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  1. #1
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    Burlington KY
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    2002 Merc 175 EFI - AGM Ok?

    Motor: 2002 Mercury 175 EFI OT566138

    From what I gather I can use an AGM battery for starting this motor, it does have a serpentine belt at top of motor. Correct?


    The reason I am changing batteries is when I start my motor all my sonars turn off, I have a small lowrance unit that shows battery voltage and when I crank the engine it drops to 10.0 volts. Also while the motor starts fine it doesn't turn over "snappy" if that makes sense. The battery I have was for my last motor, a Merc 115 elpto and is a marine starting from Advance Auto (#M24-3 AutoCraft) and is a group 24 has 800 CCA, 1000 MCA and a 135 minute reserve.

    I really would like to use an AGM in a group 31 size from Sears since they are close to my house, but I will go with whatever is the best bang for the buck. Also a lighter weight battery would really be nice due to the fact that my nitro has a big booty...weight wise
    Last edited by bnt55; 05-21-2017 at 11:47 AM.
    2005 Nitro NX 882
    2002 Mercury 175 EFI

  2. Member
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    Jul 2013
    Location
    Galena, Kansas
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    824
    #2
    I would not base voltage off your volt meters on your graphs. You really need to use a voltmeter at the battery to take the measurement when starting.


    It is very possible it is not your battery at all.....but it does sound like it may be time for a new battery if it's not "snappy" like you say. I would stay with flooded battery personally. I would also verify your battery charger would be happy with AGM battery if you went that route. Should not an issue as most chargers only have issues with Gel batteries. But depending on charger may just be a simple push button change for chemistry types.

    The NX882 was smaller gauge wire to the fuse block under the dash. Not only that but the dash power switch can have a lot of voltage drop. Mine had nearly a 1.5 volts drop thru that switch no load. Example:12.7v at battery... measured at input to switch 12.5v...measured at output of switch 11volts. The switches get dirty and worn out overtime.

    When graphs lights pumps and everything, on it was even worse.

    I recommend upgrading the wire to the fuse block and bypass the factory dash power switch. Or like I did, install a sealed automotive relay so the console switch does not handle the load of the fuse block. The console switch just turns the relay on and off and handles the fuse panel load.

    So... to recap... Run 10awg from the battery (fused) 20amp at battery to the relay, then from relay to fuse panel.
    Your console graph, live well pumps, navigation lighting, dash lights all go to this fuse block. From the fuse block to the bow run 12awg to the bow graph.
    Remember you have to upgrade both positive(red) and negative(black) to the same guage wire. Or it would be pointless.

    Once you do this, unless your battery is going bad. You will likely not have any more reboots when starting engine. Aditionally your livewell pumps and nav lights, dash lights will be brighter.

    Good luck.
    Last edited by moetorola; 05-22-2017 at 02:08 PM.

  3. Member
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    #3
    Wiring going from battery to fuse panel is 8 AWG, I have a 20 amp fuse on the positive side. One thing I do not have have is the relay. I do have the master power switch at the console that turns everything on. What kind/size relay would I need? Thanks for the tip, very informative!
    2005 Nitro NX 882
    2002 Mercury 175 EFI

  4. Member
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    Jul 2013
    Location
    Galena, Kansas
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    #4
    Any kind of automotive hermetically sealed relay will do fine. Get the one with slip on terminals.