I keep the batteries on my riding lawn mower and my DR brush hog fully charged all the time. I purchased one of the inexpensive solar panels for each, available on EBay. They are made to keep a car battery charged, the panels are about 4"x10", and they work great. Just clip the cigarette lighter plug from the wire, noting which wire side is positive.  Extend the wire with more wire & a couple wire nuts.  My wires are up to 30 feet long.  Add a couple alligator clips to the ends of the wires, marking the positive one in red.  Now mount the solar panel on the sunny side of the building or area you store the mower.  My panels are under an eve, so they get some rain, but not too much. Run the wire to the mower, and leave the clips on the battery of the mower (between uses).  Be sure to remove the clips before starting mower.  This only works on a good battery, a bad battery will not hold a charge.  I've never had a battery overcharge, and the fully-charged battery will last longer (save money there). Everything works even after all winter in storage.      P.S. Use fuel stabilizer in the gas in the fall, and your fuel will be fresh in the spring!