Solar photovoltaic (PV) modules are ideal for charging batteries. The electronics are minimal, and the costs are low because of it. When a vehicle sits around without being started up for a while, its battery grows weak. If the battery is old, it's even worse. If you have an old truck, car, or RV that doesn't get started very often, a solar charger may be a very cheap and effective way to recharge those neglected batteries.
For $40, you can get a solar battery charger. A PV module plugs in to the cigarette lighter jack, and you lay the module out on the dashboard in the sun. While you're gone, it trickle charges your battery. A solar charger won't overcharge your battery, so you don't have to worry about removing it, even when the vehicle is running. But it probably won't revive a dead battery, although it won't hurt anything to try.
Auto batteries cost around $80. With the $40 solar charger, you get more lifetime, up to 50 percent more, so there's a reasonable payback. But how much is it worth (in peace of mind, not to mention the costs of a tow-truck visit) not to get stranded when you can't get your car started?
Standard, off-the-shelf batteries cost around $0.75 apiece. The cost for 100 throw-away alkaline batteries is $75, plus trips to the store to buy them. And batteries have nasty chemicals, such as lead and acids. You run across an environmental issue with 100 batteries.
Rechargeable batteries cost $4 apiece, and the good ones issue the same charge as a throw-away. A solar charger costs $40, but the charge cycles are free. You can charge a good rechargeable battery over 500 times. (Note that the number 500 applies to devices like remote controls, which don't take much current; if you're drawing a lot of current and running the batteries down to their minimum, expect more on the order of 200 times.)
The cost for a rechargeable battery and 100 charges is $44, which is already a better deal than alkaline. But here's the best part: The cost to charge the next 100 times is zero. And the next, and the next. After 500 free charges, you may need a new battery, but that's only $4.
Alkalines cost about 20 times as much as quality rechargeables to operate. (Cheap rechargeables have much worse payback, so avoid them.) Spend some extra money on quality batteries, and it's a good investment.
The time to charge batteries (most devices charge four at a time) depends on how much direct sunlight you receive. Setting up near a window is often good enough if you don't need a lot of batteries. If you use a lot, you need direct sunlight. Kitchen bay windows are convenient candidates.
Keep a reserve of charged batteries. Buy twice as many as you need; it doesn't cost more because you go twice as long before you need to buy new batteries.