by Brent Peterson
When running the inverter or a large electrical load, it is never a good idea to assume the alternator will charge the domestic batteries through extensive engine idling or during drive time. Exposing the alternator to this burdensome task will likely overheat the alternator system, and result in a costly repair. This is why you should rely on shore power, the generator, or solar panels to supply a charge to the RV house batteries.
A bulk amount of current is withdrawn from the chassis battery when the RV engine is started. The remaining charge left in the chassis battery will depend on the battery’s initial state of charge as well as the amount required to engage the engine. Even when you are fairly certain the chassis battery’s initial state of charge is good, a number of factors, including outdoor weather conditions and engine oil viscosity, may require a high degree of chassis battery power to get the RV engine to crank.
The alternator is designed and installed to replenish the chassis battery while the engine electronics, headlights, windshield wipers, and the dash heater are operating within that battery system. Presuming the RV chassis battery is in proper working condition, maintained and regularly charged, by the time the vehicle reaches its next destination the alternator will likely have done its job, and the chassis batteries will be charged and ready for the next day’s drive.
What Not To Do
If you hope to use the alternator to charge house batteries by allowing the RV to idle for long periods of time, you will probably end up with nothing more than a surface charge and a substantial fuel cost. The alternator is not capable of producing enough power to do this.
Depending on the condition of the chassis battery, as well as drive time, alternator charge may exceed requirements for the chassis batteries and engine electronics. In that instance, the alternator will send the excess charge to the house batteries. However, this charge is minimal and totally inadequate to replenish house batteries in a low state of charge or to assist with heavy AC electrical loads while traveling down the road.
Increased electrical loads will generate heat inside the alternator and excess heat can damage it over a period of time. A fan blade is installed underneath the pulley to cool the alternator system from heat generated during normal operation, but there are some heat demands it can’t cope with. For example, undertaking a long drive with the batteries in a low state of charge will place a larger demand on the alternator. Frequently exposing the alternator to this kind of overwork in an attempt to charge the house batteries may overheat the component and result in damage to the bearings and diodes located inside of the alternator.
What To Do
Protect the alternator from irreversible failure by maintaining the chassis batteries in good working order. Refill the chassis battery plates with distilled water on a regular basis, and clean away corrosion from the chassis battery bank. Taking steps to keep the chassis batteries in working condition will ensure efficient and reliable engine start-up during most outdoor weather conditions.
The voltmeter, located in the dash area, is a quick and easy reference for checking the charge rate of the alternator. When the engine is at idle, the charge rate will read lower. Before checking the dash voltmeter, raise the engine to idle up between 1,000 and 1,200 RPM. Remember this is only a quick reference point. For better accuracy concerning the charge rate of the alternator, determine the DC charge rate at the chassis battery. This requires fully charged chassis batteries and removal of the surface charge. A simple way to remove the surface charge is to turn on the RV headlights, when the engine is off, and leave them on for a minimum of three minutes per chassis battery. Turn the headlights off and test the chassis batteries using a quality handheld voltmeter.
Maintaining the house batteries on a regular basis will eliminate the need for the alternator to send its excessive charge to them. Keep the battery banks clear of corrosion and in a healthy state of charge. Use only distilled water to cover the plates of liquid lead-acid batteries. Get in the habit of frequently checking the house battery control panel and recharge the batteries when they fall below 65% state of charge. If you find it necessary to replace a defective battery, all batteries in that bank should be replaced at the same time to prevent a weakened battery from diminishing the charge of one that is new. The inverter is a good tool for managing large electrical loads, but will quickly drain the house batteries without a reliable back up power sources, such as shore power or generator.
If you desire to run a large electrical load when driving down the road, start the generator for a backup power source. The few gallons of fuel required to operate the generator is minimal compared to the hundreds of dollars needed for a battery and alternator repair.
Occasionally inspect the alternator, always when the engine is off. Pay particular attention to the alternator belt, confirming that tension is snug. Oil splattered around the engine and onto the belt will decrease strength and create a hazard of belt slippage during charge rate. Have the alternator annually inspected by a qualified technician to confirm that all cables and connections are tight and free of corrosion.
The alternator can lend a hand in ‘maintaining’ the RV house batteries, but should never be considered capable of charging those house batteries or running a heavy electrical load. Keep the RV chassis and house batteries in good working order and use the generator to help the house batteries run heavy loads while driving. By employing these practices, you will leave the alternator free to perform the job it is intended to do and avoid the need for an expensive roadside repair.