The problem
The original fan in the Victron Multiplus only kicks in when the unit is working hard and is loud when it does. Robbert's fix: two small fans tucked into the bottom cover that provide gentle continuous airflow, then spin up automatically when the Multiplus warms up. The goal is to keep the factory fan from ever needing to engage.
Important: the modification lives inside the bottom cover, which is removable. The Multiplus itself is not opened. Warranty stays intact.
Parts
| Item | Purpose |
|---|---|
| 3D printed cradle | Holds two 80 mm fans, mounts inside the bottom cover. Free STL in the description. |
| Buck converter | Drops 48V battery voltage to 12V for the fans. |
| Temperature controller board | Reads a thermistor and outputs PWM based on temperature. |
| Two 80 mm fans | PWM PC fans, or cheap 12V fans with one extra transistor (see below). |
| TIP120 transistor + 2.2 kΩ resistor | Only needed if running cheap non-PWM fans. |
Robbert picked LED fans because he likes the blue glow under the Multiplus and uses the brightness as a visual cue for which fans are running.
Mounting the fans
Each 80 mm fan has an arrow on the side showing the airflow direction. The fans need to push air upward into the Multiplus, drawing cool air from outside. Mount each fan with three of the four screws supplied. Save two screws: those mount the printed cradle into the bottom cover at the end.
Driving cheap 12V fans with PWM
If you bought proper PWM fans, plug them straight into the controller and skip this section. You need a splitter cable because the controller has one PWM output and there are two fans.
For cheap 12V LED fans (no PWM input), one small modification turns the controller into a PWM-controlled 12V power switch:
- Take a TIP120 transistor
- Bend the middle pin backwards
- Bend the leftmost pin 90 degrees to the left
- Solder the transistor across the 12V minus pad on the front of the controller (the rightmost pin straight onto the 12V minus, middle pin bent back, leftmost pin bent left)
- Solder the 2.2 kΩ resistor between the leftmost PWM-output pin of the connector and the base of the transistor
The transistor now switches the fan minus line on and off at the controller's PWM frequency. The fans see effectively variable voltage.
Wiring summary
48V comes into the buck converter from the battery. The converter outputs 12V which feeds the buck plus on both fans. With cheap fans:
- Fan plus wires connect together to the buck 12V output
- Fan minus wires connect together to the transistor's bent-back pin
- Transistor switches the minus path at PWM rate
With PWM fans:
- Fan plus and minus go to the splitter, splitter goes to the PWM output on the controller
How the controller behaves
The temperature controller has DIP switches. With switch 1 off, the minimum fan speed is 20%. Switch 1 on, minimum is 40%. The cheap fans run dead-silent at 20% but still move air.
Switches 2 and 3 set the temperature thresholds for spin-up:
| Switch 2 | Switch 3 | Spin-up | Full speed |
|---|---|---|---|
| Off | Off | 35°C | 45°C |
| On | Off | 40°C | 55°C |
| Off | On | 50°C | 70°C |
| On | On | 60°C | 90°C |
For the Multiplus, leave both off. The fans should already be ramping up by the time the unit hits 35°C, well before the factory fan engages.
Switches 4 and 5 are stall alarms, which only function with PWM fans that report RPM back. If either fan stops, a piezo buzzer (included in the kit) starts beeping. Optional.
PWM has one side effect worth noting: the factory Multiplus fan uses a low PWM frequency that you can hear in the motor itself, especially at low speeds. The small fans in this mod do not have that problem.
The thermistor
The controller ships with a thermistor on a short cable. Extend the cable so the sensor can sit directly on the inverter transformer (the spool) inside the bottom cover. That is the hot spot, so that is where the reading needs to come from.
Final assembly
Bolt the printed cradle into the bottom cover using the two fan screws set aside earlier. The cradle uses the two inner mounting holes on the underside of the cover. Black fan screws blend in cleanly: from the outside the Multiplus looks unmodified except for a subtle glow if you chose LED fans.
Connect a 48V line into the buck converter input. The buck outputs 12V to the fan loop. The controller monitors the thermistor and steers the fans.
What you get
- Continuous gentle airflow whenever the battery is connected
- Automatic spin-up when the Multiplus actually warms
- The factory fan stays idle in most conditions
- Everything reverses: remove two screws and the cradle, and the Multiplus is back to stock
- No warranty implications
Robbert demonstrates the ramp-up by holding a soldering iron near the thermistor. The fans spin up visibly within seconds, then settle back down once the heat is removed.
Takeaways
- Removable bottom cover means no warranty hit.
- Two cheap LED fans + a buck + a temperature controller is the entire bill of materials. The 3D print holds it all in place.
- For cheap non-PWM fans, one transistor and one resistor on the controller board adds PWM behavior.
- Set the spin-up threshold low (35°C). The point is to keep the factory fan from ever needing to engage.
- The thermistor must reach the actual hot component (the transformer), not float in air.
