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Author Topic:   Dual Engine, Dual Battery Primary Power Distribution
jerm2008 posted 07-25-2011 09:04 AM ET (US)   Profile for jerm2008   Send Email to jerm2008  
I am just learning my way around a 2001 [Boston Whaler 26-foot CONQUEST], and I am confused about the battery setup. [This Boston Whaler 26-foot CONQUEST] has the usual dual-battery selection switches. Here is my confusion:

--[with the Port battery] switch on "1" and [Starboard batttery] switch on "2", [a voltmeter located on the on-board electrical] panel reads 12-Volts, but [neither] engine will start due to sluggish cranking;

--[with the Port battery] switch [set to] "OFF" and [the Starboard battery] switch [set to either "1" or "BOTH" or "2"] [the same voltmeter on the electrical] panel [indicates] 12-Volts;

--[with the Starboard battery] switch on "OFF" and Port switch on [either "1" or "BOTH" or "2"] [the same panel meter now reads] 0-Volt;

--[an independent] Voltmeter reads 12.6-Volts on each battery;

--[both] engines fire right up when both [the Starboard and Port battery] switches are [both] set to "BOTH" [and there is] much stronger turn over;

--once we get going I put the switches back to the running configuration [which is] the Port switch to "1" and the Starboard switch to "2"; and,

-whenever I need to re-start through the day [either engine will] fire right back up without moving [either] battery switches back to "BOTH".

Why does the Port battery switch not show any [voltage] on the inside panel?

I know [the primary power feed for the panel is] from [the Starboard] battery [switch] only when engines are off. Is the panel isolated from the port side selector?

Even though the manual states to start the engines in "BOTH" I am hesitant to rely on this in the event of a dead battery.



Chuck Tribolet posted 07-25-2011 09:40 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
What engines?
boatdryver posted 07-25-2011 10:24 AM ET (US)     Profile for boatdryver  Send Email to boatdryver     
I wonder if the Port battery switch is controlling a battery bank that is dedicated for starting only and not connected to the house 12-VOlt circuits.

Is the port battery a cranking type battery and the starboard battery a deep cycle?

Also how new are all the batteries? It is odd that you have to use "BOTH" to start one engine at a time.

Other things to consider: loose or corroded connections, undersized cables.

Can you get any info from the previous owner? if not, a marine electrician can sort this out for you.


jimh posted 07-25-2011 08:39 PM ET (US)     Profile for jimh  Send Email to jimh     
There really is not a "usual" configuration when it comes to dual batteries, two switches, and two engines. Most likely the wiring for the primary power distribution is something like that shown in my REFERENCE article, which is common on Boston Whaler boats. See

I suspect the configuration you have similar to that shown in this illustration:

Schematic diagram: two batteries, two switches, two engines.

but it sounds like the secondary power distribution is being fed by the Starboard battery switch.

Now to explain your observations:

Power to the secondary distribution panel is controlled by the Starboard battery switch. The panel voltmeter shows the voltage on whatever battery is selected by the Starboard battery switch.

At the initial start-up the engines will not start unless they are being fed by both batteries. This is due to weak batteries or an unusually high demand for starting voltage. If you have the Mercury two-cycle motors that use the Orbital Combustion Process under license--Mercury calls them OptiMax motors--these engines are infamous for needing a cranking battery with a rating of 1,000-Amperes. If there is any voltage sag during engine cranking these Mercury motors are infamous for not starting. Their electronics won't run properly if the voltage is low, so the engine can never start. If you have those Mercury motors, you will need to have cranking batteries that are in excellent condition, are fully charged, and can deliver a very high cranking load without voltage sag.

After you have run the engines for a while, the charging current from the engines will top off the charge on the batteries, and the batteries can provide enough power to start each engine without being connected in parallel. A warm engine is also probably easier to crank over and faster to start than a cold engine.

I recommend you check all the connections in the primary power distribution system. To check connections you need to disassemble the connections and carefully clean and burnish the actual contact surfaces. I usually do this by careful wet-sanding with emery cloth of 600-grit and using WD-40 as the wetting agent--never water. Also carefully check the connections of any terminal lugs for signs of corrosion. Usually Copper when in contact with water will produce Cuprous Oxide, a green oxidation. If there is any sign of this on the terminal lugs, the connection is suspect.

I agree with your observation: you don't want a set-up where it is always necessary to parallel both batteries to get an engine started. That is just asking for trouble.

jerm2008 posted 07-25-2011 08:59 PM ET (US)     Profile for jerm2008  Send Email to jerm2008     
The engines are Mercury Optimax 200-HP engines that were standard for this model.

The diagram that you posted seems spot-on, Jimh, and I agree with your diagnoses about the initial cranking load.

I didn't have a chance to pull the batteries out of the protective boxes yet to get the model, age, and cold cranking amp info. I did inspect the terminals for loose connections and obvious corrosion but didn't see anything. None the less, I agree that a cleaning is in order.

Also, the port side battery selector can be near impossible to move at times. Does this have any relevance?

Thanks for the help,


jimh posted 07-25-2011 09:53 PM ET (US)     Profile for jimh  Send Email to jimh     
Plan on replacing the Port battery switch if it seems to be mechanically difficult to operate. They are not very expensive, and a problem with primary power distribution could be disastrous.

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