RV Alternator to Lithium Battery Charging: Complete DC-DC Wiring Guide for Safe Installation

Bottom Line Up Front

For your 2007 Holiday Rambler lithium battery installation, YES - the yellow-circled wire labeled "12V BATTERY CHASSIS" is the correct place to interrupt the alternator feed to prevent charging your lithium batteries. However, you must keep both wires connected to your 12V distribution panel for proper operation. The yellow wire feeds power from your chassis battery through the panel to your house systems when the engine is running, while the purple wire ("12V HOUSE NON SWITCHED") provides power from your house batteries. Both circuits work together to ensure your RV's 12V systems have power whether you're driving, parked, or using shore power through your new inverter/charger.

Root Causes

Your concern about alternator charging lithium batteries is absolutely valid. Most RV alternators output 13.8-14.8 volts, with some reaching 15+ volts during cold starts, but lithium batteries require precise charging profiles with specific voltage stages and temperature monitoring. Direct alternator charging can cause lithium batteries to accept too much current too quickly, potentially leading to thermal runaway or permanent damage to the battery management system (BMS).

The confusion about which wire to disconnect stems from Holiday Rambler's dual-charging system design. Your 2007 model uses a battery isolator or separator system where the alternator charges the chassis battery, then a separate circuit feeds power to the house side. The yellow wire carries this alternator-fed power from the chassis battery through your distribution panel to charge house batteries and power 12V loads while driving.

The purple wire represents your house battery bank's direct connection to the distribution panel, providing power when parked or on shore power. These two feeds work in parallel - the chassis feed supplements house battery power while driving, and the house feed provides power when stationary. Removing either connection would create operational problems in different scenarios.

Before You Start

Before making any modifications to your Holiday Rambler's electrical system, photograph all existing connections and label every wire. Your car stereo installation experience will serve you well here, but RV systems are more complex with multiple power sources and safety systems. Test your multimeter on known good circuits to ensure accuracy, and verify you have proper 12V test equipment.

Confirm your lithium battery specifications and BMS requirements. Most quality lithium RV batteries have built-in protection that will disconnect if they receive improper charging, but this protection shouldn't be relied upon as your primary safety measure. Document the current draw of all your 12V loads - this will help you size your future DC-to-DC charger properly.

Plan your temporary charging strategy carefully. Relying solely on solar and shore power through your inverter/charger means you need adequate solar capacity for your usage patterns. In Arizona's winter months with shorter days, two standard solar panels may not provide enough power for extended boondocking without supplemental charging.

Gather the necessary materials: appropriate gauge wire for any extensions, marine-grade terminals and heat shrink tubing, electrical tape, and zip ties for securing connections. Since you're planning a future DC-to-DC charger installation, consider running a dedicated wire path now to avoid duplicate work later.

The Repair Process

Step 1: Install a Battery Disconnect Switch
Rather than permanently disconnecting the yellow wire, install a manual battery disconnect switch rated for at least 125% of your circuit's actual maximum amperage (determine this by checking existing fuse/breaker size or measuring actual load). Verify that existing circuit fusing is adequate for the modification, or install appropriate fusing between the chassis battery and disconnect switch if none exists. This allows you to isolate the alternator feed when using lithium batteries but restore the connection if you ever switch back to lead-acid or install your DC-to-DC charger. Mount the switch in an accessible location near your distribution panel, using wire sized according to the circuit's amperage requirements - typically 10 AWG for 30A circuits or 8 AWG for 40-60A circuits, following manufacturer specifications and local electrical codes.

Step 2: Modify the Charging Circuit
Connect the incoming yellow wire (from chassis battery) to one terminal of your disconnect switch, and run a new wire from the other terminal back to the distribution panel connection point. This creates a controlled interruption in the alternator charging path. Use marine-grade terminals and heat shrink all connections to prevent corrosion in Arizona's extreme temperature conditions.

Step 3: Test System Operation
With the disconnect switch in the OFF position (alternator feed interrupted), verify that all your 12V systems operate properly when connected to shore power through your new inverter/charger. The purple wire should provide adequate power from your lithium batteries to all house loads. Test lights, water pump, furnace fan, and any 12V outlets throughout the RV.

Step 4: Verify Isolation
With your multimeter, confirm that no voltage passes from the chassis battery to your lithium batteries when the disconnect is OFF and the engine is running. WARNING: Never physically disconnect battery cable connections while the engine is running, as this can damage the alternator - use only the installed disconnect switch. This ensures complete isolation of the alternator charging circuit. Also verify that your chassis battery still charges normally when driving, indicating the alternator circuit to the engine battery remains intact.

Step 5: Document and Label
Create a clear label for your new disconnect switch explaining its purpose: "ALTERNATOR TO HOUSE BATTERIES - OFF FOR LITHIUM." This prevents future confusion if someone else works on your electrical system. Update any electrical diagrams you have and store them with your RV documentation.

Professional Help Needed

While your car stereo installation background gives you excellent foundational knowledge, consider consulting with an RV electrical specialist before making final connections. They can verify your Holiday Rambler's specific charging system design and ensure your modification won't affect other systems like your battery monitor, solar controller integration, or emergency breakaway switch.

If you encounter any unusual readings during testing, or if your 12V loads don't operate properly with the alternator feed disconnected, stop work immediately and consult a professional. Some 2007 Holiday Rambler models have integrated charging systems where the alternator feed affects more than just battery charging - it may also power certain loads or provide reference voltage for other systems.

When you're ready to install your DC-to-DC charger, having a professional verify the installation is wise. These units require proper programming for your specific lithium battery chemistry and must integrate correctly with your existing solar charging system to prevent conflicts. The investment in professional consultation now can prevent expensive component damage later.