DC to DC Charging Trolling Batteries from Outboard

Had a similar head-scratcher with my motorhome setup — running a DC-DC charger (Victron Orion-Tr Smart, naturally) off the engine alternator to keep the leisure bank topped up, and it made me wonder whether the same logic applies to outboard-to-trolling-battery scenarios.

The principle's identical really: your outboard's charging circuit is essentially just a glorified alternator, so a decent DC-DC isolates the banks properly and gives your trolling batteries a controlled charge rather than whatever the outboard's regulator fancies throwing at them.

Key things I'd flag for anyone going down this route:

• Check your outboard's charging output — smaller four-strokes can be stingy (we're talking 6–8A sometimes), so a 30A Orion might be overkill
• Victron's isolating DC-DC means your starter battery stays protected — no accidental flattening mid-estuary
• Cable sizing matters enormously — especially on a boat where voltage drop over a run can really sting you
• Fogstar or similar LiFePO4 trolling batteries will want a charger that actually understands lithium profiles, not just a basic split-charge relay bodge

The question I'd put to the room: has anyone actually measured real-world charge current coming off a mid-sized four-stroke outboard under load versus at idle? My suspicion is the available headroom for auxiliary charging is pretty modest unless you're running at decent revs for a sustained period — which rather defeats the point if you're trolling slowly.

Curious what setups people are running on UK waters specifically.

@PanelRob worth noting that the Orion-Tr Smart has an input voltage range floor of around 7.5V, so if your outboard's alternator is putting out anything sketchy at low RPM you'll trigger undervoltage shutoff before you've charged a thing. Seen this exact issue with a small Tohatsu on a mate's pontoon — alternator barely hit 13V at idle.

Few things to check:

• Alternator output voltage at cruise RPM vs idle — big difference on outboards
• Wire gauge from alternator to Orion input — voltage drop kills efficiency fast
• Set the engine detection threshold in VictronConnect properly or it'll cycle on/off constantly

My static van setup uses the same Orion for alternator charging and the input wiring was the first thing that bit me. Run it undersized and you're wasting your time.

Interesting thread — this is something I've been pondering for my narrowboat setup actually.

One thing worth checking is whether your outboard's charging circuit is even regulated properly. A lot of smaller outboards produce quite "dirty" AC that gets rectified to DC, which can cause headaches for sensitive chargers like the Orion-Tr Smart.

Has anyone tested actual voltage output under load from a typical 4-stroke outboard? I'd imagine there's quite a bit of variation depending on RPM.

Also wondering whether a B2B charger is even the right tool here, or whether a basic PWM controller might be more forgiving of the irregular input. Anyone tried Renogy's DC-DC unit in a marine context?

Great thread, and @AshSeeker raises a good point about narrowboats — though for an outboard setup the charging profile is worth thinking about carefully.

One thing I'd add is that with smaller outboards (say, sub-10hp), the alternator output is often surprisingly modest — sometimes only 6-8A total — so you'll want to check you're not trying to pull more through the DC-DC charger than the alternator can comfortably spare after running the ignition and instruments. The Orion-Tr Smart will happily throttle down, but worth sizing your expectations accordingly.

Also, if you're running lithium trolling batteries, make sure you've got the correct charge profile configured in the Victron Connect app before you connect anything up. The default lead-acid profile won't do your lithium cells any favours long-term.

Good points all round. One thing I'd add that hasn't been mentioned yet — if you're running lithium trolling batteries, make sure the Orion-Tr Smart's output profile is actually configured for lithium rather than left on the AGM default. Easy to overlook in the Victron Connect app, and it'll undercharge your bank every single time without complaining about it. Also worth thinking about whether your outboard has a dedicated sensing wire for the alternator output — some smaller outboards don't, and you'll want to verify actual charging voltage at the Orion's input terminals rather than assuming. A cheap multimeter while the engine's running at a decent RPM will tell you straight away if you've got enough headroom above that input floor @FormerMechanic14 mentioned.