Victron Buck-Boost 25A wiring diagram

The Buck-Boost DC-DC 25 A is a configurable, non-isolated converter used for battery-to-battery charging, including vehicles with variable-voltage alternators. It accepts 10–30 V input and has 10–30 V adjustable output. The conversion table, printed page 9 distinguishes 25 A at 12→12 V, 15 A at 24→24 V, and 10 A at 12→24 V. The catalog's 25 A value describes the 12 V-to-12 V use case, not every conversion.

The physical high-current connections are IN, GND, and OUT, using M8 connections. Crimp draws IN − and OUT − separately, but both represent the same GND connection; there is no isolation. The manual requires connecting all grounds to the converter, battery, and chassis before either positive lead. It warns that connecting both positives without GND can produce uncontrolled currents. There is no reverse-polarity protection.

The symbol omits the USB configuration connection, engine-detection controls, and small signal pins. The 25 A model does not have the CAN-bus connection offered on the larger models; do not plan a CAN-bus temperature-sensor setup around this unit.

The input/output fuse table specifies 16 mm² cable for runs below 5 m on the 25 A model. Use 40 A protection on a 12 V side and 30 A on a 24 V side, selecting each side's row independently. Thus a 12→24 V arrangement uses different input and output fuse recommendations. Position protection to protect the cable from each battery source; an input fuse does not protect the service-battery end of the output cable.

Use the Windows TSConfig application over USB to review output voltage, maximum charging current, input limits, and engine detection. Choose settings approved for the service battery and its BMS. The unit can use engine-running detection or an appropriate control signal; verify behavior on the actual vehicle, especially when the alternator voltage falls at idle.

Test engine-off shutdown, restart, and charge current after installation. Do not lower the on-board voltage threshold simply to force charging, since that can discharge the starter battery.

Mount in the manual's ventilated location, protect wiring from vibration, and allow cooling around the enclosure. Thermal limiting can reduce charging current below its programmed maximum. Check both batteries' voltage, conductor heating, and actual output under the intended load; the 25 A label does not guarantee that output in a hot compartment or step-up configuration.