Tuesday, May 29, 2012

Solar output when the Sun doesn't shine

(And while underway)

Yesterday we left Sucia  under Cloudy Skies.  It gave me a little opportunity to log our new Solar Panels output under conditions that were not a Sunny May Day.  We had two deviations:  1) Cloudy Skies, and 2) an interaction between the Solar controller and the main motor alternator.

Here is a chart that shows in Blue the average out of our panels over the past three full-sun days, while the Red shows a portion of yesterdays output.

From approx 8:00am to 11:00am I logged the production while under Cloudy Skies (it was raining).  The 11:35 data point is while we were underway with the mains alternator putting 150A into the batteries.   Note the dramatically reduced output from the Solar panels, even though over the next hour the skies fully cleared.  At 12:40 we stopped (idled) while awaiting a tug and tow to pass before us in  a  VTS traffic lane and you can see the Solar output popped up to 396w – Right in line with the average for full sun production.  Then when we got underway again Solar output dropped to 27W (about 2 amps) while the mains alternator was putting 101A into the batteries.

This VERY LIMTED sample set indicates two things to me:
1) Solar output is reduced to about 1/2 while under cloudy (rain) skies.
2) There is an interaction between the controller and the mains alternator – with the Solar controller in effect shutting down when we are underway.


  1. The solar controller interacts with the alternator in that they both are charging the same battery. The battery has a limited acceptance rate and every charging source with a regulator will respect this and tail off its output as the batteries acceptance rate is reached. Usually the most powerful charging source continues output at near full power (so long as the batteries can accept the amps) while the others will back off. This is why you can independently connect an alternator, solar, wind, and other regulated charging sources to the same battery bank without some sort of "central brain" to control them all. They just play traffic with each other, yielding to the big boss output and to the batteries themselves.

    I don't know if that cleared anything up or not. But its pretty awesome to see those solar panels kicking butt! =)

  2. Tate, You have accurately described the situation once batteries get towards their Acceptance phase - indicated by the system voltage approaching 14.5v (in our case). However, I observed this while still well in the Bulk phase - system voltage was around 13.5v - and as such I would have expected all sources to be supplying full amps in order to 'get the voltage up' :-)

    There is something else going on here I suspect. But it is a new addition to our system so more learning to do.