The recommended procedure is always to disconnect the battery. Disconnecting just the negative terminal though should suffice, since there can no longer be a circuit through the car's electrics. (Make sure you know the code for your car's radio!)
You can definitely charge up the battery without disconnecting it. I have done it. As long as you are using a fairly modern and up to date charger. Disconnecting the battery on a modern vehicle causes all sorts of problems. The thing not to do is connecting the charger with the terminals the wrong way round.
Just a thought if the battery is flat , I mean dead flat, it is as well to use a charger that has a setting for that, or the ability to recognise a flat battery automatically.