Onan 5500The generator auto-start is a great option to have. When enabled it can automatically start the gen-set whenever the house bank requires recharging and shut it down when the charging is complete. In addition, if so equipped, it can monitor the air conditioning demands and start up if no shore power is present. Again, it will automatically shut-down when the set temperature has been reached.  What a great device! But, it will only operate correctly providing it is configured properly.

So, what exactly has to be set? Well, generally you need to be sure the device is configured to the battery type (Flooded cell, AGM, etc) and bank size (total amp hours). Next you need to set the low voltage start up point.  For that we need to know the voltage fluctuation that will work best for the batteries. That is, at what voltage should the start signal activate?

Well, we know that a deep cell battery will perform best if never discharged below 50% of full charge. The battery manufacturers state that a 12 volt system is at full charge at 12.7 volts or has a specific gravity of 1.277 per cell. The voltage at 50% of charge is 12.10 volts or a specific gravity of 1.172 per cell. So, setting the start-up voltage should be a no brainer! Just set it at the 12.10 volt value.

Onan RemoteWrong! If you adjust the automatic start to intervene at 12.10 volts you will be unhappy will the results. You will find the generator starting constantly while you are occupying your RV. So, you will have to lower the setting of 12.10 volts to more like 11.7 volts.

But, 11.7 volts calculates to about 10% of full charge, far below the target of 50%. The fact is, 11.7 volts will actually translate into about a 12.1 volts of a rested state battery. Your RV draws DC power constantly when not supported by shore power. Things such as an inverter, fridge, TV, lights, etc. all draw power, thereby not representing the actual rested state of the batteries.

Now let’s look at some of the advantages this option can deliver.

  • If you leave your pets in the RV in camp, the auto-start will keep them cool should a power failure occur.
  • If boondocking on a hot night, close the bedroom door and run only the rear air on the auto-start. It will keep you comfortable while only running the generator as needed.
  • If you need to leave your RV for a couple of days the auto-start will look after maintaining the batteries.

Onan also offers a “Plug and Play” auto-start for existing rigs.  Now, just a safety note.  Be sure to deactivate this feature prior to doing any work on the gen-set or if it is going in for service.

Leave a Reply


  1. Peter Mercer

    Yes, the full AGS system monitors both the house battery bank voltage and the AC demand. This has been a feature for many years and works exceptionally well for many applications. Thank you for your participation on this topic.

  2. Anonymous


    How do you set up an AGS to turn on the Air Conditioner based on room temperature?
    Does the AGS connect to the Thermostat?

  3. Peter Mercer

    Glad you found the topic of interest. Hopefully the “Plug & Play” kits are still available as there was talk that they were going to discontinue their sale. Thank you for your comment.

  4. Anonymous

    That is a great option, thanks for the info, i am going to get the kit from a Onan dealer.

  5. Peter Mercer

    Well, yes. If you think the auto-gen start is just to save “Labor”, don’t opt for it. For those however, that fully understand the need of such intervention to maintain batteries in the owner’s absence or to support air conditioning for a pet that is in an RV on a hot day when there is a electric grid issue, it’s great.

  6. Anonymous

    Definitely, because holding down a toggle switch for 10 seconds is more work than any sane person should ever be forced to do, huh?

  7. Anonymous

    I Have

  8. Anonymous

    All Onan add-on auto starters were discontinued