The essentials in 30 seconds
- Four DC-DC uses onboard: protect the alternator of a lithium bank, convert 12 ↔ 24 V for mixed-voltage equipment, maintain the starter battery from the house bank, and provide stable power to sensitive electronics.
- Isolated vs non-isolated: isolated separates input/output grounds — useful for voltage conversions and clean buses; non-isolated, simpler, is fine when both sides share the same ground.
- Size with margin on the actual downstream current draw: a converter running at its limit overheats and shuts down.
- Heat is the real enemy: efficiency is not perfect; the difference is dissipated as heat — a ventilated location is mandatory.
- Poorly chosen, a DC-DC causes intermittent, invisible faults; properly sized, it runs for a decade without a thought.
Contents
The DC-DC converter is the least glamorous piece of gear onboard — yet one of the most critical for electrical reliability. On a modern refit mixing lithium house bank, lead-acid starter, and sometimes 24 V gear, there are two to five units. Poorly chosen, they cause intermittent faults that are hard to diagnose; properly sized, they run the system without a second thought. For the bank they protect, see our article on why lithium plug & play is a dangerous myth.
The 4 key applications
1. Protect the alternator of a lithium bank
A lithium bank can accept everything an alternator can deliver continuously — an alternator originally sized for lead-acid runs flat-out all the time, overheats, and wears out prematurely. The DC-DC charger sits between and limits the current drawn to a safe value for the alternator. This is application #1 in lithium refits.
2. Convert 12 ↔ 24 V for mixed-voltage equipment
24 V boat with 12 V gear (fridge, electronics, USB), or the reverse: the converter bridges cleanly with high efficiency, though not perfect — the difference is lost as heat.
3. Maintain the starter battery from the house bank
A DC-DC charger between the house bank and the starter battery keeps the latter charged with the correct profile, without a dedicated shore charger.
4. Power sensitive electronics with stable voltage
Boat voltage fluctuates constantly with load and consumers. An isolated DC-DC with a regulated output prevents random reboots of displays and onboard computers.
Isolated vs non-isolated: the rule of thumb
- Isolated: input and output grounds are separate — no ground-loop current, less electrical noise, no ground loops. Required for voltage conversions and clean buses.
- Non-isolated: common ground, simpler and cheaper. Suitable when both sides share the same ground reference — the standard case for alternator protection or battery-to-battery charging at the same voltage.
Workshop rule of thumb: use isolated for voltage changes or clean buses; use non-isolated for alternator protection and battery-to-battery charging at the same voltage. When in doubt, isolated never hurts — it just costs a little more.
Victron Orion line
The reference line for pleasure craft, available by voltage, current, and isolation. The units we install most often:
- Orion-Tr Smart 12/12-30A non-isolated — battery-to-battery charging (maintain starter from house bank).
- Orion-Tr Smart 12/12-18A isolated — clean bus for displays and onboard PCs.
- Orion-Tr Smart 24/12-20A isolated — 12 V gear on a 24 V boat.
- Orion Smart 24/24-17A isolated — clean bus on a 24 V boat.
- Orion XS 12/12-50A — the alternator protection unit for lithium refits, our workshop standard; and the Orion XS 1400 for higher loads (exact ratings on datasheets).
The rest of the line (12/24, 24/48, IP67 versions…) is in the DC-DC collection.
Mastervolt DC Master line
The direct competitor, with the argument of MasterBus/CZone integration: on units shipped with CZone, the DC Master is monitored from the existing digital display. Our datasheets: DC Master 12/12-6 isolated (sensitive electronics) and DC Master 48/12-20 (48 V boats, electric propulsion). The Victron vs Mastervolt choice follows the boat’s ecosystem more than the datasheet.
Sizing voltage and current
- Identify the maximum downstream load — the sum of what can draw at the same time (e.g., display + autopilot + fridge).
- Add margin to that current: a converter running continuously at its limit overheats and eventually shuts down. Margin equals reliability.
- Choose a model with a nominal current rating above that calculation, in the correct voltage/isolation combination.
- Plan for heat: even high-efficiency models dissipate a few dozen watts at full load — the mounting location must be ventilated.
Installation: wiring and protection
- Wire sized for the actual current, cross-section and length per the model’s manual — undersized input wiring drops voltage and triggers undervoltage shutdowns.
- Fuse in series on the input side, using the type and rating specified in the manual. It protects the wiring.
- Ventilated mounting location, never inside a closed locker.
- Keep it short on the input side: mount the converter close to its source to minimize losses.
- Commissioning settings (VictronConnect for Orion Smart): output voltage per use case (clean bus vs lead-acid charge profile), input undervoltage threshold to protect the upstream bank, and trigger mode (continuous or engine-running detection). These settings are part of installation, not optional extras.
Three workshop cases
Case 1 — Oceanis 38, lithium conversion
200 Ah lithium bank installed on a boat whose original alternator (80 A) was not designed for it. Protected by an Orion XS charger in series, current limited to a safe value for the alternator: the lithium charges at the alternator’s sustainable rate, eliminating prolonged overheating.
Case 2 — Bavaria 50 in 24 V, 12 V fridge
24 V boat, Vitrifrigo C115 fridge in 12 V (Danfoss BD35 compressor). A dedicated Orion-Tr Smart 24/12-20A isolated converter for the fridge: stable conversion, correct duty cycles, measured draw of 4–5 A on the 24 V side.
Case 3 — 38-foot sailboat, clean bus for displays
Displays and onboard computer that rebooted with every voltage fluctuation. An Orion-Tr Smart 12/12-18A isolated on a regulated output: no more reboots in 18 months.
What we rule out, and why
- No-name low-cost converters: optimistic efficiency ratings, weak thermal management, unpredictable overload behavior. On gear that interfaces with the alternator and the bank, the savings aren’t worth it.
- Automotive/caravan converters not marine-rated (unprotected enclosures): fine on the road, rapid aging in marine humidity and salt.
- Parallel wiring to increase current: without load-sharing designed by the manufacturer, it’s a recipe for trouble — pick the right model directly.
FAQ
How many DC-DC converters on a typical 40-foot sailboat?
Two to four depending on the architecture: alternator protection (lithium refit), clean bus for displays, maintenance of the starter battery, and a 24/12 conversion if the boat mixes voltages.
Do DC-DC converters make noise?
Small units cool passively and are silent. Large units have a fan that is audible nearby at full load — a factor when mounting near a cabin.
Is the Bluetooth on Smart models useful?
Yes: voltage, current, temperature, and history are read from a phone, and configuration is done without disassembly. For diagnosing intermittent faults, it saves hours in the workshop.
What’s the difference between Orion-Tr Smart and Orion XS?
The Orion-Tr Smart covers common uses (clean bus, conversion, battery-to-battery charging). The Orion XS is the high-power charger designed for alternator protection on lithium banks — two ratings in the catalog; exact capacities on datasheets.
Can two DC-DC converters be wired in parallel?
Technically sometimes, but not recommended without load-sharing designed by the manufacturer: better to choose a single model with the right rating.
Further reading
- Switching to lithium: why plug & play is a dangerous myth
- Marine lithium BMS comparison 2026
- Sizing a lithium bank — workshop method
- Pure-sine inverters: sizing and installation
- DC-DC catalog · Converters
About the author
Pol Conin, Digital Manager at Skysat. Software engineer and tech entrepreneur, Pol joined to bring Skysat into the digital age. See his author page for the full list of articles.
Transparency: Skysat distributes Victron Energy and Mastervolt and sells the products cited; the links point to our catalog. This does not change the rules of choice or the cases where the small model is the right one.
