If you are stuck somewhere and you have access to a 12V power supply but no regulator, you can use a car headlight to act as a rough regulator. The resistance of a headlight filament (measured with a multimeter) is extremely low. When power is applied the high current heats the filament and its resistance increases dramatically and limits the current to approximately its rated level.
Wire the globe in series with the flat battery and apply the power across them and the globe will light up, limiting the current to the max that the globe will draw. The current through the battery slowly charges it. As the battery voltage rises, the current decreases, causing the filament to cool and glow less brightly, and this reduces the resistance. As the filament cools down, so more and more of the voltage is being applied to the battery, which is itself now limiting the current. When the filament is cold, almost all of the voltage is being applied to the battery. Depending upon the nature of the power supply and the type of battery, you may want to disconnect before it gets to this stage, but the battery will get charged without the current being too excessive. For example, a 60W globe will limit the current to about 5 amps on a nominal 12V supply.

Coop