I have a 20 watt solar panel going on the roof of the bus and would like to know how long I could expect it to take to charge my 75 amp leisure battery. I read a healthy 13.5 volts on my multi meter on a cloudy day but i'm not sure how to read amps being output (assuming that's correct). Will a 20 watt panel always generate 20 watts or will the volts be more of a constant? I know there's the watts/volts = amps (or something) formula but i'm a bit blank beyond that. Help! ???
Given a theoretical max of 22/13.5 = 1.4 amps the real charge rate will be more like 1.25 amps on a sunny day. A 70 amp/hr battery will take around 56 hours of constant daylight to charge up to full from completely flat (if it reads less than 11.4 volts on a meter then consider it fully discharged).
Thanks for that. I should just add I don't plan to charge the battery from flat, i'm just using it to supplement the leisure battery when running my cooler. I have very little else draining the juice at the moment.
Good call - it will keep the battery topped up and in good condition. I assume that you are running a 'proper leisure battery' ie one that says it's a leisure battery? The reason I ask is that they have thicker lead plates so they won't 'slump' and create a short in a cell if fully discharged (unlike a starting battery that has thin plates and will struggle if fully discharged).
