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The short answer. Yamaha’s F300 page lists the alternator at 50 to 70 A at wide open throttle and doesn’t say which version gets which. Plan around 50 A. That’s room for one 30 A DC-DC charger per engine, between the start battery and the lithium house bank.
What Yamaha publishes for the F300
| What | Figure | Source |
|---|---|---|
| Alternator output, W.O.T. | 50-70 A (“up to 70 amp”) | Y2 |
| Maximum generator output, F300SB manual | 70 A, no rpm | Y1 |
| Starting battery, CCA/EN | 640-1080 A | Y1 |
| Starting battery, 20-hour rating | 80 Ah | Y1 |
| Main fuse / isolator fuse | 100 A / 100 A | Y1 fuse box page |
| Accessory battery fuse | Size it to local rules; in the USA, ABYC E-11 | Y1 |
Our manual is Yamaha’s European edition for the F300SB and LF300SB. It doesn’t list CCA/SAE or MCA figures. Get your own manual by serial number from Yamaha and use its battery table if it differs.
Can the F300’s isolator lead charge a lithium bank?
Not by itself. The F300 can charge an accessory battery separate from the starting battery using an optional isolator lead, with over-current protection. Yamaha’s electrical page says the engine charges the starting battery first, then switches the charge to the house battery.
The trouble is the second half. With a lithium bank on that lead, nothing limits the current. Lithium takes everything offered, and Victron’s Orion-Tr manual says direct charging like that can overload an alternator. The 100 A isolator fuse protects the wiring. It doesn’t shape a lithium charge.
A DC-DC charger does. It takes up to 30 A, charges with your battery maker’s voltages, and stops when the engine stops. The long version is in Charging lithium from a boat alternator: 3 ways compared.
One charger or two on an F300?
One per engine. The math is short.
Victron’s manual says an alternator at idle can give about half its rating, less when hot. Take Yamaha’s low figure, 50 A, and idle could be around 25 A. A single Orion-Tr 12/12-30 already asks for more than that at full output. Two chargers off one F300 would stall at the dock every time.
Victron’s rule for paralleling is to add up every charger’s input current plus the boat’s other loads and check it against the alternator at idle and hot. On one F300, that sum says one charger.
Running twins? Each F300 has its own alternator and start battery. A charger off each start battery, into the same house bank, keeps every engine inside its own numbers. Ask your Yamaha dealer to confirm the twin layout against your rigging; the manual shows twin wiring separately.
You’ll need
- One Victron Orion-Tr Smart 12/12-30 isolated DC-DC per engine
- Fuses at each battery end, sized from the Orion manual
- Tinned cable from the Wire gauge calculator
- Victron SmartShunt 500A
- Victron Smart BatteryProtect 100A for the house loads
How to wire it
- Battery switches off, engines off.
- Check the start battery against Yamaha’s table. The manual’s warning is plain: too little voltage and it won’t start.
- Decide what the isolator lead does now. If it charges a lead-acid thruster or windlass battery, leave it. If it went to the old house bank, have your dealer take the lithium bank off it.
- Mount the charger in a dry spot near the start battery, not right above it.
- Input from the start battery, fused near that battery.
- Output to the house busbar, fused near the busbar.
- House negative to the SmartShunt’s system side.
- Load the lithium profile in VictronConnect.
- Test at idle and at cruise. Watch the SmartShunt. If the charger pauses at idle with big loads on, that’s the idle math above at work.
Anything on the AC side (shore power, inverter-charger) goes to a licensed electrician.
Does this fit your boat?
Fits: – Single or twin Yamaha 4.2L F300 on a 12V house system. – A lithium house bank with lead-acid or AGM starting batteries that meet Yamaha’s table.
Doesn’t fit: – The 4.3L F350 or an F300 with a different manual. Check your own figures. – A 24V house system, or a big inverter bank wanting more than 30 A per engine. That’s a bigger alternator job for a marine electrician. – Your lithium maker’s manual says otherwise. Follow it.
Products
| Product | Use on an F300 |
|---|---|
| Victron Orion-Tr Smart 12/12-30 isolated DC-DC | One per engine, 30 A each. |
| Victron SmartShunt 500A | Proves what each charger is putting in. |
| Victron Smart BatteryProtect 100A | Drops house loads before the bank runs flat. |
| Victron Cyrix-ct 12/24V 120A | Lead-acid thruster or windlass battery only. Never the lithium bank. |
Prices show live in the store.
Bundles for this job: Dual-battery DC-DC charging kit, Boat lithium upgrade kit, large
Quick answers
How many amps does a Yamaha F300 alternator put out? Yamaha’s V6 4.2L page says up to 70 A, with 50-70 A at W.O.T. in the spec table. The F300SB owner’s manual lists 70 A maximum generator output.
What fuse protects the F300 isolator lead? The F300SB manual’s fuse box shows a 100 A isolator fuse beside a 100 A main fuse. For a fuse at the accessory battery, Yamaha points US owners to ABYC E-11.
Can I run two DC-DC chargers off one F300? We wouldn’t. At idle, an alternator can drop to about half its rating, which on a 50 A rating won’t feed two 30 A chargers.
Sources
- Yamaha Owner’s Manual F250/LF250/F300/LF300, 6KA-28199-70-E0: https://cdn2.yamaha-motor.eu/prod/owner-manuals/Outboard%20Engines/6KA-28199-70-E0.pdf
- Yamaha Outboards, 300-225 HP V6 4.2L: https://yamahaoutboards.com/outboards/350-150-hp/v6-4-2l
- Yamaha Outboards, Electrical System Maintenance: https://yamahaoutboards.com/maintenance-matters/electrical-system
- Victron Orion-Tr Smart DC-DC Charger Isolated manual (Rev 14, 02/2026): https://www.victronenergy.com/upload/documents/Orion-Tr_Smart_DC-DC_Charger_-_Isolated/34439-Orion-Tr_Smart_DC-DC_Charger-pdf-en.pdf
