2017 g23 Battery / starting concern.

[MK Deuce]

Just upgraded to 2 new group 27 agm batteries. This boat has the factory blue sea relay with battery switch. Every time we go out we run a wake or surf session for the first 30 mins or so then just sit and take a break with the engine off and music at very low volume. Naturally this first 30 mins puts some charge back into the batteries just from running the boat.My boat will barely start/ turn over on the "battery on" location. I mean it bogs the battery down bad trying to start on the single battery. I have to put it to combined to start the boat every time. My old fairly new Interstate batteries did the same thing last yr. No way that sitting for 5-10 mins running the stereo at super low volume could ever draw down a new group 27 agm battery. Is something wired wrong from the factory, im 99% sure this isnt a battery issue.

[angrygiraffe]

That really doesn’t sound like a battery issue, especially with brand new AGMs. More likely it’s the wiring or how the relay is set up from the factory. A lot of people end up starting in “combine” for that reason. Might be worth double-checking grounds and cable connections too.

[MK Deuce]

For more reference these are brand new group 27 agm batteries with 650 cca. I know thats lower end of the cca but i have never had a battery not start any of my old boats with smaller group 24 batteries after sitting for 5-10 while running the stereo at super low volume.

[bturner]

For whatever reason my boat always sounds like it's turning over slowly but that's just how it's always been. My buddy's MC with an Ilmor sounded different but turned over much slower than I was used to or expected as well. I've never had a no start but it just seems like it turning over slower than expected. For reference, my 2019 Tahoe sounds like it's turning over a third faster.

As suggested above, I would closely inspect both the hot and ground cables going to the starter circuit and at the battery while cleaning and checking all the connections for any corrosion or loose/damaged connections. The last stop if still no joy would be to replace the starter. It wouldn't be the first starter that has had issues after it's gotten warm.

[MK Deuce]

My limited starter knowledge from the automotive world tends to show a bad starter as either making a clicking sound or not turing the motor over at all. Will a starter that is going bad on a boat act like a low voltage issue like this is doing? Obviously when i combine batteries it isnt an issue because its introducing a fresh battery into the system.

[bturner]

You're actually not adding a fresh battery per sei, you're electrically combing 2 batteries in parallel which is increasing the current to the starter in this case. I 've had starters that were fine when cold then had difficulties when they were hot. This was mainly on cars that I was cranking on trying to get started back in the 70s - 80s.

The clicking sound it typically from a bad starter solenoid.

IMO, the most likely cause based on the symptoms you describe would be in the electrical circuit. Adding that second battery is increasing the available amperage to overcome a deficiency in the circuit. Boats are notorious for grounding issues as there is no metal chassis like with a car for the circuit to ground to. Everything is dependent on the wiring to supply both the hot side and the return to ground. I've owned quite a few older boats as I've only bought used throughout my boating life and with the exception of my current boat I've chased down electrical issues on all of them at one time or another. Any time I look at a boat and the seller states anything about adding a stereo or doing any electrical work I'm mentally prepared that I'm going to have to fix their "custom" work. This includes work done by a dealer or marina.

Most boats are engineered to meet electrical spec and that's it. Very little head room is designed into the electrical system as this adds cost to the build. Add a big stereo or electronics to the boat and you could be in for some difficult gremlins to chase. Some boats are also known for their electrical deficiencies. ~2000 - 2004 MasterCrafts for example were known for weak grounding that would cause all kinds of gauge issues which I have considerable experience with based on 2 that I personally owned. And don't think for a minute this is just a MasterCraft thing. Do enough searching on any of the boat forums for any manufacture and you'll find plenty of electrical issues.

With all this said, it doesn't take a lot to have the type of issues you're having. A poor/loose ground, a loose connector end or corrosion that is wicking up a cable will all stress an already "meets requirements" electrical system. This is why I always first recommend an in depth inspection of the circuit in question as well as the base electrical system at the battery before even thinking about getting out the parts cannon. I've fixed far more electrical issues cleaning and tightening connections than I have with parts replacement. The problem most people have doing this is that it is no fun and takes time. It's much easier to change a battery or plug in a new part.

One last place I would recommend looking at which should be included but often is overlooked in the wiring inspection is the battery switch itself. Disconnect the battery and pull the back cover off the switch and inspect the contacts. I've seen these burned up and had a couple fall apart in my hands when I took the cover off.

From Brave AI.....

Internal problems within the starter itself can also cause slow cranking. Worn brushes, a dirty commutator, or dirty contacts in the starter solenoid can increase resistance and reduce the motor's efficiency. A heat-soaked starter solenoid can also exhibit increased resistance when hot, leading to slower operation. If the starter has been replaced multiple times without resolving the issue, the problem may lie in the electrical circuit rather than the starter unit itself.