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Thread: 1991 Johnson outboard 120hp

  1. #1
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    Default 1991 Johnson outboard 120hp

    I'm wondering how I can find out if my outboard has an alternator....I can't find it in the manual anywhere or online either. I'm just looking to run dual batteries and want one to be able to charge the other while the boat is running. Reasoning for dual batteries is i'm running a stereo in my lake boat. Any information would be appreciated, thanks!

  2. #2
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    Those outboards dont have an alternator, they have a charging circuit. Same thing basically, I think the charging circuit puts out about 14V. It does charge your battery(s) though, so I think you should be ok. I basically had the same outboard on one of my boats and had a stereo in it as well. I had a CD deck and 2 - 6" sony x-plode speakers from wally world. I could run that thing for an easy 4 hours and crank the motor back up and I only had one battery (trojan deep cycle 1000 CCA).

    The way to make sure if your charging circuit is charging the battery is to use a volt meter. Check the battery voltage with the motor off... Record or remember that reading, then start the motor and leave it at a high idle, wait about a minute or so and check the battery voltage again. It should read higher with the motor on than with it off.

    Let me know if you have anymore Q's.

    G

  3. #3
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    Default Thanks

    Still about a week or two before I pull the boat out but I will definetly try that. Just to clarify, the volts should read higher when the motor is running because the battery is being charged correct? So if I have 2 batteries with a battery switch. leave the switch on both batteries while im motoring it should charge both, switch to bat 2 for stereo when the motor is off and use batt 1 for starting power and I should be good to go all the time right?

  4. #4
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    Quote Originally Posted by bowdy15 View Post
    Still about a week or two before I pull the boat out but I will definetly try that. Just to clarify, the volts should read higher when the motor is running because the battery is being charged correct? So if I have 2 batteries with a battery switch. leave the switch on both batteries while im motoring it should charge both, switch to bat 2 for stereo when the motor is off and use batt 1 for starting power and I should be good to go all the time right?

    Yes, the volts will read higher when the engine is running since they are being charged. And yes, if you hook up your battery switch right, both will charge and you can run the stereo off one and start with the other and then switch back to the "deader" one or both while running to get them charged up again.

  5. #5
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    I will get it all set up and try it out. Thanks again!

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