All guides · Batteries and charging
I would start with the generator's capabilities and the house battery's requirements. A DC-DC charger provides control over the charging process. A standard voltage-sensing relay connects the battery banks once the conditions are met.
Two solutions with different tasks
A voltage-sensing charging relay can connect the starter and house batteries during charging and disconnect them again when the voltage drops. It does not in itself change the generator's charging voltage to a specific charge profile for the house battery.
A DC-DC charger manages the current between the two sides. On a Victron Orion XS, for example, input current and charging settings can be customized. This is relevant when the house battery requires different charging conditions, or when the generator's load needs to be limited.
| Question | Charging relay | DC-DC charger |
|---|---|---|
| What is regulated? | The connection between the battery banks. | The charging of the house battery. |
| What must match? | The generator's charging conditions and both battery banks. | Input, output, charge profile, and current limits. |
| What am I comparing in terms of price? | Relay, cables, fuses, and connection. | Charger, cables, fuses, any necessary controls, and setup. |
A lithium battery changes the calculation
When switching to LiFePO4, I want documentation on how the generator, battery, and BMS work together. A new battery is therefore not just a question of how many Ah can fit in the battery box. Also, clarify how charging is stopped when the battery's BMS requires it.
This does not mean that all boats must have the same DC-DC charger. There are various system solutions available. An existing installation should be evaluated based on the specific components.
Calculation example: 30 A is not automatically 60 Ah in two hours
If the charger actually delivers 30 A for two hours, and the boat's consumption is simultaneously 6 A, the net input will be approximately (30 − 6) × 2 = 48 Ah. The example assumes constant current. The charging process, temperature, and available generator output can change the result.
Choose based on available power
The generator's rated current is not a guarantee of available capacity at all RPMs and temperatures. I would therefore like to know the engine's other electrical consumption and the generator's documentation before I propose a charge rating. An input limit must match the source; the battery's permitted charging current also sets a limit on the output.
Send this information before you buy
- Engine and generator, preferably with a photo of the nameplate.
- Starter battery and house battery: voltage, chemistry, capacity, and model.
- Existing relay or charger, as well as cable lengths.
- How long the engine usually runs and how much power you use while underway.
With this information, I can help with a proposal that includes both the charger and the installation parts.
Sources and manufacturer documentation
This guide was written by SejlStrøm. Technical information was verified on September 22, 2026. Always use the documentation for the exact model and version.