Alternator output on an outboard motor

[The Rooster]

How good is the alternator output on an outboard motor? The pontoon boat I'm getting has a 20HP Tohatsu outboard and it has an alternator, and at first I will just have one battery that will be shared between the outboard and trolling motor until I can do some upgrades.

As for finding myself in a dead battery situation, I'm not too worried cause the outboard also has a pull start cord and can generate enough electricity for its own fuel injection. This is more of a curiosity question since I'm planning a fairly nice electrical system upgrade with lots of lights and a fish finder (already bought the Humminbird Piranhamax 4 DI). I'm also gonna add a second battery and have an isolator switch between them to choose which one the outboard uses to start and also recharge as it's running. There will also be a 2 bank onboard charger for when I'm off the water, but if I were to stay out a couple of days and camp off of the boat then the outboard would be the only charging source I would have. Is it more of a maintainer rather than charging up from a depleted state? I know on a car, if I have a dead battery, I can jump start it and the alternator can bring it back up to full charge on its own, but that's not a deep cycle battery.

[Mike Cork]

Well it's going to maintain very well for the starting battery but will have trouble trying to keep up with the trolling motor. It's a matter of time as well, it takes time to recharge. So if you run it dead, you're going to need the motor running for a while before it will charge it full.

I wouldn't recommend camping on the boat for more than a night especially if the trolling motor is going to work hard (IE wind, water current, wave action)

[The Rooster]

Well it's going to maintain very well for the starting battery but will have trouble trying to keep up with the trolling motor. It's a matter of time as well, it takes time to recharge. So if you run it dead, you're going to need the motor running for a while before it will charge it full.

I wouldn't recommend camping on the boat for more than a night especially if the trolling motor is going to work hard (IE wind, water current, wave action)

Thanks for the reply. Since I'm gonna have 2 batteries eventually (probably a winter time upgrade project before next spring) which battery would you use to start the motor? In my head I see 2 deep cycle batteries, one for trolling, and one for everything else (all lights which will be a lot, fish finder, radio, horn, USB ports, and anything else.

The dealer has a video on this motor stating that it takes very little to crank it over and he runs it from a deep cycle battery instead of a starting battery. He uses Interstate 27 series deep cycle batteries and said it has enough cranking amps to do it. I'm getting one battery with the boat but did not want to pay $150 per battery to get two when I feel that $75 for an Everstart from Walmart would work well enough for the second one. I've never had issues with those and I've had several.

I'm just wondering if I should move the trolling motor to the new battery and run everything else from the first one along with starting the motor. For daytime use this would not be a problem and eliminate any issues with not being able to key start from having a low trolling battery. But at night when I'm sitting still and have lights turned on then I'm going to be using power (not sure how much though). I will be using this boat for camping so there may be an overhead light on as much as 2-3 hours while we play games, plus the anchor light that has to stay on all night even while we sleep. When we have the boat out for July 4th fireworks, it will be anchored and running the anchor light, and also be running some under deck LED lights for hours at a time, and other interior mood lights. If all that is on the "starting" battery, this could cause it not to crank later. Again, I have a pull start motor so I know I can get back. It's more about convenience using the key than anything else.

Also, ALL of my lights will be LED so they're low draw compared to incandescent lamps. The amount of lights I'm going to have will be: standard navigation lights, two sets of 18 watt dock lights in front, one set in the rear (3 sets total), interior mood lights along the rails, one overhead spreader light, under deck LED light strips along each side, interior storage box lights,  6 LED lit switches across the dash, and two voltmeters to monitor battery status. There may also eventually be a fuel gauge and a trim gauge. I know these are fairly low draw loads and not all of it will be on at once. To me there's just a LOT, and I'm planning one battery to run it all.

[Mike Cork]

I wouldn't go cheap on the batteries. Getting stuck in the middle of no where is no fun. However you can carry a small jump pack to start the motor should a battery fail.

Sounds like you've got a good game plan to me. Get the jump pack just in case and I think you'll be fine.