The topic has been sort of nibbled around the edges, but to give a fuller explanation for the next person who comes across looking for solar charger info, here we go:
A simple solar panel with NO electronic controls on it whatsoever must be of a very small wattage, say <10w, to avoid damaging a typical lead acid battery over time. As a side effect of that, it cannot effectively 'recharge' a drained battery for a couple of reasons. One reason is that lead acid batteries 'age' and deteriorate very quickly when left in a discharged state (sulfation) so any recharging would preferably be done in a fairly quick manner to avoid taking chunks off the lifespan of the battery. Sulfation is constantly forming in a lead acid battery but one of the ways that a charging setup typically combats that is that it puts enough current into the battery to cause some slight plate growth/deformation through heating, and causes a 'healthy' amount of 'gassing' or bubble formation, and the bubbles themselves gently 'jostle' things on the surface of the plates as they float to the top, as well as causing gradual circulation of the electrolyte which prevents 'stratification' where the more acidic electrolyte settles to the bottom, leaving the upper parts of the battery plates with 'weaker' electrolyte. These minor movements of the plates and electrolyte help prevent young, 'soft' sulfate from hardening into crystalline forms which are basically impossible to 'shake loose' from the plate and block whatever parts of the plate they cover, making the battery 'act' smaller and smaller until it is too weak to do its job and must be replaced.
So a real charger should be able to charge lead acid at a certain minimum level of aggressiveness usually described as ~0.12C or 12% of the amp-hour rating. If you have a 100amp-hour battery (large car battery, typical boat battery) you would ideally charge it at 12amps or more until it was full or nearly full, for example. 12amps X 14volts (what you typically charge a 12v lead acid with) = ~170watts. So you'd need a 100-200w solar panel to be an effective 'charge source' for a drained lead acid.. but then if you just left it hooked up all the time it'd kill the battery in no time!
So the cheapest solar 'maintainers' are simply small enough that they can't overcharge the battery. The battery can dissipate the ~5w of energy as heat without suffering any harm even when it is 100.00% charged.
So be wary of large 'solar maintainers' of 10+w of power.. they may do as much harm as good over the long term. If you want to use a larger solar panel you would really need a 'solar charge controller' to go with it. Some of the nicer solar maintainers may have these electronics built in, but if not you would need to add one separately and set the voltage limits appropriate to your battery so that it would transition from the 'charging' voltage to the 'floating' voltage when the battery was charged, and if the battery didn't need any power it won't give it any!
Basically, more watts isnt necessarily better, and most of the manufacturers/sellers know that the typical buyer doesn't know jack shi*t about lead acid battery charging so they don't even bother to be transparent about what their devices do, electronically. Be skeptical, and on some level a 5w solar panel with no electronic controls is safer than a 20w panel with electronics that do.. indeterminate, unstated things.
