RV Battery Charging: Underhood Generators vs Engine Idling Guide

The Quick Answer

Based on your reference to NYC's response regarding idling restrictions, you're absolutely right to be concerned about using your main engine for battery charging. Many cities now prohibit extended idling, with fines ranging from $350-$2,000. A dedicated under-hood generator or upgraded alternator system is indeed a better solution than idling your main engine. Modern under-hood generators (which produce 120V AC power that's converted to 12V DC through built-in converter/chargers) or a high-output alternator setup (160-200 amp) will charge your house batteries efficiently without violating anti-idling laws and without the fuel waste of running your main engine.

What's Causing This

The push for under-hood generators and upgraded alternator systems stems from several converging factors. First, anti-idling legislation has become widespread - NYC, California, and many other jurisdictions now heavily fine RV owners for extended idling. Your main engine burns 1-2 gallons per hour just to run the alternator, which is incredibly inefficient compared to a small generator using 0.3-0.5 gallons per hour.

Additionally, modern RVs have significantly higher electrical demands than older units. LED lighting, inverters, residential refrigerators, and multiple device charging require consistent 12V power that your standard 130-amp alternator simply can't maintain during travel stops. The thermal stress of extended idling also damages your main engine - transmission overheating, carbon buildup, and premature wear of seals and gaskets are common problems.

Under-hood generators solve these issues by providing dedicated power generation without legal complications, while upgraded alternator systems (like a 200-amp unit with smart charging) can better handle the increased electrical load during actual driving periods.

Tools You'll Need

WARNING: Working with fuel systems and high-amperage electrical systems can be dangerous. Ensure proper ventilation, disconnect all power sources before working, and wear appropriate safety equipment.

For installing an under-hood generator system, you'll need a comprehensive set of tools. Essential items include a socket set (10mm-19mm), combination wrenches, wire strippers and crimpers, multimeter, torque wrench (30-150 ft-lbs), drill with metal bits, and a reciprocating saw for mounting bracket modifications. You'll also need electrical supplies: 10-12 AWG wire, ring terminals, heat shrink tubing, electrical tape, and wire loom protection.

For alternator upgrades, the tool list is similar but add an alternator pulley puller tool, belt tension gauge, and possibly a serpentine belt routing diagram for your specific chassis. Safety equipment is crucial - jack stands rated for your RV's weight, wheel chocks, and safety glasses. A creeper will make under-hood work much more comfortable.

Specialty items you might need include vibration dampening mounts for the generator, flexible exhaust tubing, and depending on your installation, potentially a hole saw kit for ventilation or exhaust routing. Having a helper for lifting and positioning components is highly recommended.

Step-by-Step Fix

WARNING: Before beginning work, ensure adequate ventilation when working with fuel systems and never smoke or use open flames in the area. Always disconnect battery power before working on electrical systems.

Start by evaluating your current electrical system and determining your actual power needs. Calculate your typical daily amp-hour consumption - this determines whether you need an under-hood generator (typically 2.5kW, 3.2kW, or 4kW) or if a high-output alternator upgrade will suffice. For most Class A motorhomes, budget $3,500-$6,000 for a complete under-hood generator installation, or $800-$1,500 for a quality alternator upgrade.

1. Disconnect the main engine battery first, then the house batteries before starting any work. Ensure the engine is completely cool before proceeding. Remove the engine cover and inspect available mounting space in the engine compartment.
2. For under-hood generators, mount the unit using the manufacturer's brackets, ensuring adequate clearance from heat sources and moving parts. Most under-hood generators require 6 inches minimum clearance on all sides - verify specific requirements for your chosen unit and chassis compatibility.
3. Install the control panel inside your RV - typically near other generator controls. Run the control cable through existing wire passages, using protective loom throughout.
4. Connect the generator's built-in converter/charger output to your house battery bank using appropriately sized wire (typically 4 AWG for 30+ amp charging over short distances - consult voltage drop charts for longer runs). Install an appropriately sized breaker at the battery end (see manufacturer specifications for proper sizing with 4 AWG wire).
5. For exhaust routing, use flexible stainless steel tubing and ensure the outlet directs fumes away from air intakes and windows. Secure all exhaust components with proper clamps torqued to manufacturer specifications.
6. Connect fuel lines to your main tank using the generator manufacturer's fuel pump kit. This typically ties into the fuel return line with a T-fitting (verify your chassis has a return line system - consult manufacturer specifications). Connect the generator to the engine's coolant system as specified by the manufacturer - most under-hood generators require coolant circulation for proper cooling.
7. Test the complete system with a load bank or actual house loads, verifying proper voltage regulation and amperage output.
8. For alternator upgrades, remove the old unit, install the high-output replacement (correct part for your model - verify with dealer), and upgrade the charge line to the house batteries with heavier gauge wire and proper isolation.
9. Complete the installation by securing all wiring with appropriate clamps and loom protection, verify all connections are tight and properly insulated, and perform a final system test under load to ensure proper operation and charging rates.

When to Call a Pro

Call a professional installer if you're uncomfortable working with fuel systems, as improper fuel line connections can create serious safety hazards. The fuel pump and line routing for under-hood generators requires specific knowledge of your chassis fuel system. Similarly, if your RV requires modifications to the engine compartment structure or firewall penetrations, this work is best left to experienced technicians who understand weight distribution and structural integrity.

Electrical work involving high-amperage charging systems should be handled by qualified RV electricians if you're not experienced with 12V systems. Improper wire sizing or connections can create fire hazards. Additionally, warranty considerations are important - having professional installation may be required to maintain your chassis warranty.

Finally, if your RV is still under warranty or if you're dealing with a newer motorhome with complex engine management systems, professional installation ensures compatibility and prevents potential issues with emissions systems or engine control modules. The labor cost of $800-$1,500 for professional installation is often worth the peace of mind and warranty protection.