I have a 2002 225 optimax i replaced the alternator with a rebuilt wilson it charges fine then 3/4 thru the day after running depth finders and acces. battery voltage goes from high 13t volts down to 11.8 volts. So i went and bought a brand NEW seconds alternator put it on the motor took off went a couple miles 14 to 14.2 volts, shut down the motor opti. fished a while it got as high arounf low 13 s volt and slowly keept dropping. so i started my 30hp kicker motor and at first there was around 11.8 to 12 volts working its way up to low 13s volts after running kicker for about 10 min. went back to opti and started around 12 volts and soon down to 11.5 volts. could both alternators be bad since the 30 hp charges charges the battery. I had a mercury dealer check it on there computor and said nothing was wrong though there was a low voltage fault. It seems to me you need to run it @ high rpms to get the voltage to drop.

Need to check all electrical connections on engine and engine grounds, check battery cable connections at battery and on engine are clean and tight. make sure battery meets Mercury requirements, minimum 750 cold cranking amps or 1000 marine cranking amps

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I think that the battery connection and engine's electrical connections should be checked.may be due to this the problem is happening.