I've been using 2 different 10w panels to keep batteries charged - one I bought off of Amazon and one I scavenged from an old solar electric fence charger I had for years. I had to purchase 2 charge controllers from Amazon to use these panels and so far all has worked well for several years now. I have 1 panel & controller on my garden fencing with a deep cycle battery and it has worked very well for about 4 years now. The second one I have in a window in my shed and use it to keep my mower & boat batteries charged thru the winter inside for about 6 years now. Both charge controllers were inexpensive - maybe $10 each. Can't remember what I paid for the one 10w panel I bought off Amazon but it wasn't over $20. These 2 controllers say you MUST have a battery attached BEFORE you attach the charge wires from the panel, I just put a disconnect in and make sure the battery leads are attached FIRST like the directions say. They say it will fry the controller if you don't do it this way and I don't want to test this on mine … Anyway, the 10w panels are plenty for keeping batteries up to charge.