2011 175 Optimax ProXS Voltage issue??

[Riverhunter55]

2011 175 Optimax Pro XS SS#1B847035
I bought this motor with low hours on it in 2012 and have 400ish on it now. I have always had a group 24 interstate cranking battery with the bilge also hooked to it. This boat is in the water all summer so the bilge runs whenever it rains. I have never had any kind of starting problem with this motor ever. Yesterday after running around and then hanging out on the sandbar it would not start and I was not able to jump start it so I was towed in.
Today I purchased a group size 27 Interstate 27M-XHD 1000 MCA battery thinking bigger is better, after researching tonight it seams I did not go big enough. After installing the new battery I thought now would be a good time to isolate the starting battery and also installed a deep cycle for all accessories. I also hooked up my onboard charger which i should have been using all along but haven't. The battery was fully charged and still will not start the motor. Starter will spin just not fast enough. I plugged in my onboard charger and when the cranking battery got to 13.9 it started the motor, seemed to run fine tied to the dock. The voltage went up as the RPMS increased so I assume the alternator is working. I checked all connections which are stock and they seemed tight. My battery cables are less than 6' with stock ends. Once the battery went below 13V it would not start the motor, just spin. Even when it did start it didn't just take right off. After thinking back this motor has never started real fast, I am assuming because I was not using the proper battery.
I am not one to skimp on necessary things like battery's but it seems strange that this brand new battery will not start the motor when a smaller one would before.
Is there something else I should check before dropping $250 on a new battery?
I read that it could possibly be the Main Power Relay, would it start at all if this was bad?
I know just enough to be dangerous with electrical, so it does make me a bit nervous to mess with it too much but I am a curios person and would really like to know the why.... :)
I appreciate any and all help, I don't give much information back on here because wrenching is not my forte but I sure do enjoy reading about the talent of others.
Thanks again.
Kurt

[SilverFox579]

Be sure to check and clean all engine cables, both ends, and ensure you've connected engine cables to battery first, then any additional loads largest to smallest.

I'd be interested to know actual voltage at the battery as measured with a DMM, engine off, engine running. You should see close to 14.1v at idle.

Btw, this forum covers your engine . Mercury - 2.0/ 2.4/ 2.5 Liter High Performance

[Riverhunter55]

Sorry about the wrong forum!
As far as actual voltage with a multi meter at the battery with just the motor leads no accessories it was 12.6 when it would not start the motor just slowly turn the starter. When the battery had over 13.9 it would start the motor. Idle it had over 13.5, revved to 3k it had 14.4 ish.

[JOE54]

RIV I have that exact batt in mine running 3 big depths and 250pxs without any probs, sounds like your alt is working but sounds like you may have starter issues ,how old is battery

[Riverhunter55]

This group 27 battery is brand new

[JOE54]

should be fine works good on mine ,may have to have amp draw tested on starter

[SilverFox579]

+1. Should start without issue at 12.6v.

[Riverhunter55]

I pulled it in the shop this morning, I think I am just going to have to work from my battery back and see where I am losing voltage. An amp meter would probably be helpful but I will start with my multi meter and see if I can see a voltage drop.

[Riverhunter55]

Well after testing the voltage everywhere I figured out I was losing volts from the front to the back side of soleniod. I ended up just buying a complete new starter and it started right off...
Hopefully that is all it was.

[EuropeanAM]

Hopefully you bought a new OEM Starter (not a knock-off or aftermarket).

[Riverhunter55]

Yes sir I did buy an OEM Starter. So far so good!

[EuropeanAM]