If a battery is completely empty (11 volt or less) the internal resistance is very low. Attaching a fully loaded battery may result in a very high current, enough to kill the battery (or both). Potentially dangerous, some batteries just explode!!

The safe way is to use a good charger.
If not available a headlight lamp is series with the battery maximizes the current to +/- 5 amps. Just leave it connected until the battery has at least 12 volts. Using both high and low beam maximizes the current to +/- 10 amps, still safe.

Rob