New starting battery.

I currently run a dual purpose group 31 for starting and my electronics. Looking to add a stand alone starting battery just for the merc 75. Any recommendations? Every time I start my boat currently my graphs beep, my lights go dim and yesterday playing with it while on a battery meter it takes most the juice the battery has to offer. So I want a stand alone cranking battery. What are you guys running on a 75hp motor.

https://www.interstatebatteries.com/products/27m-xhd?productline=marine

Five years on this starting a 175 Yamaha. It also runs my electronics but I only have a single graph since the Garmin is in a shuttle.

Bear,that merc shouldnt take much juice to start.
Perform a voltage drop test on both battery cables.
Put one of your volt meter leads on the positive cable at the battery,the other lead goes to the positve cable where it attaches to the starter solenoid.
Attempt starting the motor,if you see a large reading over half a volt that cable or connection is corroded.

Do the same with the negative cable,test lead at the negative post,other lead to where the cable bolts to the block.
If it reads over half a volt that cable is bad.
Ive seen corrosion in the middle of the cable you cant see until you slice the insulation,look for swelling along the length of the cables,that will give you an idea where the corrosion is.

From there if they test good either the starter needs serviced or the battery is weak.

I use a group 31 dual purpose run 2 helix 10s . Will go for hours without the graphs shutting off when hitting the key to start a 150 . Has it always done this ? If not run some 10 gauge wire to a fuse block and run your graphs off that .

Iowaboy1 wrote:

Bear,that merc shouldnt take much juice to start.
Perform a voltage drop test on both battery cables.
Put one of your volt meter leads on the positive cable at the battery,the other lead goes to the positve cable where it attaches to the starter solenoid.
Attempt starting the motor,if you see a large reading over half a volt that cable or connection is corroded.

Do the same with the negative cable,test lead at the negative post,other lead to where the cable bolts to the block.
If it reads over half a volt that cable is bad.
Ive seen corrosion in the middle of the cable you cant see until you slice the insulation,look for swelling along the length of the cables,that will give you an idea where the corrosion is.

From there if they test good either the starter needs serviced or the battery is weak.

Bought my current boat and the console depth finder was wired to the trolling motor battery. Everyone told me I shouldn’t do that. So I rewired it back to the starting battery. But if the graph is on when I start the boat, it turns off the graph and I have to unplug the df, plug it back in, and then start the df after the motor is running.

So people told me to get a big, dual purpose battery. So I did. Still have the same issue.

The wiring is new. I’m considering just getting another battery just for the console df. Only thing that sucks is I’ll be short a slot for the onboard charger.

I run 2 12 inch and a 9 inch and have a 250 all on 1 group 31. Something else is going on.

I use a size 27 starter/cranker with 2 graphs and a 75 hp outboard without issue.

I agree with others here, I think there is a problem going on with your setup/wiring.

How old is the battery BC? That would be the 1st thing to check.

I’m going to call Sheldon after I get all the kids rallied up and busy doing stuff so I have time to dink around.
But I’m wondering i have a some kind of a ghost drain some where. The battery is only a year old. I am going to have napa check my battery says my warranty is good through this month

Well a couple of things. First I experienced this many times with my old Series 31 AGM’s. Very frustrating, yes the motor would start but the electronics didn’t like the voltage drop. This happened when the battery was about 1 year old. This was especially prevalent if I sat in one spot for a while, the electronics would bleed down the voltage. My first solution was to buy a DC – DC boost converter. It would take any DC input voltage down to 8.5VDC and boost it up where you set it, I set it at 15.5 volts. It is on a switch and absolutely solved the low voltage problem with my starting battery. Here is what I used https://www.amazon.com/XIITIA-Converter-DC8-5V-50V-DC10V-60V-Equipment/dp/B0C6KKYCQH/ref=sxin_16_pa_sp_search_thematic_sspa?content-id=amzn1.sym.95e4d6bd-d93f-4ee1-9766-ff64f54d2f71%3Aamzn1.sym.95e4d6bd-d93f-4ee1-9766-ff64f54d2f71&crid=FKHAQGIQ4BW3&cv_ct_cx=boost+converter&keywords=boost+converter&pd_rd_i=B0C6KKYCQH&pd_rd_r=1fae74b7-5ad7-47c1-bea9-673c6e2f7ab5&pd_rd_w=C3Snn&pd_rd_wg=smH5V&pf_rd_p=95e4d6bd-d93f-4ee1-9766-ff64f54d2f71&pf_rd_r=S4YBGAMCW9KJ4RP6DJH5&qid=1742062832&sbo=RZvfv%2F%2FHxDF%2BO5021pAnSA%3D%3D&sprefix=Boost+co%2Caps%2C513&sr=1-3-6024b2a3-78e4-4fed-8fed-e1613be3bcce-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9zZWFyY2hfdGhlbWF0aWM&psc=1
In 2021 I switched to Ionic LFP series 31/125AH and my problems ended. I still have the converter in my boat with a switch to turn it off when not in use, but a good solution if one doesn’t want to spend the money on a lithium battery. All the other suggestions are good however. I see your in North Branch, if you are interested, I think I have an extra converter if you want to go that route.

Had a weird one a while back doing something similar and it was the main ground on the outboard under the cowl. Never hurts to clean a ground, even if it looks good.

I’m running a 31 AGM with a Yamaha 300 and Solix 15 dash & 12 bow. With live well, dash solix 15, and radio, locator doesn’t shut off when starting. I do have the locator running direct wired with 10ga wiring to a fuse block direct wired to starting battery.
Does your charger have a specific agm setting, if not that could be some of your issues as there is a different charge profile for AGM.
After fully charging, your AGM battery should register between 12.75v to 12.85V after disconnecting the charger and letting it rest for 3-4 hrs before checking. Less than 12.75V your battery is not fully charged.

First thing I would check is the battery I have had way too many new batteries that were bad, never made 18 months some only 6-12 months. Charge battery unhook everything check voltage let it sit for 24-48 hours check voltage if not about 12.5 the battery is toast. Many times a cranking test will not show a bad battery, just ask me going round and round with the local parts store personnel.

I would go through all the power cables and clean all grounds and check the connections. A new 31 size battery should be able to start a 75hp with out loosing voltage.