Cranking battery

[Redngoo]

2boo9069. I'm using the big northstar 31. I have something going on and guess im reaching for straws. Start off the day 13.3 volts yes its a hot battery. Load test verifys this. I run 2 12 inch graphs 1 10 inch graph with gls10 black box. 6 gauge wiring to the console 10 gauge to the bow off of a quality buss bar. Now my problem 2 hrs in the day im down to 12.2. After fishing first spot. Crank up and move watching the monitor to verify charging, charges fine. Again shortly after fishing voltage down again. All voltages are checked at the battery with a volt meter and using my graphs. Are there anything that "could be causing this on the motor side". Purchased battery last march. Kept complaining where i bought the battery. Made up in Nov. Still same issue.

[Double Trouble]

Have the battery load tested could be a dead cell or something on the boat is draining it.Try shutting everything off and put meter on battery is being drained.

[EuropeanAM]

May also want to look at the connection types you have at the battery. My preference is a CLAMP style terminal on that battery, then place the ENGINE and CHARGING leads on that post.

Remaining lugs can go on the factory threaded posts.

Also- if you have a BATTERY SWITCH- bypass it temporarily to see if the problem persists.

[Redngoo]

20210221_173016.jpg

This the set up. These are the T+h marine style . 2 breakers one goes to boat acc one goes to electronics. The batterry has been load tested

[EuropeanAM]

That's actually a great setup you have there in terms of connections.

Unfortunately, I suspect that your setup is simply depleting the battery due to the amount of load you're running from the cranking battery.

If the below checks do not reveal anything, it may be necessary to look at a "house" battery and a battery isolator setup.

Further Checks:

-Test the AC voltage between the Positive and Negative terminals, with the engine idling. If your meter "auto ranges" you will need to lock it into one range (20 VDC or lower). AC voltage should be less than .25 VAC.

-Inspect/Check/Clean the ENGINE cable ends of the battery cables. Sometimes the ground cable may not be pulled down well (ie: loctite under the bolt preventing it from clamping the cable tight). Positive Cable stacks differently that you might expect (intentionally atop a lock-washer).

-Inspect/Clean the GROUND strap for the alternator.

-Check the red-output lead for the alternator (alternator end of fusible link lead). Is it DISCOLORED? Clean? CAUTION: This is a brass stud, do not over-tighten (it will break or strip if you do).

[Redngoo]

Am i understanding you correctly? A battery isolater to charge both batteries from the engine alternator? If so do you prefer a brand? Also if i understand correctly the alternator wire that connects with the starter is ran straight to the isolater to the input side with a fusable link inline before connection to the isolater

[lpugh]

I think are seeing normal voltage reading due to a continuous load. After this occurs, turn everything off and watch the voltage slowly rise. Probably about .4 to .6 volts, after about an hour you will see the true state of charge on that battery
13.3 volts would indicate a surface charge not actual state of charge, I think that battery 12.9 @ 100% though some AGMS are 13.1 such as Optima Bluetops
Surface charge must be removed to get an accurate state of charge. A 50 amp load for 15 second will do the job

[EuropeanAM]

If you're going to go the "dual battery" route, I would strongly recommend going with one of Mercury's setups that is designed for your engine/charging system/battery configuration.

Our Parts Department can assist- see Rhonda's post, pinned at the top of the forum.

BUT... before you start spending $$$, take note of the comment Leon made above (Post# 7).

[Redngoo]

After finishing all checks. All those seem fine nothing outta the ordinary. After fishing for 8 hrs i had no issues cranking the engine. Though did see some lower than i like voltages. For know l just going to keep a close eye on it untill i decide watch direction i will go