How to Fix Firefly Battery and Victron System Conflicts in Your RV

Short Answer

Your Firefly-Victron conflict in your 2020 Discovery LXE 40M stems from communication protocol incompatibility between the two systems. While your dual ATS workaround solved the A/C shedding issue, the remaining low voltage warnings and non-functioning Auto Gen Start indicate the systems are still fighting over power management decisions. The best solution is typically implementing a CAN bus isolator or switching to a Victron-compatible battery monitoring system that can properly communicate with both your SOK batteries and the Firefly system.

Understanding the Problem

The core issue you're experiencing is that Firefly and Victron use different communication protocols and power management philosophies. Your Discovery's Firefly system expects to be the master controller for all power decisions, but your Victron Cerbo GX is also trying to manage the same functions. When you installed the dual ATS setup to make Firefly think you're always on shore power, you essentially bypassed Firefly's load shedding logic, but you didn't resolve the underlying communication conflict.

The "low voltage" warnings you're seeing are likely coming from voltage reading discrepancies between the two systems. Your SOK batteries are communicating their state to the Victron system via CAN bus, but Firefly is reading voltage differently, possibly from a different measurement point in your electrical system. The Auto Gen Start failure is probably because Firefly can't properly assess your true battery state when it thinks you're always on shore power, so it never triggers the generator start sequence.

Your electrical engineer's solution was clever but created new problems. By fooling Firefly into thinking you're always on shore power, you've disabled several important safety and convenience features that rely on accurate system state detection. This is why you're still experiencing functionality losses despite solving the immediate A/C problem.

What You'll Need

For the most comprehensive solution, you'll need to choose one system as the primary controller since CAN bus protocol converters between Firefly and Victron are not available due to their incompatible architectures. Most RV technicians recommend either fully integrating with Victron or maintaining Firefly as the master system. You'll need proper wiring materials for whichever integration approach you choose.

Alternatively, you might consider replacing Firefly's battery monitoring with a Victron BMV-712 Smart Battery Monitor (around $200) that can properly interface with both your SOK batteries and provide accurate data to both systems. This approach requires rewiring your battery monitoring setup and possibly adding a Battery Protect unit for load management ($100-150).

For the Auto Gen Start issue specifically, you may need a Victron Auto Gen Start controller or similar device (around $400) that can work with your existing Victron ecosystem, effectively replacing Firefly's gen start function. Tools needed include basic electrical testing equipment, crimping tools for proper CAN connections, and possibly an oscilloscope for troubleshooting communication issues.

How to Fix It

Start by disconnecting batteries and following proper DC electrical safety procedures, then choose which system will serve as your primary power management controller. If keeping Victron as primary, you'll need to bypass or disconnect Firefly's power management functions while potentially retaining its monitoring displays. This requires running new CAN bus cables from your electrical bay to wherever your Firefly control panel is located, typically in the cockpit area of your Discovery.

Decide whether to maintain your current dual ATS workaround or implement a different solution based on your chosen primary power management system. With proper CAN communication in place, Firefly should be able to make appropriate load shedding decisions based on actual system state rather than assuming constant shore power. This involves rewiring the ATS control circuits back to their original configuration while maintaining the physical dual-ATS setup for power routing.

For the Auto Gen Start, program the CAN bus converter to send appropriate battery state information to trigger Firefly's gen start sequence, or alternatively, disable Firefly's Auto Gen Start entirely and implement Victron's solution. Configure voltage thresholds in the converter to match what Firefly expects to see for proper low voltage warning management. Test the system thoroughly by disconnecting shore power and monitoring both systems' responses to changing battery states.

Finally, calibrate both systems to ensure they're reading consistent voltage and current values. This may require adjusting shunt locations or adding additional monitoring points. Document all settings and create a troubleshooting guide for future reference, as these hybrid systems can be complex to diagnose later.

Leave It to the Experts

This is definitely a job for professionals with specific experience in both Victron and Firefly systems. The complexity of CAN bus communication, power system integration, and safety considerations make this beyond typical DIY capabilities. Your original Victron-experienced technician sounds like exactly the right person to tackle the CAN bus isolation solution, as they understand both systems' requirements.

Expect to invest $1,500-3,000 for a professional solution, including parts and labor. The work typically takes 2-3 days and requires specialized diagnostic equipment to verify proper communication between systems. A qualified technician should provide documentation of the final configuration and ongoing support for any future integration issues.

Look for technicians certified in both Victron and RV electrical systems, particularly those with experience in Class A motorhomes with complex electrical systems like your Discovery LXE. Many Victron dealers now have experience with these hybrid installations as they become more common. Make sure any solution maintains your warranties on both the Victron components and your Discovery's electrical systems.