Upgrade Your Outboard Motor to Charge Your Battery

The luxury of onboard electronics and electrical devices cuts both ways. What it gives in terms of convenience, it takes in terms of a load on your battery bank that needs recharging. There is tons of good info available on the net and in books on sailboat electrical systems. I especially like Sailboat Electrics Simplified by Don Casey. The information is concise and clearly presented in a way that makes it easy to refer to when I’ve forgotten some important point. I won’t bore you by rehashing all that but cut right to what I did and why.

Summer Dance has a single group 24 deep cycle battery. With the addition of accessories like a music system, LED strip lights, and a tiller autopilot, a single battery charge was no longer enough for a two-day cruising weekend, let alone those occasional great three-day weekends. I needed to find a way to put some amp hours back into the battery while we were out on the water. While the boat sits in her slip, I plug in a small, onboard charger.

At first, I considered the most popular solution, a solar panel. But after seeing the cost and complexity of a quality system and how little charge we could expect, especially at the northern latitudes where we sail, the results we could expect looked disappointing and my focus turned to our outboard motor. The winds are usually light in the North Idaho lakes in summer and we sometimes spend a lot of time motor sailing, more than I’d like, actually.

Most outboard motors (even those without an electrical starter) can charge a battery just like your car’s motor can charge its battery. Large displacement outboard motors do this as a rule.

The major components needed are:

  • Generator coil(s) that create electrical current (unregulated AC) from the motor’s rotation
  • Regulator/rectifier to convert the coil output into regulated DC current suitable for charging the battery

The good news was that our outboard motor, an 8HP Yamaha 4-cycle (model F8MLHC), already had generator coils installed (on top of the flywheel) at the factory. The manual states that the output is 80 watts maximum, or 10 amps at 12 volts DC. All it needed was a regulator/rectifier. 

If your outboard motor doesn’t have generator coils installed already and you would rather consider a solar panel solution, check out Solar panels for boats: an easy installation guide.

Get a charge out of your outboard

I started shopping at eBay (where else?) and bought a used regulator/rectifier (Yamaha part no. 68T-81960-00-00). I had it mounted (to factory supplied holes for this purpose on the side of the motor) in 5 minutes and connected to the engine as shown in this diagram. The last step was to connect it to the battery onboard.

My goal for the onboard wiring was to be able to easily connect and disconnect the motor for trailering with a heavy-duty waterproof connector. I wanted the process to be as simple as connecting the fuel line. But I didn’t want to mount a cable connector in the hull that required drilling another hole.

At the motor, I made a short 2-wire harness out of 12 AWG cable. One end connects to the rectifier/regulator output and to ground on the motor. It then exits the motor at the front grommet and terminates in the male half of a 2-pole Delphi Packard Weatherpack connector.

Front of motor showing wiring connections and male connector
Front of motor showing wiring connections and male connector

In the boat, I ran a 12 AWG duplex (2-wire) cable from the battery along the bottom of the port side of the cockpit sole to the aft vent fitting. I ran the wiring right inside the short vent hose and out the scoop with enough cable to reach the outboard motor where I crimped on the female half of the connector. I covered all exposed cables with flame-retardant woven loom. When disconnected, the boat cable folds neatly out of the way along with the gas line. I spliced a 10 amp fuse holder on the battery end of the positive wire and connected it to the battery’s positive terminal. The negative wire is connected to the negative bus bar nearby.

Wiring harness connected for use

The proof is in the put-putting

At cruising throttle, the motor puts out 2-3 amps to the battery, more than enough to offset the autopilot, GPS, and music system that are typically on at the same time. At full throttle, the battery gets a 5 amp charge.

I’ve been using this setup for five years now and I haven’t even had a low charge condition yet. I’m glad that we don’t have a big clumsy solar panel hanging off the stern. We might need one someday for week-long cruises, but even then it will be nice to know that we have more than one charging option.

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27 Comments Add yours

  1. Hi, I check your blogs like every week. Your writing style
    is awesome, keep up the good work!

  2. wewhite74 says:

    I would suggest using an Anderson sb type connector with boot for the wiring and a buss type on the motor

  3. Richard Weston says:

    Question – You say in your boat you trailer a lot. I’ve been wondering if you trailer with the boat on the motor mount or take it off? On the Trailer Sailor forums the consensus is split in my very unscientific survey. Half of the people say they take the motor off while trailering because they are afraid one good bump will damage the transom. None of them have actually tried this or ever had it happen to them. The other half leave it on all of the time and boast of trailering hundreds of miles with the motor still on and never having a problem. Guess which side I’m leaning towards 😉

    1. Thanks for your question, Richard. I’ve seen those debates (and others) too. The first year with Summer Dance, I dismounted the outboard for trailering. We have to trailer 30-90 miles, some of it on bumpy county roads, and I didn’t want the extra wear and tear on my transom, motor mount, or motor. But at 90#, it’s a chore to mount/dismount our motor and I have a weak lower back. Starting the second year, I left the motor mounted. I had 1/4″ thick aluminum angle brackets as backup plates inside the transom already, but I also take the leverage off of the motor mount with two small ratchet straps. They hook onto the outboard and lead forward at angles to each side of the cockpit. With that setup, all of the motor weight is directly downward on the transom. I haven’t seen any negative affects of this setup other than reducing the tongue weight of the trailer accordingly. It saves time and back pain.

