Mastervolt Mac Plus 24/12-50 DC-DC Converter Charger

The Mastervolt Mac Plus 24/12-50 is a DC-to-DC converter and charger built for boats that run a 24-volt house or start system but still need clean, well-managed 12-volt power on the service side. Feed it 24 volts and it hands you a properly charged 12-volt bank at up to 50 amps, using the same 3-step+ charge algorithm Mastervolt has trusted for years. If you've ever watched a service battery limp along because a smart alternator wasn't giving it a real charge, this is the box that fixes it.

Here's the problem it solves. Modern engines with Euro 5 and Euro 6 alternators, along with start/stop systems, throttle back charging voltage to save fuel. That's great for emissions and lousy for your house bank. The Mac Plus sits between the source and the service battery and boosts things back up, delivering a quick and consistent charge even to a completely flat battery. It also watches for engine-run status, so it only pulls when the engine is actually turning. Your starter battery never gets drained to feed the house.

We like this unit for repower and refit jobs on trawlers, sailboats, and cruisers that are moving over to lithium. It handles flooded, Gel, AGM, spiral, traction, and Lithium Ion chemistries, including LiFePO4, and you set the profile with the on-board DIP switches or through MasterAdjust. A 24-volt sailboat converting its fridge and pumps over to 12 volts is the classic case. Just plan on a small 12-volt sub-panel for those loads rather than tying straight into the main panel. Anglers running a big 12-volt electronics stack off a 24-volt boat lean on it too, since it keeps the sounders and plotters fed without dragging the main bank down.

The build is what you'd expect from Mastervolt. Aluminium housing, IP23 protection against dust and spray, and a fanless design that cools naturally, so there's nothing to hum in a quiet cabin at anchor. Efficiency runs better than 95 percent, peaking around 97, which means very little of your charge current gets wasted as heat. It's rated from -25 to 60 degrees C, though it starts derating above 40, so give it some breathing room when you mount it. Vertical, with clear space around the housing, is the way to go.

Wiring is straightforward but don't skimp on it. The Mac Plus uses three heavy M8 terminals for the positive input, positive output, and shared negative, and they'll take cable from 10 up to 50 mm2. At a sustained 50 amps you want heavy gauge on both sides and a proper fuse close to each battery. There's also an 8-pole screw connector for the remote on/off, the included battery temperature sensor, and the voltage-sense leads that compensate for cable drop and keep the charge accurate at the battery, not just at the converter.

Need more than 50 amps? You can run up to ten of these in parallel for well over 100 amps of 12-volt output, which is how bigger boats scale it. And because it speaks MasterBus, CZone, and NMEA 2000, the Mac Plus drops right into a monitored system. You'll see status, current, and alarms on your existing display instead of guessing. The 3-color LED on the unit gives you a quick health check at a glance too. Set it up once and it pretty much looks after itself trip after trip.

One honest note. This is a 24-to-12 volt converter. If your boat is 12 volt in and 12 volt out, or 48 volt, you want a different model in the Mac Plus family, and we're happy to point you to the right one. But if you've got 24 volts to work with and a service bank that deserves a real charge, the Mac Plus 24/12-50 is about as solid and set-and-forget as they come.