How to Protect Your RV Alternator When Charging Lithium Batteries: DC-

The Straight Answer

Safety Warning: Working with alternator charging systems involves high current electrical connections. Always disconnect the battery and turn off the engine before making electrical modifications.

Your temperature-sensing alternator protection idea isn't bad at all - it's actually a solid approach that many experienced RV techs use when dealing with LiFePO4 conversions. The concept of using a temperature switch to open your battery isolator solenoid when the alternator gets too hot is mechanically sound and relatively simple to implement. The main concern I'd have with your specific approach is that completely disconnecting the house batteries while the engine is running could cause voltage spikes that might damage sensitive electronics, but this can be managed with proper implementation. A better variation would be using the temperature sensor to trigger a current-limiting relay or activate a resistive load bank rather than completely isolating the batteries.

Common causes of alternator failure with LiFePO4 batteries include their aggressive charge acceptance compared to traditional batteries, especially when they're depleted. Where a traditional battery bank might pull 30-40 amps from your alternator, those lithiums can easily draw 80-100 amps or more until they reach about 80% state of charge. This sustained high current draw generates serious heat in your alternator windings and can cook the internal components. Your temperature sensing approach addresses the root problem by giving the alternator a break when it needs it most.

Breaking It Down

Let me walk you through why alternators often fail with lithium batteries and how your temperature protection scheme fits into the bigger picture. Traditional lead-acid batteries have internal resistance that naturally limits charging current as they approach full charge. They also have that familiar charging curve where current tapers off gradually. LiFePO4 batteries, on the other hand, maintain a very low internal resistance throughout most of their charging cycle and will happily accept whatever current you can throw at them until they're nearly full.

Your typical RV alternator was designed assuming it would see intermittent high output loads (see manufacturer specifications for duty cycle ratings) - charging depleted batteries for a while, then coasting at lower output once the batteries start to saturate. With lithiums, you're looking at potentially 70-90% duty cycle at maximum output, which generates heat faster than the alternator can dissipate it. The windings start to break down, the rectifier diodes fail, and eventually you're looking at a $300-800 alternator replacement.

The temperature sensing approach you're considering tackles this by monitoring the actual heat buildup rather than trying to predict it. A typical setup uses a bimetallic temperature switch mounted on the alternator case or a thermistor that feeds into a control module. When the alternator reaches elevated temperatures, the switch opens and interrupts the charging circuit. This gives the alternator time to cool down before resuming charging.

Safety Warning: Suddenly disconnecting a load from an alternator can create voltage spikes that may damage your engine's ECU, radio, or other sensitive electronics. The alternator doesn't know the load disappeared and for a split second it's trying to push current into nowhere. Some alternators handle this better than others, but it's a risk that requires proper circuit protection.

What You Need

For a robust temperature-based alternator protection system, you'll want to gather several key components. The heart of the system is going to be either a simple bimetallic temperature switch or a more sophisticated temperature controller with a thermistor probe. I'd recommend going with a temperature switch from a reputable manufacturer - verify the correct part for your model (verify with alternator manufacturer or parts supplier). The simpler switches are dead simple - they open at a set temperature and close again when it cools down by about 15-20 degrees below the opening temperature (differential). More sophisticated controllers give you adjustable setpoints and cost more but let you fine-tune the operation.

Instead of directly controlling your main battery isolator solenoid, I'd suggest using the temperature switch to control a secondary relay that either activates a current-limiting resistor bank (to reduce charging current) or switches in a dummy load (to maintain alternator loading). You'll need a heavy-duty automotive relay rated for at least 40 amps continuous. For the resistor bank approach, you're looking at some high-wattage ceramic resistors, with resistance values calculated based on desired current limiting (see manufacturer specifications for proper sizing). These will limit the current flow to your batteries while still providing a load path for the alternator.

The dummy load approach uses a bank of resistors or even a small 12V heating element that gets switched in when the alternator gets too hot. This maintains a load on the alternator while reducing the current going to your batteries. You'll need resistors or heating elements that can handle 20-30 amps safely - think along the lines of heavy-duty brake resistors from industrial suppliers or 12V heating elements designed for automotive applications.

