2018 Mercury 150 4Stroke voltage issue

[catfish4767]

Hey there, I have a 2018 Merc 150 4S that has been losing power recently. I will start with a fresh battery at 12.5 volts and fish a tournament but then ultimately get the low voltage beep after a few hours of running around. I replaced the alternator in March 2023, and just replaced that one last night thinking it was the same issue. Last night I started the motor and saw 12.14v while idling - I did not take it off of the lift. I left it idling for several minutes but the voltage only went to 12.15. I turned everything off and tested the voltage on the battery terminals again but saw it rising to 12.36. This afternoon I went down to check again and it was at 12.51, so I took the boat out for a spin up and down the lake. This time I turned on a graph to see voltage readings and saw 12.1 and 12.2; nothing higher. I brought the boat back and checked the battery terminals again and saw 12.22. The battery is a group 34 X2 AGM that seems to work fine. I have used another battery a few weeks again and same reading. Should I be testing something else?

I do have a MEP auto-pro charge system on the boat that is disconnected since I don't normally need to charge the trolling motor batteries. Could this be an open circuit or something and it is drawing power for some reason? Anyone have a wiring diagram on how to connect the wires? Seems crazy to ask but it has but I have 5 wires coming out of the box – a pos/neg in an insulated shield that I connected to one trolling motor battery on a 24v system. The other 3 wires are 2 pos and 1 neg – I connected those to the other TM battery but when I connect the neg, it shoots sparks everywhere and the terminal visually looks to heat up. Not sure if that is correct or if I have something crossed.

Thanks for the help!

 

[Andrebass]

Check and see if your trolling motor needs to be hooked up parallel or in a series. Do you have a 12/24 switch at the trolling motor pedal? why didn't fuses pop when you connected the neg. Did you check the voltage at the leads before you hooked up the batteries.

This won't solve the main engine power issue, You should have 14 plus volts when the motor is running through the alternator. I would check with a qualified technician and not risk any damage to main ecm components. Too new of a boat to have this many issues electrically speaking.

 

[catfish4767]

Check and see if your trolling motor needs to be hooked up parallel or in a series. Do you have a 12/24 switch at the trolling motor pedal? why didn't fuses pop when you connected the neg. Did you check the voltage at the leads before you hooked up the batteries.

This won't solve the main engine power issue, You should have 14 plus volts when the motor is running through the alternator. I would check with a qualified technician and not risk any damage to main ecm components. Too new of a boat to have this many issues electrically speaking.

I found someone selling the same charger on ebay and asked them to send a copy of the installation instructions. Got that one figured out!

My father-in-law has the same 2018 merc on his pontoon and had the same parasitic draw on this batteries a couple summers ago. Changed the alternator and he was good to go. I did the same and fished a summer without an issue, but this year something is off. I am going to check my fusible link to make sure its not blown. Thanks for the input