THE IDEA
Use the energy already on board before the starter battery becomes a problem.
Our Unimog can remain parked for long periods. Parasitic loads slowly draw energy from the starter battery while the house battery usually still has ample reserve. We did not want jump-starting at the press of a button, but autonomous maintenance charging: react in time, protect both battery systems and make the entire process visible remotely.
The automation detects gradual discharge after long periods of inactivity.
The house battery only supplies energy while its state of charge allows it.
Monitoring, switching and follow-up checks run without intervention.
GX and VRM show operating states and faults remotely.
THE COMPLETE SYSTEM
Four building blocks. One clearly interlocked task.
The Victron GX system running Venus OS Large supplies the starter voltage, house-battery SOC and device states. Node-RED evaluates these values, runs the timers and decides which charging path may be enabled. VRM provides the remote view.
The Orion XS charges the house battery while driving. When parked, the VOTRONIC StandBy-Charger Pro provides limited maintenance charging for the starter battery. To prevent opposing DC/DC paths from operating together, Node-RED locks the Orion first and waits for confirmation before enabling VOTRONIC.
Starter voltage · house SOC · timers · feedback · fault states
Remote access and history through VRMInterlock: The Orion XS is confirmed off before every VOTRONIC charge and is only released again afterwards.
WHY NODE-RED?
A single threshold would not be enough.
A brief voltage dip must not trigger a charge cycle. The automation needs to debounce measurements, retain AUTO mode through restarts, wait for feedback and stop safely when signals are missing. That is why the process runs as a state machine rather than a loose chain of triggers.
AUTO only means that the automation is allowed to operate. Whether it is waiting, confirming undervoltage, locking the Orion, charging, resting or reporting a fault is a separate state. This keeps its behaviour unambiguous after restarts and delayed device feedback.
THE FINAL LOGIC
Check first. Interlock next. Then charge.
- 01Read AUTO mode
The selected operating mode is loaded reliably at startup and retained.
- 02Confirm undervoltage
The starter voltage must remain at or below 12.5 V for five continuous minutes.
- 03Protect house SOC
Charging stops below 20%; the automation is not released again until 30%.
- 04Lock the Orion XS
Node-RED switches the Orion off and waits for confirmed feedback.
- 05Enable VOTRONIC
The 30-minute maintenance charge only begins after the interlock is confirmed.
- 06Switch back cleanly
VOTRONIC turns off, then the Orion XS is released again.
- 07Rest for ten minutes
The starter battery stabilises before the next assessment.
- 08Repeat if required
If the starter voltage is still below 12.7 V, another cycle begins.
ORION XS FIRMWARE 1.15
Emergency reverse charging and automatic maintenance solve different problems.
Victron's emergency reverse charging temporarily reverses the Orion XS charging direction. The 12/12-50A model can supply up to 25 A. It is started manually in VictronConnect, charges in reverse for 15 minutes and then lets the starter battery rest for five minutes. It is practical assistance when starting energy is already insufficient.
Our solution intervenes earlier: it monitors continuously, starts autonomously, respects the house-battery SOC and can repeat after a rest period. VOTRONIC performs the gentle maintenance charge, while the Orion remains available for its main job while driving. The new Victron function therefore does not replace this automation.
TWO FAULTS, TWO CAUSES
The software switched correctly – yet the charger still did not charge.
In the first version, AUTO did not start by itself. The mode was visible but was not passed to the state machine as a usable event during initialisation. Only a manual OFF → AUTO change produced the required impulse. Once corrected, the switching sequence started reliably.
VOTRONIC then remained physically in standby despite an active GX output. Its own UINLO input threshold was set too high and blocked charging. Only a careful adjustment to the actual switching point brought the software release, relay path and charger together.
A visible mode is not yet a processed event.
A switched output does not yet prove real charging current.
Do not turn it blindly to minimum: UINLO and UOUT must suit the battery type, cable losses and desired house-battery reserve. Node-RED's SOC protection complements the hardware protection; it does not replace it.
THE REAL-WORLD TEST
What matters is whether the complete sequence works without intervention.
The test began at about 12.21 V starter voltage. After five minutes of confirmed undervoltage, the automation locked the Orion, enabled VOTRONIC and began the 30-minute charging phase. Once UINLO had been corrected, charging current actually flowed. VOTRONIC then switched off automatically, the Orion was released and the rest period completed.
- START
- 12.21 V starter battery
- CHARGE TIME
- 30 min automatic cycle
- FINISH
- 13.06 V after the cycle
- HOUSE
- 73.6% SOC at test end
After ten minutes of rest, no second cycle was needed: the starter voltage was already above 12.7 V. The automation returned to standby.
SAFETY NOTE
This is a practical report – not a universal wiring guide.
Work on vehicle batteries can involve very high short-circuit currents, cable fires and damage to control units. Fuses, cable sizes, grounding, battery types and EN/DIS connections must suit the individual system. If in doubt, have a qualified specialist check the installation and final voltage thresholds.
THE CONCLUSION
The best automation quietly preserves starting capability.
The decisive factor was not one threshold, but the interaction of measurements, time, interlocking, feedback and hardware protection. The Unimog now uses its available energy deliberately without losing sight of the house reserve – and we can follow the system through GX and VRM.
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