    2. Mike hurley says:

      I have a Tohatsu 9.9 four stroke. At idle, it puts out 2 to 3 amps. I can start it by the rope starter or a small lawn tractor battery. I can’t decide on two 12v dual purpose AGM batteries in parallel or a small start battery and 1 AGM or LiPo4 battery. The LiPo4 means using a DC to DC converter in place of a charge controller. I could always add a solar panel as it’s a sailboat.

      1. Hi, Mike

        Why do you feel the need for two batteries when one would suffice, be simpler, and less expensive?

      2. michael j hurley says:

        You’re correct. I was thinking I might want a larger AH battery in case I decide on using some type of small refrigerator or an electric kettle for hot water.

      3. Got it. In my opinion, there are better ways to do those. Here’s a few things to think about. A refrigerator will be heavy, generate heat and noise in the cabin, and there isn’t a good installation location for it in most trailerable sailboats. I’d consider a high efficiency cooler with regular or dry ice that you can move wherever it’s out of the way. An electric kettle will also generate heat in the cabin and is a big energy draw. Consider a small, gas backpack stove like a JetBoil that you can use in the cockpit and cook entire meals, if necessary. But the choice is up to you. Neither is wrong, just different.

      4. michael j hurley says:

        You are correct. I or anyone can over engineer anything. I should just use the kiss formula and get sailing. Thank you

      5. michael j hurley says:

        Also another battery for lights and instruments.

  4. Ron says:

    Hi I have enjoyed your blog for some time and you have saved me quite a bit of $ and opened up quite a few new ways of looking at taking care of my boat. Thanks… I recently purchased a 32′ Clipper Marine with a Yamaha 25hp 2 stroke outboard motor. I have been looking for information on installing an alternator to help with my battery charging. I can’t seem to find anything except your blog. Can you possibly tell me where I can find some info? I see you have done it on a Yamaha 8, so I seem likely I could do it on my 25 but where do I start looking, what do I need, how do I go about it, etc. Thanks for what you are blogging and any help you can give me..

    1. Hi, Ron

      I’m glad you’re finding the blog useful.

      Find your outboard starting here. Compare the diagrams to your motor and see what parts you have and what you’re missing to complete the charging circuit. You might already have the coils installed on your motor. In that case, you just need the rectifier/regulator and wiring harness like mine did. If you can’t find it, a local marine outboard dealer’s service department should be able to help you figure out what’s possible with your motor.

      Good luck and let me know how it works out.

  5. Ben says:

    Can the negative wire be connected to the negative post of the battery or does it HAVE to go to some non-battery ground point on the boat?

    1. The negative wire from the rectifier/regulator can be connected to the battery but you should not have more than 4 connections total at the battery post. A small ground bus bar makes maintenance and additions much easier and more reliable.

  6. Garland says:

    I know my limitations when it comes to electricity, but I think that the idea of fuses is to protect the wiring by putting the fuse close to the source of the current, which in this case is the motor. So perhaps you should install one there as well, if there isn’t one there already.

    1. Good point, Garland.

      Generally speaking, that’s true. But think about this; how much of the time is the motor a current source versus how much of the time is the battery a source? The battery is a source as long as it has a charge, right? The motor is a source only when it’s running and it only puts out a handful of amps. The battery can dump hundreds of amps instantly. And where is a short most likely to happen, across the battery poles or in the outboard wiring? Imagine a following swell swamping the motor. Then the motor becomes an infinite load on the battery. The wiring between the battery and the motor could fry anywhere along its length in an enclosed space possibly containing gasoline fumes. Wouldn’t you want a fuse to blow first and as close to the battery as possible? If the battery gets swamped to the point that it shorts out, you’ve got bigger problems and the outboard probably isn’t going to be much of a help.

  7. bmcd47 says:

    I have a Yamaha 9.9 on a Catalina C22. I have a solar panel charging my house battery and plan on installing a starting battery and charging it off of the solar panel. Does the Yamaha send enough current to damage the solar panel. What precautions should I take.

