CONTINUOUSWAVE

[Moved to SMALL BOAT ELECTRICAL for discussion.]

I have a 2011 Brunswick Commercial products Guardian 25 boat. The starboard engine will not start using the starboard battery.

The starboard engine only gets enough power to crank when the boat is connected to shore power or when the battery switch is moved to the EMERGENCY PARALLEL position that connects together the starboard and port batteries.

The boat also has a house battery in addition to the port and starboard batteries.

There is a battery switch for each battery and its circuit.

To test the starboard battery, I moved the port battery and starboard battery to each other's position, I did that to eliminate a potential bad battery [at the starboard position]. However, with the batteries swapped the problem persisted.

I have checked the switches and all of the visible wiring I can access with no solution. I reached out to the Brunswick Commercial Products. They saild there was no wiring diagram available since [producing a wiring diagram for this custom-ordered bpat] was not specified in the build sheet.

I found a maintenance and operations manual [at the hyperlink below], but it lacks detail.

https://www.manualslib.com/manual/10168 ... =93#manual

Any help would be greatly appreciated, particularly before I have to take the boat to a Boston Whaler dealer.

That "manual" looks like it is composed mostly of boiler plate.

I will offer more advice when I have a moment.

Your test conducted by swapping the batteries between starboard and port was a good idea. The results tend to indicate that the cause of the low starting power for the starboard engine was NOT due to the original starboard battery having poor deliver of 12-Volt power.

The most important element in your narrative is that the starboard engine would crank over when the EMERGENCY COMBINE switch positions were used. This suggests that the wiring to the engine when the two batteries are combined in parallel is sufficient to start the engine. This could really mean that there is something wrong in the wiring that connects the starboard battery to the starboard battery switch. That is the element of the circuit that would not change when you swapped battery locations.

The fault could be in a conductor that brings the starboard battery positive terminal to the starboard battery switch, but the fault could also be in the NEGATIVE wiring from the starboard battery to the engine. Both of those paths did not change in your test.

On that basis, my recommendation for you is:

--carefully check the wiring and connections in the NEGATIVE circuit from the starboard battery negative battery terminal to the starboard outboard engine, and;
--check the wiring from the OUTPUT terminal of the starboard battery switch to the starboard engine.

You should be aware of the possibility in wiring involving low voltages--like 12-Volt circuits--that the visual appearance of a connection may look normal but that connection may have a very poor electrical connection. It takes only a very thin layer of oxidation to stop the flow of current at 12-Volts.

You should inspect all the connections in the two circuit paths suggested above, including unfastening the connections at terminal posts, spraying the ring connectors and the terminal post with WD-40, and if necessary wet sanding those surfaces with emery paper of about 400-grit wetted with WD-40 to remove any oxidation.

Avoid stacking ring terminals on one terminal post as much as possible. If you must stack two or three ring terminal connectors on a terminal post, make sure that both sides of all the ring terminals are free from oxidation.

Never use a wire brush to clean electrical contacts or ring terminals. You can use an old toothbrush and WD40 to scrub terminals and ring connectors. If any doubt, use 400-grit emery cloth wetted with WD40 to clean the surface of the ring terminal connectors. Do not remove the tin plating on the ring terminals.

The connection of the any ring terminal connector to the actual wire in the conductor is also a possible source of problems. Inspect all of those connections.

In an article in the REFERENCE section I give a list of all possible connection points in the HIGH current portion of the engine starting circuit that could cause trouble in engine starting. See

Electric Starting Circuits
https://continuouswave.com/whaler/refer ... Start.html

Deducing the wiring of the primary power distribution from the batteries through the battery switches to the engine and other loads should be easily done by a careful visual inspection of the wiring and taking very clear notes.

A wiring of dual batteries for dual engines that is often seen in Boston Whaler boats is described in my article at

Dual Battery Wiring
https://continuouswave.com/whaler/refer ... ttery.html

under the heading of "Boston Whaler Factory Installation."