What controls the battery from either cockpit in the T-6?

The battery switch is the correct answer because it is the dedicated control that allows pilots to manage the battery system from either cockpit in the T-6 aircraft. This switch is part of the electrical system, enabling pilots to connect or disconnect the battery, which directly influences the power available to the aircraft's systems and instruments. Pilots can use this switch to ensure that the battery is engaged when needed for starting the engine or operating other electrical systems, and to disconnect it during certain circumstances to prevent battery drain.

In the context of the other choices, the battery charger is not a control mechanism; rather, it serves to recharge the battery when the engine is running. The circuit control panel displays various electrical system states and allows for monitoring but does not serve to manage the battery directly. The power management system coordinates how electrical power is distributed within the aircraft but does not allow for manual control of the battery from the cockpit. Thus, the battery switch is vital for direct interaction with the battery's functionality in the T-6 cockpit environment.