  8. Hi, BMCD47
    Most solar panels that I’ve seen have built-in backfeed protection, so check your panel or controller. My Yamaha 8HP makes up to about 5 amps, for reference.

  9. kootenayEVfamily.ca says:

    I’m an electronics novice (planning to get that book to read!), and after reading your post I am still a bit unclear on something… bought my boat last fall and it came with two starter/”deep cycle” Pb acid batteries, both the “maintenance free” wet-cell variety. One is toast, and the other is probably a few years old too, so I’m contemplating replacing both with either two AGM cells or one (to start with) Li-ion battery (probably LiFePO) – boat has a 2-way Perko switch.

    What I can’t figure out is where the charger regulator lives on the boat, or maybe there isn’t one? Boat came with a 2009 20hp 4-stroke Mercury with electric start and 12A “charger”. After reading your post, I’m guessing it is just a rectifier without any way to limit charging to the battery(s)? If that is the case, I would fry my AGM or Li-ion pretty quick right? Is there a type of smart charger to install in between? Thanks for any help!!

    1. If the “charger” is built into the motor then it also has regulation circuitry to prevent overloading the battery bank. If it’s an external box connected to the battery bank then it should also have built-in protection. In that case, research the specs of the charger and find an owner’s manual to learn how it regulates charge and its proper operation. In either case, confirm with the mfr. that the charger is compatible with the types of batteries that you want to install.

  10. Andrew McGowan says:

    Great article and guidance. I am updating the wiring and connector (weather pack) from my 5 hp 2 stroke tohatsu to my west marine new battery. I got a battery busbar with integrated and replaceable fuses (the busbar has four connectors and fuses). Do you keep the outboard charging cables connected at all times or just when charging? Is there any effect if you are charging while using the house batteries (VHF, electronics)? I haven’t seen much online guidance on used of this set-up. Thanks, Andrew McGowan (C-22 “Sea Life” Virginia)

    1. I leave it connected unless I’m working on the outboard or the electrical system. The effect of using devices while charging is that, depending on how much the total device load is, the battery rebuilds its charge slower. If outflow is more than inflow, the battery will slowly drain. If the other way around, the battery slowly recovers or maintains its charge.

  11. Tim says:

    Great article & you’re spot on about all but one thing: 80 watts is 6.6 amps at 12 volts, and 5.7 amps at 14 volts, which should be the actual output of the regulator. This is why you’re only seeing 5 amps @ full throttle. Power is calculated by the formula: Volts times Amps equals Watts (V x A = W). This is why, on transformers, you often see the power rating expressed as “VA” (volt amps). The reciprocal equation would be Watts divided by Volts equals Amps (W ÷ V = A). If you divide 80 Watts by 14 Volts , you’ll come up with 5.714 Amps.

    I would recommend adding a fuse to the output of the charging circuit close to whatever the rated output of the charging coil is to protect the coil. The reason being is that it’s very possible to overamp your charging coil. You’re never getting more than the rated max out of your coil because your particular engine has the lighter magnets in the flywheel. Many older engines had 2 different flywheels, one for no charging system & one with charging system. The flywheel for charging system motors has more & stronger magnets. Many manufacturers these days have decided not to produce two different flywheels. When you add the charging coil to the engine, the magnets are already there & are capable of exciting the coil beyond it’s maximum output rating.

    Now, you CAN overamp your charging coil for a short time without damaging it. Say about 20% for 5-10 minutes isn’t likely to harm it, provided it gets a nice long chance to cool off afterwards. If a boat does have a higher draw than the coil was rated to supply, you will need to install a current limiting (ballast) resistor in series with the charging circuit output.

    Here’s a good source to get single package rectifier/regulators on the cheap:

    Many years ago when I was a small engine mechanic, I added the charging coil from a Lawnboy to my Johnson 9.9. (Hey, OMC is OMC!) I built my own bridge rectifier as they didn’t have packaged ones in those days. I didn’t bother putting a regulator on it. I just wanted something to keep the lights going on my 8′ inflatable when I was running at night. It worked great, but if I touched any of the lights I got lit up like a christmass tree! I still have that old Johnson, but I’ve added a rectifier/regulator package to it since then.

    1. Ah, I wasn’t aware that the output voltage was 14V, that’s good to know. Thanks for your thoughtful comments.

  12. Bob says:

    You can use rectifier from china made scooters bring ac to battery comp mount rectifier near battery $10. Works fine have 92 johnson 8 hp

  13. BoatingList says:

    Yamaha is a nice engine manufacturer. I remember I had one boat with the same engine when I was a young man. That was a while ago. Would you mind if I add your site to my web directory?

    1. None of the links on your site seem to work for me, does it require registration?

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