenance/Electrical/Starting Circuits

The problem
The boat is a 37 aft cabin, with twin 440 Chryslers. I tried to start the engines (just to make sure they start) and the STBD engine won't turn over. If I use the parallel switch, it starts fine. Worked fine all summer up to now (mid-July).

I am not certain of how my three batteries are arranged--haven't had to worry about it until now. I have two batteries behind the STBD engine and one battery behind the PORT engine. For this discussion, we can ignore the port battery.

I disconnected the outboard battery behind the STBD engine--completely--NO wires connected to it. Had my wife try the STBD engine, and it turned over normally. So, obviously, the battery I disconnected is not the STBD starting battery. However, when it is connected, I cannot start the STBD engine without using the parallel switch...almost. Immediately after re-connecting the previously disconnected battery, I flipped the starter toggle and the STBD engine turned over. I fliped the switch off, then on again--nothing, except some buzzing from under the black cover with red reset switches on it that is mounted on the engine (don't know what it is called). No way to turn over the STBD engine without using the parallel switch.

I am wishing I had my wife try several times with the aforementioned battery disconnected to see what happened with multiple attempts, but we were rushed for time and had to get home, so now I have all night to ponder this problem.

Batteries were (so I am told by previous owner) brand new last season. Water covers the plates in all the batteries. The battery I disconnected reads 13.56 VDC.

Possible answers
Your batteries are probably arranged with two batteries on the STB connect in parallel (12.5 volts) The port bank is a single starting battery. On most Marinettes the majority of the load is on the stbd bank of batteries. At least on my the factory wiring has almost everything but one bilge pump.. nav. lights and port starting on the STB bank. It varies depending on age.. configuration and what might have been added over the years.

The problem with having 2 batteries tied together in parallel is that the bank is only as good as the weakest battery. A bad cell for example will draw amperage from the good battery and pull them both down. Charging is the opposite.. the highest voltage battery will tell your regulator to shut down.

Assuming the STB will start without parallel switch with only one battery connected on the STB bank, it sounds like a bad battery. A bad battery could easily read 13.5 volts after being energized by the alternator but will have no reserve cranking capacity and will pull down quickly dragging the other battery down with it. Generally a good battery will hold a static charge of approx 12.5 volts at rest. A battery tester is the best way to test the capacity. The buzzing at the solenoid is low voltage caused by high current draw. Also check your wires under the black plastic cover at the solenoid. The wires often work loose. Make sure Contacts are clean and all wires are secure on the lugs. Do not knock off the yellow wire.. it is easy to do and it is your neutral disconnect switch.. if it falls off you will not be able to turn engine over at all.

For information I added a 4th battery to my Port bank using a ACR switch. This switch is sophisticated combiner switch. It allows current to flow into the 4th battery when charging at 13.6 volts or more. When voltage drops below.. (like when engines are off) it disconnects the 4th battery and holds it safe from discharge. You can over-ride it by flipping a switch.

The simple solution
The terminal connection (a multiple-type fitting with multiple leads on it) was loose, corroded and dirty on the positive side of the inboard-starboard battery. Cleaned connection with wire brush, tightened clamp fitting, and like magic, the problem was solved.