backchannel: charging the Winnebago Solis

This is a response to Charging the Solis, a user-created doc on the WinnieVans site.

The Solis is a Promaster-based class B. The stock power setup appears to be

  • 2x 105Ah AGM
  • 190w solar panel
  • Xantrex 30A PWM solar charge controller
  • some kind of combiner for charging from alternator1 It may be a latching solenoid.


don’t be fooled by the voltage increase you get after 20-30min of charging as the battery isn’t “rested” and will show a higher voltage for up to 3hrs (and then drop dramatically) after charging for a short amount of time.

This is worded oddly, but generally correct. A charging battery will show an apparent voltage higher than its actual voltage, and that voltage will normalize after reesting for a while. Two complications here:

  • campervan batteries are rarely resting
  • the Solis’ stock house bank is AGM, which has lower internal resistance and therefore resists apparent voltage rise.

Charging Sources and Times

There are many, many variables to Solar.


The general idea is that it will top off the batteries when in full sun. Taking an 85% charged battery to 100% during a full sun day.

It is possible for a well-designed solar setup to handle all charging/load duties under even inclement conditions. The design of the Solis’ setup (which I generally agree with) is alternator + solar where the alternator does the heavy lifting and solar handles higher voltage duties and load maintenance.

The MPPT charge controller should be able to charge the battery much more efficiently than the stock PWM controller.

Sloppy wording. MPPT are actually less efficient (DC-DC conversion losses) but generally make more power.

Solar times are wild guess.

Not if we do some insolation modeleing.

Figuring out Bulk, Absorption and Float rates would be a bit of work.

  • Bulk
    • theoretical 69A (210Ah x 0.33C)
    • real-world ≤61A (50A alternator + 11A PWM (14.2v x 11.21Isc))
  • Absorption - ≤61A tapering to ≤4.2A (210Ah x 0.02C trailing amps)
  • Float - ≤0.5A to offset self-discharge

I beleive the alternator is dumb and just spits out 50amps @ 14.2v all the time and it is up to the battery to accept it or not.

Close. It is a dumb alt that puts out 14.2v. The current is dependent on loads, including house battery acceptance. If the overall system needs 10A to maintain 14.2v it will output 10A. If it needs 120A it will output 120A.

  1. this is what inspired the article. Next campsite over is a fellow with a Solis Pocket with a broken combiner. I wanted to know what/where it was. For now he is using a switch to manually combine the banks.