Mercury and FourStroke are trademarks of their owner. Amp Hour Supply isn’t affiliated with Mercury Marine.
The short answer. Mercury rates the 75-115hp FourStroke alternator at 35 A. A 30 A DC-DC charger takes most of that, so it works best at cruise and will pause at idle. It’s still the right way to charge a lithium house bank from a 115. Just keep your expectations at trolling speed honest.
Start with the 35 amps
Most lithium upgrade advice assumes a big engine. The 115 isn’t one, electrically. Mercury’s spec row for the 75-115hp FourStrokes reads 35 amps, or 441 watts. The 150 gets 60.
Victron’s Orion-Tr manual adds the catch: at idle an alternator can give about half its rating, and less once it’s hot. On a 115, that’s not much. A 30 A charger running flat out would want nearly everything the alternator makes at full throttle.
What happens when it can’t get it? The input voltage sags. The Orion-Tr reads a long sag (over a minute below its shutdown level) as “engine off” and stops charging. Then the voltage recovers and it starts again. Annoying, not harmful. The charger is doing its job: keeping the start side safe.
What the 115 does to help itself
Mercury built in idle charging compensation. The manual’s spec table lists it at 700-800 rpm and says the engine may raise idle on its own, up to 800 rpm, when the battery is low, so it charges faster.
One thing to know: turn on the optional troll control and it overrides that low-battery feature. If you troll for hours with electronics running, the alternator gets no idle bump. Plan your house bank for that day, not the cruising day.
Mercury’s battery and cable rules for the 115
- Starting battery: 1000 MCA, 800 CCA, or 65 Ah (75-115hp manual).
- Factory cables: 12 ft of #4 gauge. Go longer and Mercury steps the size up: 2 AWG to 40 ft, 1 AWG to 50 ft, 1/0 to 64 ft, 2/0 to 80 ft. Add the positive and negative lengths together.
- Copper cable only. Mercury says no aluminum in any marine install.
- The vessel (accessory) harness fuse is 2 A. Don’t take charger power from it.
- Keep a nonconductive shield on the battery terminals.
Why not just parallel the batteries?
Because lithium doesn’t taper the way lead-acid does. Wire it to the start battery through a switch or a plain combiner and it pulls as hard as the 35 A alternator can push, for most of the charge. Victron says that can overload an alternator. The charger caps it at 30 A and uses a lithium profile. More on the three options in Charging lithium from a boat alternator: 3 ways compared.
You’ll need
- Victron Orion-Tr Smart 12/12-30 isolated DC-DC
- A fuse at the start battery and one at the house busbar
- Copper, tinned cable, sized with the Wire gauge calculator
- Victron SmartShunt 500A so you can see the pauses for what they are
- Victron Smart BatteryProtect 100A on the house loads
Wiring it, step by step
- Shut the engine down and turn the battery switch off.
- Confirm your starting battery meets Mercury’s figure above. Leave it as your start battery.
- Mount the charger close to the start battery, upright, with space around it.
- Fuse and run the input from the start battery positive.
- Fuse and run the output to the house positive busbar.
- Run the house negative through the SmartShunt.
- Enter your lithium maker’s voltages in VictronConnect.
- Test at cruise first. Then idle. If it cycles at idle, cut house loads while trolling and let the cruise run do the work.
Shore power or an AC charger: hire a licensed electrician.
Is a 115 the right engine for this?
Good fit: – Mercury 75-115hp FourStroke covered by the 8M0145552 manual, single engine, 12V. – A modest house bank: fish finder, lights, a livewell pump, phone charging.
Poor fit: – Long days on troll control with heavy loads. The idle bump is off, and 35 A is 35 A. – A big inverter or fridge bank. Add solar or shore charging, and talk to a marine electrician about the engine side. – Your lithium maker’s manual says something else. Follow it.
Products
| Product | Why on a 115 |
|---|---|
| Victron Orion-Tr Smart 12/12-30 isolated DC-DC | Caps the charge so a 35 A alternator isn’t run flat out. |
| Victron SmartShunt 500A | Tells you if you’re keeping up between runs. |
| Victron Smart BatteryProtect 100A | Saves the bank on a long troll day. |
Prices show live in the store.
Bundles for this job: Dual-battery DC-DC charging kit, Boat lithium upgrade kit, large
Quick answers
What’s the alternator output on a Mercury 115 FourStroke? Mercury lists 35 amps (441 watts) for its 75-115hp FourStrokes.
What does idle charging compensation do on the 115? It can raise idle on its own, up to 800 rpm, when the battery is low, so the alternator charges harder. Troll control overrides it.
Will a 30 amp DC-DC charger work on a 35 amp alternator? Yes, but expect it to pause at idle. It charges properly at cruise.
Sources
- Mercury Operation Maintenance and Installation Manual, 75-115hp FourStroke, 8M0145552 (2018): https://servicelit.mercurymarine.com/mnetdata/service/mermar/owner/18/8m0145552.pdf
- Mercury FourStroke 75-150hp: https://www.mercurymarine.com/us/en/engines/outboard/fourstroke/fourstroke-75-150hp
- 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
