A B737-300 Captain reported losing battery power in cruise flight. An emergency was declared and a divert to nearest suitable airport was accomplished.

Narrative

We were level at FL330 on leg 2 of first day of a three-day trip. Both DC Fail lights illuminated; and autopilot and heading failed. We referenced the QRH; battery charger circuit breaker (behind First Officer) and switched the battery bus circuit breaker (behind the Captain). Both checked okay and all other circuit breakers checked okay. We checked TR1; 2 and 3. All okay. We checked A/C power; it was okay. We checked the battery; It showed zero (0) Volts and Amps. ZZZ was directly in front of the aircraft about 80 NM and in sight. We decided to divert there and not continue. I briefed the Flight Attendants. I informed the passengers we had a problem and were going to stop to have it checked. The Center was helpful; however; I declared an emergency to receive priority handling. All checklists were accomplished. The visual approach; landing and taxi to gate were uneventful. We called Dispatch; Maintenance Control; and the Chief Pilot on Call. All were debriefed and all questions were answered. We got a new aircraft; and continued to our destination. There were possible altitude errors due to the aircraft malfunctions and high workload of both Pilots.