Piper PA-30 pilot reported a complete electrical system failure while in the traffic pattern; necessitating the use of an alternate method of extending the landing gear prior to landing.

Narrative

I recently purchased this aircraft and the aircraft had just completed its annual. Both generators performed satisfactory during the annualWe departed ZZZ for pattern work. After 2 rounds in the pattern we saw flickering of the radio during departure when retracting the landing gear. About 15 seconds later on downwind all electrical power was lost. We tried to recycle the generators and master switch but had no success.Due to the increasing darkness we decided to land rather than spending more time on further troubleshooting.The gear on the aircraft is electrical. To allow time for the alternate extension we continued the pattern and overflew the runway at pattern altitude. Once reaching downwind again we successfully extended the landing gear. The gear light was out but the mechanical sound was sufficient confirmation that the landing gear was down and locked the other pilot also confirmed the nose wheel in the mirror.We visually cleared final and turned base turned base and received a steady green light gun signal. We landed with out flaps slightly faster than normal and vacated the runway without any issues.The root cause is still unknown but the battery was severely depleted when it was checked the next day.The other pilot and I worked well together and exchanged controls to allow trouble shooting and alternate gear extension as necessary. For the landing I took controls since I have more experience in this aircraft. He called out the airspeed since the airspeed indicator was hard to see due to the progressing darkness.