Victron Cerbo GX Relay: Generator Start, Fans, Heaters

The Victron Cerbo GX relay outputs turn a monitoring screen into a system controller. With them, a Cerbo can start a generator when the battery runs low, run a fan when a compartment gets hot, or switch on a battery heater before lithium cells get too cold to charge. This guide covers what each relay can do, its limits, and how to set it up, all from Victron’s Cerbo GX manual.

Electrical work can be dangerous: follow the manual, fuse and size every cable correctly, and use a licensed electrician for anything connected to household wiring.

A Cerbo GX generator auto-start video.

Victron Cerbo GX Relay Specs

	Cerbo GX MK2 / MK3
Relays	2, each NO/NC (potential-free)
DC rating	6A up to 30V DC; 1A up to 70V DC
AC rating	6A at 125V AC
Temperature sense inputs	4
Digital inputs	4

Source: Victron Cerbo GX manual, Technical specifications.

The relays are dry contacts. They switch your circuit but don’t supply power. For anything over those ratings, like a big fan, a heater pad or a generator starter, use the Cerbo relay to drive the coil of a larger external relay or contactor.

What Each Relay Can Do

Relay 1: alarm relay, generator start/stop, connected genset helper, tank pump, temperature-controlled or manual.
Relay 2: temperature-controlled or manual (needs firmware 2.80 or later).

You set the function under Settings → Integrations → Relays → Function. The default is normally open (NO). Switching to normally closed slightly raises the Cerbo’s own power draw, according to Victron.

Generator Auto-Start

Most generators accept a potential-free start/stop contact. Closing it starts the generator and opening it stops it. Some generators need a pulsed signal instead, which means an extra timing relay. Victron says to check your generator’s manual.

Once it’s wired, the GX can start and stop the generator based on:

Battery state of charge or voltage
AC load
Inverter high temperature or overload
Tank level
A periodic run or manual start

You can also set quiet hours with different thresholds, plus warm-up and cool-down times, run-time counters and service intervals.

Fans and Battery Heaters

Use a temperature-controlled relay. First, connect a temperature sensor and check that it shows up in the Device list. Then set the activation and deactivation temperatures for each relay. For lithium batteries, Victron’s manual covers heating and cooling control. Run a heater when the battery is near freezing, because most LiFePO4 batteries can’t be charged below about 5°C.

Wiring Checklist

Fuse the circuit the relay switches.
Keep within the 6A at 30V DC limit, and use an external relay for bigger loads.
Label the NO, COM and NC terminals.
Test the generator start manually from the Switch pane before turning on auto-start.

For the complete spec sheet and current manual, see the Victron Cerbo GX page.

Example: Generator Auto-Start in an Off-Grid Cabin

A 48V cabin with a MultiPlus-II, solar and a generator that accepts a two-wire start contact. Relay 1 is wired to the generator’s remote start terminals and set to “Generator start/stop.”

Setting	Example value
Start when state of charge falls below	30%
Stop when state of charge reaches	80%
Start when AC load exceeds	3,000W for 1 minute
Quiet hours	10pm–7am, start only below 20%
Warm-up / cool-down	60 seconds each
Periodic run	1 hour every 28 days

Example values; set them to suit your battery, generator and neighbors. Stopping at 80% instead of 100% lets the solar finish the charge the next morning.

Example: Battery Heater in a Winter Van

A 12V van with a lithium battery mounted in an unheated area. A temperature sensor goes on the battery, and Relay 2 is set to temperature-controlled, switching a 12V heating pad through an external relay.

Setting	Example value
Activate below	5°C
Deactivate above	10°C

Example values. A heating pad drawing more than 6A goes on an external relay or contactor, with the Cerbo relay switching only the coil.

Example: Compartment Fan

A fan in an electrical locker with a MultiPlus and MPPT. A temperature sensor in the locker drives Relay 2: the fan turns on at 35°C and off at 30°C (example values). A small 12V fan drawing under 6A can be switched directly, with its own fuse.

Example: Tank Pump

With a tank sensor connected, Relay 1 can switch a transfer pump to fill a tank when the level drops to a set point and stop when it reaches the upper level. This is handy for a water tank fed from a larger reservoir.

Wiring an External Relay

Connect the Cerbo relay’s COM to a fused positive supply.
Run a wire from the Cerbo’s NO terminal to the external relay’s coil.
Connect the other side of the coil to negative.
Wire the load through the external relay’s main contacts, with a fuse sized for the load.
Use a relay or contactor with a coil voltage matching your system (12V, 24V or 48V).

Testing Before You Trust It

Use the Cerbo’s manual control to switch each relay on and off and confirm that the load responds. For a generator, test start and stop by hand, then set the conditions and watch the first automatic start.

Using the Alarm Relay

Set to “Alarm relay,” Relay 1 switches whenever the GX raises an alarm, such as low battery voltage or an inverter overload. Wire it to a buzzer, a warning light or a boat’s alarm panel so you hear about a problem even with no phone nearby.

FAQ

Can both relays start a generator?

No. Only Relay 1 handles generator start/stop.

Can a Cerbo relay power a 12V fan directly?

Only if the fan draws under 6A at 12V, and the fan still needs its own fused supply.

Can the relays work without a temperature sensor?

Yes, in manual mode, or as an alarm relay or generator relay. The temperature-controlled function needs a connected sensor.

What happens if the Cerbo restarts?

Check the relay’s normally open/normally closed setting so the connected device behaves the way you want when the Cerbo reboots.

Sources: Victron Cerbo GX manual (Technical specifications; Relay connections; Generator auto start/stop; Temperature relay configuration), victronenergy.com, accessed Oct 2026.