Mercury and FourStroke are trademarks of their owner. Amp Hour Supply isn’t affiliated with Mercury Marine.
The short answer. Mercury rates the 150 FourStroke alternator at 60 A, and its idle charge feature raises idle rpm when batteries run low. That’s enough for one 30 A DC-DC charger feeding a lithium house bank. Keep the starting battery the size Mercury asks for.
Mercury’s numbers for the 150
| What | Mercury’s figure | Source |
|---|---|---|
| Alternator | 60 A | M2 |
| Idle charge | Raises idle rpm to lift output and recharge low batteries | M2 |
| Starting battery | 1000 MCA or 800 CCA | M1 p.36 |
| Factory battery cables | 3.6 m (12 ft), #4 gauge | M1 p.117 |
| Cable material | Copper only, no aluminum | M1 p.117 |
| Accessory harness fuse | 2 A (DIAG, diagnostics/vessel harness) | M1 p.75 |
| Other fuses | Ignition 20 A, fuel 20 A, helm 15 A | M1 p.75 |
The manual is Mercury’s 150 FourStroke (3.0L) manual, © 2015. Mercury keeps current manuals behind a serial-number lookup on its site, so check yours against this table.
Will a lithium battery work with a Mercury 150?
As a house bank, yes, with a charger in between. As the starting battery, follow Mercury’s own rating table and your dealer. This page is about the house side.
Two things matter on this engine.
The 2 A accessory fuse isn’t a power feed. Fuse 1 on the 150 protects the diagnostics and vessel (accessory) harness at 2 A. That’s for gauges and data, not a charger. Charging power comes from the battery side, through the main cables.
Idle charge helps, but only so much. Mercury says the 150 bumps idle rpm on its own to lift alternator output when batteries are low. Victron’s Orion manual still warns that idle output can be about half the rating, and less when the alternator’s hot. Half of 60 A is roughly 30 A, which is the charger’s whole output. So at idle with house loads on, the charger may pause now and then. At cruise it has room.
Why not wire the lithium bank straight to the start battery? Victron’s manual says direct charging can overload an alternator, because lithium accepts current so readily. See Charging lithium from a boat alternator: 3 ways compared.
Getting the cable right
Mercury is specific here, which helps. The 150 ships with 12 ft of #4 gauge cable. If you move the starting battery further away, Mercury’s chart steps the gauge up. Length means positive plus negative added together:
| Total cable length | Gauge (AWG) |
|---|---|
| up to 12.2 m (40 ft) | 2 |
| up to 15.2 m (50 ft) | 1 |
| up to 19.6 m (64 ft) | 1/0 |
| up to 24.4 m (80 ft) | 2/0 |
That chart is for the engine’s starting cables. The charger’s own runs are smaller; Victron’s Orion terminals take up to 16 mm² (AWG6), and the manual lists a 60 A input fuse. Size those with the Wire gauge calculator. All of it copper, per Mercury.
You’ll need
- Victron Orion-Tr Smart 12/12-30 isolated DC-DC
- Two fuses, one near each battery
- Copper, tinned cable
- Victron SmartShunt 500A
- A nonconductive cover on every battery terminal (Mercury asks for one on the starting battery)
Steps
- Engine off, switches off.
- Check the starting battery: 1000 MCA or 800 CCA.
- Put the charger near the start battery, but not right over it.
- Start battery positive to the charger input, with the fuse close to the battery.
- Charger output to the house busbar, fused at the busbar end.
- House negative through the SmartShunt.
- Set the lithium profile from your battery maker’s manual in VictronConnect.
- Start up, let it idle, then run at cruise. Compare the amps on the SmartShunt.
Shore power and AC charging belong with a licensed electrician.
Fit check
This page fits if you have a single Mercury 150 FourStroke (3.0L) on a 12V boat and want a lithium house bank with a separate starting battery.
It doesn’t fit if: – You run twin 150s. Mercury wants a common ground cable between the starting battery negatives, the same size as the engine cables. Plan the charger layout with your dealer. – You have the 150 Pro XS, SeaPro, or an older 150. Check your own manual’s numbers. – Your lithium battery maker gives different instructions. Theirs come first.
Products
| Product | On a 150 FourStroke |
|---|---|
| Victron Orion-Tr Smart 12/12-30 isolated DC-DC | 30 A, inside the 60 A rating. |
| Victron SmartShunt 500A | Shows when idle charge kicks in and the charger keeps up. |
| Victron Smart BatteryProtect 100A | Cuts house loads before the lithium bank gets too low. |
Prices show live in the store. Or start with the Boat lithium upgrade kit.
Bundles for this job: Dual-battery DC-DC charging kit, Boat lithium upgrade kit, large
Quick answers
How many amps does the Mercury 150 FourStroke alternator make? Mercury lists 60 A for the 150hp. The 75-115hp FourStrokes are listed at 35 A.
What battery cable does a Mercury 150 FourStroke use? It comes with 12 ft (3.6 m) of #4 gauge cable. Longer runs step up to 2, 1, 1/0 or 2/0 AWG on Mercury’s chart, and copper only.
Can I take charger power off the 150’s accessory harness? No. That circuit is fused at 2 A for diagnostics and vessel wiring. Feed the DC-DC charger from the starting battery.
Sources
- Mercury 150 FourStroke Operation Maintenance Warranty Installation Manual, 8M0105247 (2015): https://servicelit.mercurymarine.com/mnetdata/SERVICE/MERMAR/OWNER/15/8M0105247.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
