The RV Forum Community

RVing message boards => Tech Talk => Solar => Topic started by: Frank B on July 13, 2017, 03:29:52 PM

Title: Newbie with FM80 programming questions
Post by: Frank B on July 13, 2017, 03:29:52 PM
Just installed 6 x 205 watt Solartec 72 cell panels on the roof of our RV.  Voc is 45. Vmpp is 39.  All panels are wired parallel with #12 solid to a combiner box, and with #8 over the remaining 20 feet to the solar controller.  Battery bank is 6 x GC2 flooded cell batteries from NAPA that are about a year old.  System is just installed by myself, and the FM80 is at factory default.

As a test, I have been running my aftermarket 2500 watt inverter to keep the trailer powered, and am using only the FM80 to keep the batteries charged.  I deliberately left the propane fridge running on 125V to see what my new system is capable of.  Something must be wrong.

I turned off the PV array to the controller this morning when I saw that the batteries were pretty much depleted, and were showing just under 12v.  I waited until noon, and turned the array on to see what 'maximum performance' would be, as it is bright and sunny in Calgary today. 

The FM80 beings pouring amps into the batteries in bulk mode up until it gets to about 46 amps. It then switches to absorb mode, and input current drops to 40 amps or just below.  This changeover takes less than half a minute.  Should it not run in bulk mode for several hours?

I tried resetting the controller.  Same story.  I reset the factory defaults.  Same story.

I don't understand what it is that is kicking the controller out of bulk mode and into absorb within a few seconds when the battery bank is pretty much depleted.

HELP!   (

Title: Re: Newbie with FM80 programming questions
Post by: Frank B on July 13, 2017, 04:42:30 PM
OK, I may have found the answer by poking around on the web. It appears that the absorb voltage needs to go up from the default 14.4 volts to closer to 15.3 volts for the majority of gc2 batteries on the market. I have requested specific information from the manufacturer of my batteries, but in the meantime I did put the absorb voltage threshold up to 15.3. It is now staying in bulk mode with the voltage sitting around about 14.8. We'll see what that does. :-)

I also set the absorb time up from one hour to two hours. Again, what I've seen on the web is that the absorb time for these batteries can be between two and three hours.

I am now two or three hours past solar noon here in Calgary, but I'm getting about 45 amps. We'll try it again tomorrow, which is also supposed to be sunny, and I'll do it at solar noon to see what the maximum input is.