http://www.northerntool.com/shop/tools/product_200328565_200328565

http://www.northerntool.com/shop/tools/product_200610284_200610284

I have two of the 15 watt chargers to keep batteries on "problem" vehicles charged.  They sit for long periods of time and often needed a jump when I needed them.  With a spreader with 2 group 31 batteries, 15 watts was not enough.  I replaced the 15 watt panel with a cheap 30 watt EBay panel one (even though the instructions said that was too much for the charge controller) and have had no problems since.  They were on sale for 1/2 price when I bought them.