Here`s how to make a good charger for a sealed lead-acid battery (this will NOT work with NiCad batteries) that`s faster (because it allows more current into the battery initially) and safer (because it uses lower voltage when the charging is finished). The battery can be left plugged into this charger indefinitely, and it won`t bother it in the slightest. In fact this is the `float` or `standby` charging method recommended by battery manufacturers.
Below is a circuit based around an L200 chip. (Note to US residents: the L200 is more easily available in Eurpore than in the US. I hear that they are available in the US from BG Micro. If BG has run out, you may consider an equivalent made from an LM317