Car Battery + Car battery Charger = 12 V Bench Power Supply?
There is an safety issue that should be mentioned.  Car batteries can put out huge amounts of power.  If you put together a project and you have a dead short somewhere you are going to start lighting things up.  A fuse is a must.

That said, a battery can work if you can't get access to a regular DC power supply.  It shouldn't be recommended though as no current limiting capabilities are provided.  (this is why a fuse is a must)

Another note, batteries will vary in voltage.  Don't be expecting a nice clean 12V supply