For mounting the temperature sensor, you'll want to attach it to the alternator case near the rear bearing or on one of the heat sink fins if your alternator has them. Use thermal paste or thermal adhesive tape to ensure good heat transfer. The sensor needs to be positioned where it will accurately reflect the internal temperature of the alternator windings, not just the ambient air temperature around the engine bay.

Doing the Work

Safety Warning: Always disconnect the battery negative terminal and ensure the engine is off and cooled down before beginning any electrical work on the charging system.

Let me walk you through the installation process step by step, starting with the temperature sensor placement. First, you'll need to identify the best mounting location on your alternator. Look for a flat spot on the rear housing or one of the cooling fins where you can get good thermal contact. Clean the area thoroughly with brake cleaner or alcohol to remove any oil or dirt. If you're using a bimetallic switch, you can mount it directly to the alternator case and secure according to manufacturer specifications. Apply a thin layer of thermal paste between the switch body and alternator case to improve heat transfer.

For the electrical connections, you'll need to tap into your existing charging system wiring. Locate the wire that feeds from your alternator to your battery isolator solenoid - this is typically a heavy gauge wire, probably 10 AWG or larger, with 12V present when the engine is running. You're going to interrupt this control signal with your temperature-activated relay. Run a new wire from the alternator output to one side of your control relay contacts, then run another wire from the other side of the relay contacts to your original isolator solenoid trigger wire.

The temperature switch connects to the relay coil circuit. When the alternator is cool, the temperature switch is closed, the control relay is energized, and current flows normally to activate your battery isolator. When the alternator gets hot, the temperature switch opens, the control relay drops out, and the isolator solenoid loses power. At this point, your house batteries are disconnected from the alternator, but here's where you need the safety feature I mentioned earlier.

Instead of leaving the alternator with no load, your temperature-activated relay should simultaneously switch in your dummy load or current limiting resistor bank to maintain proper alternator loading while protecting the charging system using resistors. This requires a double-pole relay - one set of contacts controls the isolator solenoid, the other set switches in your protective load. When the alternator is cool, the first set of contacts energizes the isolator solenoid to connect the batteries for charging.d and the second set keeps your dummy load disconnected. When the alternator gets hot, the first contacts open the isolator circuit while the second contacts close to engage the dummy load.

Testing the system requires some patience and careful monitoring. Start with the engine at idle and your lithium batteries depleted enough to draw significant current. Monitor the alternator temperature with an infrared thermometer or digital probe thermometer if you have access to one. You should see the temperature climb steadily as the batteries pull maximum current from the alternator. The protection system should activate when the alternator case temperature reaches your setpoint according to manufacturer specifications.

Time for a Pro

While your temperature protection concept is sound, there are several scenarios where you should consider bringing in a professional or exploring more sophisticated solutions. If your RV has a complex battery management system with multiple charge sources - solar, inverter/charger, DC-DC converters - integrating a simple temperature switch might create conflicts or unintended interactions. A professional familiar with lithium battery systems can help you design a more comprehensive approach that coordinates all your charging sources.

Another situation where professional help makes sense is if you're dealing with a high-output alternator or multiple alternators. Some newer RVs have dual alternator setups or aftermarket high-amp alternators that can put out 200+ amps. The protection schemes for these systems get more complex because the current levels and heat generation are much higher. You might need active cooling, more sophisticated current limiting, or even a complete charging system redesign.

Consider also that there are commercial solutions available that might be more elegant than a DIY temperature switch setup. Companies like Sterling Power, Victron, and Redarc make alternator protection devices specifically designed for lithium battery applications. These units typically include current limiting, temperature monitoring, and smart charging profiles optimized for LiFePO4 batteries with charging voltages typically in the 14.0-14.6V range and controlled current delivery that can extend alternator life while optimizing battery charging. Commercial alternator to battery chargers provide current limiting, multi-stage charging, and built-in alternator protection.

The bottom line is that your temperature sensing approach is fundamentally sound - you're addressing common causes of alternator failure with lithium batteries by monitoring the actual parameter that matters: heat buildup. The refinements I've suggested about maintaining a load path and using current limiting rather than complete disconnection will make your system safer and more effective. If you're comfortable with electrical work and have the time to implement it properly, this could be a cost-effective solution that protects your alternator investment while still getting good charging performance from your lithium battery bank.