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 so far. With the addition of accessories like the music system, the LED strip lights, and the autopilot, a single battery charge became insufficient 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. I don’t need a weekday system to charge the battery while the boat sits in a slip. We trailer to every launch and home again and I have an onboard charger for that already.

At first, I considered 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 return on investment looked bleak and my focus turned to our outboard motor. The winds are usually light in the North Idaho lakes in summer and we typically spend a lot of time motor sailing, more than I’d like, actually.

The good news is that our outboard motor, an 8HP Yamaha 4-cycle (model F8MLHC), already had generator coils. All it needed was a regulator/rectifier to convert the unregulated AC output into regulated DC current suitable for the battery. The manual states that the output is 80 watts maximum, or 10 amps at 12 volts DC.

If your outboard motor doesn’t have generator coils installed 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

So I turned to eBay (where else?) and bought a used regulator/rectifier (Yamaha part no. 68T-81960-00-00). I had it mounted 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 wiring harness out of 12 AWG cable. It connects to the rectifier/regulator output and to ground on the motor, 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 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.

Connector
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 two years 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.

The bottom line:

Suggested price: $120.25 + installation
$tingy Sailor cost: $71.05 installed
Savings: $76.40 + installation

What do you like most about your outboard charging circuit or solar panel?

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14 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.

  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.

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