How an Inverter/Charger Works. There are two modes of operation associated with all Inverter/ Chargers:
Inverter Mode. When you are not plugged into shorepower and not running your generator, the inverter draws 12 VDC power from the “house” battery bank and does its inversion magic to provide 120 VAC power to the electrical appliances (i.e. AC Loads) connected to the inverter’s output.
Comment: Sorry to say so, but contrary to popular belief, when the inverter is working on battery power, it does not run its “charger” side to recharge the batteries. (A “perpetual” machine it is not!)
Standby / Idle Mode. When you are plugged into shorepower or running your generator, the inverter receives its power from the RV’s power distribution panel (120 VAC) -- usually via its own 30 amp circuit breaker -- and only operates as a charger that converts the 120 VAC to 12 VDC and recharges the batteries.
Here’s what’s happening: When the inverter recognizes it is receiving shorepower or generator power, an internal, automatic transfer relay (switch) closes and the power quickly and efficiently passes through the inverter to the attached appliances. No inverting occurs.
Comment: The input 120 VAC is shared between the Charger and the Loads. That means the charger’s amp draw will be part of the maximum allowed 30 Amp flow and could lead to an unexpectedly popped circuit breaker. Remember to factor this into the total amp draw when turning on appliances powered through the inverter.
Remember: Depending on the availability of 120 VAC power, the internal, Automatic Transfer Relay switches between the Inverter Mode and the Standby / Idle Mode. If 120 VAC is present, the unit will only be a 12 VDC battery charger and the incoming 120 VAC power will pass right through to the attached appliances. If the 120 VAC power goes away, the unit instantly becomes an inverter.
Running the Generator while still attached to shorepower. My recommendation is to always disconnect from shorepower before starting the generator. Yes, the automatic transfer switch is expected to take care of this job automatically. However, over the past ten years, I usually replaced 5 to 8 ATS units during the winter months in Florida. Most common cause for failure was "starting the generator while still connected to shorepower." Hmmm...
Hope this helps.
Dale Lee Sumner
RVIA/RVDA Master Certified RV Service Technician