Dual-battery Dual-engine Wiring

Electrical and electronic topics for small boats
outrage 25
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Joined: Mon Mar 22, 2021 10:46 am

Dual-battery Dual-engine Wiring

Postby outrage 25 » Sat Aug 20, 2022 11:00 am

[Moved to SMALL BOAT ELECTRICAL for discussion from another forum.]

My 25-foot boat is rigged with twin Mercury 200-HP [two-stroke-power-cycle] EFI engines. In 2021 the left engine tachometer did not work.

The 12-Volt power on the boat is supplied by two batteries and ONE OFF-1-BOTH-2 primary power switch that allows me to select which battery to use and both engines charge the battery selected by the switch.

In 2022 the left engine [voltage] regulators [apparently there are more than one on each engine] were replaced with updated models.

On August 20, 2022 while underway, I noticed the left engine tachometer again did not work.

When I returned to port at engine idle speed the left engine tachometer began to work again. I noticed on a Garmin instrument that the battery voltage was 13.5-Volts rather than 14.2-Volts, and from that I deduced that it [something unidentified] is charging only one engine.

Then I opened the fuse box on the upper part of the engine that houses three fuses. I realized that one of the three fuses was not broken but was overheated. This is exactly what Mercury recommends to replace with the new model regulators from 20-Amperes to 30-Amperes.

I would not like to keep buying regulators and burning them all the time. I don't know if it's a coincidence but the problem happened while I was sailing with high waves and in any case the problem always appears in the left engine. Thank you.

jimh
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Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
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Re: Dual-battery Dual-engine Wiring

Postby jimh » Sat Aug 20, 2022 1:00 pm

outrage 25 wrote:The 12-Volt power on the boat is supplied by two batteries and ONE OFF-1-BOTH-2 primary power switch that allows me to select which battery to use and both engines charge the battery selected by the switch.
I believe this wiring is the cause of your problem that the voltage regulator/rectifier assembly in one of the engines burns out.

My recommendation is that you re-wire the 12-Volt primary power distribution so that there are TWO switches. Rather than describe the wiring in a narrative I refer you to the drawing below, which is the method used by Boston Whaler on their boats when they rig twin engines and two batteries:

Image
Fig. 1. The proper method for primary 12-Volt power distribution to twin engines with dual batteries

The drawing in Figure 1 comes from my earlier article on the topic of dual battery wiring. I highly recommend you read that article. The article will explain in detail the operating method to use with the battery, switch, and engine wiring seen in Figure 1. Here is a hyperlink to the article you should read:

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

jimh
Posts: 11674
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
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Re: Dual-battery Dual-engine Wiring

Postby jimh » Sat Aug 20, 2022 1:15 pm

outrage 25 wrote:I would not like to keep buying regulators and burning them all the time.
A bit of work and added expense in re-wiring the boat electrical as recommended should prevent needing to buy new rectifier/regulator components over and over.

I will explain why I recommend you follow the wiring as I illustrated above. When two engines that each have a battery charging output are connected to one battery, the effect is that the two power outputs from the engines are connected together. The outcome of wiring the engines like that has been demonstrated to me by my own first-hand experience to repeatedly cause one of the rectifier-regulator assemblies to fail. I explain in a short story.

I bought a boat with twin engines that were wired so both engines were always in parallel and running from and trying to charge one battery. At the time I bought the boat the owner acknowledged that the tachometer for one engine was not working correctly. As a buyer I thought I could probably remedy that defect easily, so I was not concerned about it. I bought the boat with one bad tachometer.

I later found that the problem with the tachometer was not in the tachometer itself, but in the tachometer signal being sent by the engine. The tachometer signal on many older two-stroke-power-cycle legacy engines of old design is taken from the voltage rectifier. If the rectifier is defective then the tachometer signal will not be correct, and thus the tachometer won't work properly. So a new rectifier/regulator was needed

I also found that the coil assembly in the alternator was damaged and needed replacement. That was beyond my capability because the engine flywheel had to be removed in order to replace the stator coil assembly, and without an impact driver I could not get the flywheel nut loose. I resorted to taking the boat on the trailer to a Yamaha engine dealer and he performed the work. The tachometer that had been not working was now working again.

But the repair was short lasting because after a few hours of running the boat with both engines still connected to ONE battery, the other engine tachometer stopped working with the exact same symptoms as the first tachometer had. To make this story shorter, I had to replace the stator coil and the rectifier/regulator assembly on the second engine. The cost of solving the original bad tachometer problem was now about eight time more expensive than I thought it would be when I bought the boat knowing it had a bad tachometer.

I was determined to solve the real problem, which I soon posited must be because the two engines and their two separate and independently working battery charging circuits were perpetually connected in parallel, and one of them would always burn out.

I won't go into the electrical details and theory of why this occurs as I am not even use my explanation is correct. Much of this depends upon the design of the circuitry involved in the battery charging output. However, I can tell you that after I rewired the 12-Volt batteries and the switch so that the engines were always charging just one battery in normal operation, there was never a further problem with the rectifier/regulator assemblies or the stator coils on the twin engines.

I believe your twin engine set up is in the same configuration: two engines run off one battery. You should re-wire as I recommend above. I am confident that will solve your problem--providing you follow the operating practices and AVOID ever configuring the system so you get back to two engines both running simultaneously and charging one battery.

Also, I would not adopt the "new" wiring circuit I improvised on my boat. While it works, the drawback is you cannot disconnect the battery from its engine with the switch. If there are any stray circuits on the boat, the battery may become discharged. My "new method was just a quick workaround, and I really no longer endorse it.