I have 2 x110 amphr batteries in my van and want to put a 20w panel on the top to keep them charged, I will be in sunnier climes and want to know what are the best type of panels? I know there are various sorts.
Typically, you will want to charge lead acid batteries at a rate of 5% their capacity, or at least 3%. So in your case, you have a 220 AH bank, and would want to charge at 6.6 A minimum, or better, 11 A. The lower figure is about a 100w panel, and the higher is about a 200w.
The 20w panel is good for maintaining batteries that were already charged from your alternator, but will be poor in getting discharged batteries to absorb charge.
As for type of panel, I would look for a crystalline silicon panel, either mono- or polycrystalline. Those flexible or thin-film panels wear out after a couple years, in my experience. I have a dead flexible panel on my roof to prove it, but the crystalline panels are still going strong after years.
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