I haven't used one myself, but from what i've read; there's different models available depending how much you want to pay, and where you buy them from. Some of them (the cheaper ones) are just trickle chargers for topping up your main battery, and they're good if you go on holiday and want to keep your battery topped up.
Some of the more expensive ones also include charging circuits which trickle or pulse charge depending on how much current the solar panels are giving. These tend to be for the more expensive panels which can actually provide more power (big panels that provide more than 5W), but buying higher powered panels and proper chargers tends to cost a lot of money.
If you want to install a second battery you'll need to buy some sort of battery management module. Don't plug the second battery into the primary battery's charging circuit as there's a chance that you'll damage the circuit.