I'm not much of an expert on solar panel systems,,, but it seems to me that using a 12v panel to charge a 12v battery is not usually how it's done. Unless you're talking about the little solar LED yard lights, that are basically just a solar cell hooked right up to a NIMH battery.... but those are not a normal occurrence, and that only works on a fairly small scale (kinda like running a LED straight off a button cell, with no dropping resistor).
What I've seen is something like this: a solar array that puts out 24 volts--or more--in bright sun, connected to a charge controller that needs at least 18 volts to work, and that in turn is set to charge a 12v battery (to about 14 volts or so).
The practical problem with solar panels is that they put out a very limited amount of current.
When you start drawing current off them, the voltage tends to drop WAY down.
