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Originally Posted by Paul_Heuvelhorst
70 watt solar, at max. output will charge the batteries at the rate of 5.8 amps per hour. So, given 12 hours of daylight, the solar panel should be putting approx. 70 amps/per day into your batteries. If your batteries are each rated at 225 amp/hr, you have 450 amp/hrs available, but you are only charging them at the rate of 70 amps per day.
Check the rating of your batteries, and if they are rated at less than 225 amp/hr the charge ratio is better, but still not enough to fully charge them. At the rate your solar panel is charging (at max. output) it would take approx. six and one-half days with NO LOAD to recharge the batteries fully. Please understand, if the daylight is subdued, your solar panel will not operate at its maximum capacity, so the charging rate would be less. Does that make sense?
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Paul,
While I totally agree with most of your analysis, there is one rather large error in your math. Specifically, when batteries are connected in series (as is done with a pair of 6 volt T-105s) , the total amp hours of the pair won't exceed the total amp-hours of the lowest capacity battery since all the current has to flow through both...and when the lowest capacity battery is exhausted, the total voltage drops very rapidly. Therefore Tony's max capacity is 225 amp-hours and his time to recharge fully depleted batteries with a 70 watt panel is around 3 days of full sunshine.
Only if he had a pair of 225 Amp-hour
12 Volt batteries connected in parallel, would he would have 450 amp-hours total.
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Please understand, if the daylight is subdued, your solar panel will not operate at its maximum capacity, so the charging rate would be less. Does that make sense?
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This, I suspect, is the chief problem. Although I have a 2 panels totalling 170 watts on my TM's roof (a 120 and a 50), I've found that unless the entirety of a given panel is in full sunshine, the output drops off to nearly nothing (maybe 1 amp). Even a single branch of a pine tree that shades as little as 10% of a panel is usually enough to almost totally kill output from that panel. Similarly, even thin overcast, much less a full fledged cumulus cloud, will cut output by 50% or more.
Accordingly I count on only having 3 to 5 hours of full, unshaded, unclouded sunshine a day...and I camp in a very sunny state. And I also try to choose campsites that have little to no morning or even mid day shade. The trailer sometimes gets a tad warm but at least I can usually get half a day of full, unobstructed sunshine this way.
The natural tendancy of most folks is to choose a shady campsite and many campgrounds have very shaded campsites. These campsites are virtually useless from a solar generation point of view.