How Much Does Your Outboard Really Charge? Rated vs Net Output, Rpm and Extra Batteries

Check your owner's manual and the engine:

• No charging at all: some small rope-start motors.
• Lighting coil: many small tiller motors have a coil under the flywheel meant mainly to run lights. Practical Boat Owner notes its power is deliberately limited, and that it needs a rectifier to charge a battery.
• Stator with rectifier/regulator: most electric-start outboards. Coils under the flywheel make AC, and the rectifier/regulator turns it into controlled DC.
• Belt-driven alternator: many larger outboards, much like a car.

Find the charging rating (in amps) in your owner's manual or on the maker's spec page. Examples from published specs show how wide the range is:

• Yamaha lists 16 amps for its F40 jet, 17 amps for the F60 jet and 35 amps for the F90 to F150 jets. Its spec tables label the figure "Alternator Output at W.O.T.", meaning at wide-open throttle, which is exactly why you won't see it at idle.
• Mercury lists 35 amps on its 75 to 115 hp FourStrokes and 60 amps on the 150. On its jet models it lists 17 amps (25 hp), 18 amps (35 and 40 hp), 35 amps (65 and 80 hp) and 60 amps (105 hp).
• Mercury says the smallest alternator on its V6 to V12 outboards (175 to 600 hp) makes 85 amps, and rates the V10 Verado at 150 amps.

Small tiller motors make far less. Practical Boat Owner's example was a 6 hp motor giving about 8 amps at the rpm a sailboat typically cruises at, and only with an improved rectifier circuit.

Charging output drops off sharply toward idle on most outboards. A motor rated at 35 amps may give only a fraction of that at trolling speed. Some newer engines help: Mercury's 150 hp FourStroke and its 105 hp jet have an "idle charge" feature that raises idle rpm on its own when the battery is low, to boost alternator output. If yours does that, it's normal, not a fault.

Make a simple list of everything that runs while you are out, and its current draw in amps (from the label or manual). Typical items: fish finders and chartplotters, livewell and bilge pumps, navigation and courtesy lights, a stereo and amplifier, and the engine's own electronics. Multiply each by the hours you use it. The battery bank sizing guide walks through this amp-hour budget in detail.

Then compare:

• At cruise: most engines out-charge typical loads.
• At idle or trolling for hours: the loads often win, and the battery slowly drains even though the system works fine.

With the engine off and the battery rested, measure the voltage at the battery posts. Then, with the engine in the water, measure again at idle and at a fast idle or cruise rpm. Voltage that rises above the resting reading means current is going in. The charging system test guide lists typical ranges; your service manual has the exact ones. Write down the rpm next to every reading.

Voltage tells you charging is happening; amps tell you how much. Two owner-friendly ways:

• DC clamp meter: clamp it around the engine's main positive cable or charging lead (not around both positive and negative together). Make sure the meter can read DC amps; many cheap clamp meters read AC only. Zero it before clamping.
• Battery monitor with a shunt: shows the net amps going in or out of the battery all the time. It is the easiest way to see what really happens on a fishing day (see the battery monitor shunt guide).

With a helper driving, record amps (or voltage) at:

1. Idle in neutral.
2. Trolling speed.
3. Around 1,500 rpm.
4. Normal cruise.

Do it once with your electronics off and once with everything you normally run turned on. The difference shows how much your loads eat into the charge. Log the results; they become your baseline for spotting a failing stator later.

On most outboards, the engine charges the starting battery first. Extra batteries only get charged if the boat is wired for it:

• Battery switch: charges whichever bank is selected (see the dual battery guide).
• Automatic charging relay or isolator: shares charge once the starting battery is up (see the ACR and start isolation guide).
• Built-in charging leads: Yamaha says many of its outboards of 150 hp and up have separate charging leads for house batteries that charge the starting battery first, then switch to the house bank.

Splitting a modest output between two or three batteries means each one fills more slowly.

A 12-volt outboard can't directly charge a 24- or 36-volt trolling motor bank, and at trolling speed it has little to spare anyway. Most anglers recharge the trolling bank with an onboard shore charger between trips (see the onboard battery charger guide). Some add a DC-to-DC charger that can step up voltage; Boating Magazine notes these come in 12-to-12, 12-to-24 and 12-to-48-volt versions. Size any such charger to what your engine can actually spare.

Lithium (LiFePO4) batteries charge differently from lead-acid. Before connecting one to your outboard's charging system, check that your engine maker approves it, and follow the battery maker's instructions. Battle Born warns that if a lithium battery's management system shuts the battery off while an alternator is charging it, the alternator and boat electrics can be badly damaged. A DC-to-DC charger between the engine and the lithium bank is the usual safeguard. Boating Magazine notes you will still need a lithium-capable shore charger to bring the bank to full between trips.

If your measurements show you use more than you make:

• Turn off what you don't need while idling or trolling.
• Charge fully at the dock before every trip.
• Add solar to keep batteries topped up between trips.
• Run at higher rpm for a while on the way home.
• Fit a separate house or trolling bank with its own shore charger.

Don't add home-built charging circuits to an outboard or try to "upgrade" a stator yourself. Ask a dealer whether a higher-output charging kit exists for your model.

If your measured output at cruise is far below what similar engines make, if voltage never rises, or if it climbs too high, the problem isn't usage. Go to the stator and rectifier test guide and the charging system test guide.