Came across what looks like a real handy battery minder / solar powered charger from Canadian Tire. Coleman brand so that's a good start. this little rig has 3 different plug in options and operates on indirect light as well. It purports to have an auto-shut-off built in to prevent overcharging and charges as the top rate of 2.5 watts. With a 5 year guarantee and costing $19.95 it's worth a look and I'm trying out a few of them. I often park a vehicle outside my place for over a month or so when I'm gone and this should take care of the battery depleting itself while running the alarms etc.
It's also nice to have some of the bikes plugged into it and not have the usual charger and cables going all over the place.
we'll see