Going to dig into this.
On my 292 Formula, there is an almost 2v difference between port and starboard engines while running. Looked for obvious load differences(something on) and did not find anything. Batteries are new...and after a trip yesterday, I quickly measured the voltage on the two batteries with the boat on the trailer..13.0 and 12.5v.
Gonna start with looking at all connections I can find, but wanted to get some advice/ideas to help find the issue. The boat is a 2014, with only about 20 hours on it, so my hope is the hardware is good...just need to fine the source of the problem. Of course I could bring it to get it repaired under warranty, but hate the idea of dropping it off at a shop.....and I can learn some more about the boat.

Thanks.

it sounds like you might have an alternator problem or a battery issue.i would do a load test on the battery,s first,if they check out good i would check alternator output.

A charged battery should have a voltage of at least 12.8v.

verify that the purple wire is getting a good switched on 12v.

Had this come up with my 6.2s...the wire to the alternator had some corrosion on it. I probably could have cleaned it but I decided to put a fresh connection on it and it fixed it immediately.

Which wire? Thanks for the advice...hope it's a simple fix

Past years - Merc had a service bulletin about the routing of the alt wire and wanted it changed, most of the time the connector ended up bad inwhich had to be changed. back thenI think I redid at least 75 of these in a 2 year span including the bad connector. 2014 engines did not have this problem compared to the past. With that said it could be loose or a form of corrosion at the alt side.

Also for your app as Mike said (battery issue) weak cell in the battery - a fully changed battery is 12.7 to 12.8. Also there is specific gravity testing for wet cell batteries.

Also ALL the current marine engine manu's recommend a cranking battery for the engine only not a deep cycle and NO WING NUTS for the terminal connections. We have not installed wingnuts in years for connections at the battery especially for EFI / MPI engines.

I would load test the battery first and check all connections. Next is to scan the engines and read battery voltage that way to see what voltage output readings

Lastly what engines and engine serial numbers - sometimes it is best to at least call an OEM dealer ( have your engine serial numbers to report to them ) because service bulletins and recalls along with federal recalls are issue per engines and engine serial number groups that possible your engine falls under that needs to be fixed ASAP.

Thats great you want to fix it yourself but alot of times with new engines still under warranty or just coming out of warranty not really the best idea. Just saying as this could be a good way to void your warranty if it is still active.

That black wire on the back of the alternator. But like BUP said, check all connections. All it takes is one a little loose or corroded. It could also be the positive.

Well I did type this in my post but for some reason after I edited some missed spelled words it is gone.

Anyways the jist of it again.

The number one area for loose connections and or corroded connections is at the starter (12 V connection). Even with new boats very common problem.

check battery connections, back side engine ground connections in which is the second worst problem area for either loose and or corrosion along with the starter connections and of course at the alternator connections,

Take a hard look first in load testing the batteries for any problems there and how accurate are the Volt gauges at the dash ? Did
you use a quality volt meter for testing at the batteries when the engines were running at 1500 to 1800 rpms and at idle to read voltage output ?

Well, I think I have it figured out. Had both engines on the hose in my driveway, and was second guessing myself about what I saw yesterday....a difference in voltage from side to side.

The starboard engine battery was 12.98 v at rest. After starting, it jumped to 14.65 at the battery and 13.8 at the gauge on the helm. I was puzzled...but thought I had the engines switched in my mind.
Hooked the hose to the port engine and with engine off, batt was at 12.55v, and after starting, it went to 14.46 at the battery, and 13.8 on the gauge at the helm. Now I was really confused. Put hose back on starboard, and low and behold, I did have a voltage drop.....when the Corsa exhaust is engaged. All 4 solenoid are tied to the starboard power and switch....as you would expect I guess.
After unhooking all of the solenoids and looking for one that was misbehaving, I found that they all will drop the voltage from the gauge on the helm, you start at 13.8, manually plug one in, drops to 13.5, then 13.1, than 12.9, then 12.8. Reverse the whole process(unplug all, and start from the other side), and you get identical v drops. Plug any one in...and you get a drop from 13.8 to 13.5. So......all of them are the same.

What is weird, is that the battery voltage through this whole thing is 14.4v....so there is no problem with draining the battery. It must be that gauge uses an upstream source, after the accessories. Probably great for battery health, but not good for things that like 14v....like amps etc.

In any case, I am no longer worried about it too much, unless someone out there reads this and has a concern.

Thanks for the ideas...that is what is great about this site.