SAIL - - Boat Works Ask Sail -

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


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

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