A SOLAR VOLTAGE THIEF?
Q: I am using a West Marine 25W solar panel with a Nature Power charge controller. My batteries are also only a year old and in good shape. Nonetheless, after plugging in the solar, I regularly observe the battery voltage increasing from, say, 12.5 to 12.8 volts during the first 2-3 hours, after which it drops within 15 minutes or so back to 12.5 volts. There are no loads on the battery during that time, and the panel is directed consistently perpendicular to the sun’s rays. This is also happening on a sunny California day. It appears that the solar is “sucking the juice back,” which doesn’t make sense. What is going on? Bo Oppenheim, Marina del Rey, CA
NIGEL CALDER REPLIES
I doubt the solar panel is sucking back juice. The missing piece of information here is the size and type of the battery bank (e.g. wet cells, AGMs, etc.). At 25 watts, the solar panel is quite small. Assuming this is a 12-volt system, at best you will be seeing about 2 amps output. On even a moderately-sized battery bank, unless the batteries are pretty much 100-percent charged, this will not drive up the voltage by very much, hence the modest rise from 12.5 volts to 12.8 volts. The minute the solar output decreases, however (at the end of the day, because of shading of the panels, etc.), the batteries will drop back to their open circuit voltage. My guess is you have wet cell batteries (the type that needs topping up) and that when fully charged have an open-circuit voltage around 12.6 volts (AGMs are higher at 12.8 volts), in which case this is what you are seeing. s