I have a 45 watt panel and a Battery Tender solar controller that will handle 5-45 watts. I was curious how it charged so I watched the volts and amps as it was working. The panel was making about 35 watts it looks like, and when I plugged in the controller it started outputting at about 2.6 amps. It seems that's all it could make with the panel output available. The voltage slowly climbed and then hovered at about 13.8 volts, with current remaining for a time at around 2.6 amps. After a bit the current started dropping slowly, and the voltage climbing. It seems that as the battery charge increased the Tender was raising voltage to the minimum required to push the maximum current possible given the watts it had to work with and the battery's increasing resistance it was trying to overcome.
It eventually hit 14.4 volts but by this time the sun was setting and the current was well below 2 amps. It was also time to go in and help with some chores so I'm not sure where it went from there. I'm going to check it again sometime and see if it jumps the voltage up for a few minutes of equalizing at the end of the charging cycle, but that'll be another day.
Just thought I'd pass that along to anyone who might be interested. The system is hooked to a pair of 80 Ah batteries and if they have been fully disconnected from all loads it will charge 24 hours worth of "self-discharge" in about 15 minutes of full sun. Most mornings in May and so far in June, when I've been there to look, it was finished and floating by about 10 am. On a rainy day it sometimes never achieves a full state of charge by sunset. My little test was longer than that because I had them connected to base camper load for a day just to get a better feel for the charging algorithm.
I think you mean 13.8 volts not 3.8? Thanks for the report.
Regards, Don My ride is a 28 foot Class C, 256 watts solar, 556 amp-hours of Telcom jars, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.
