Aero Commander 500 Series
Flight Crew Multiengine; Flight Crew Private; Flight Crew Instrument
Flight Crew Last 90 Days 33; Flight Crew Total 4200; Flight Crew Type 3.5
Situational Awareness; Human-Machine Interface
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Provided Assistance; Flight Crew Diverted; Flight Crew Landed in Emergency Condition; General Maintenance Action; General Flight Cancelled / Delayed
Aircraft; Human Factors; Procedure
Lost all electrical power at cruise; just south of ZZZ while IFR on ZZZ1 approach. Turned south and returned to ZZZ2; 3500 ft.Just had new radios installed at ZZZ3 and initially thought that caused the failure.Made approximately 10 cell phone calls to notify ZZZ3 tower. Finally got my wife and she called Company Avionics at ZZZ3. Word did not get to the tower until about the time I landed.Circled the airport and they gave a green light; but the beam was too weak to see. I observed 3 aircraft waiting to depart and figured they were holding for me (which they were) and landed.When on the ground I discovered alternators were turned off.I have 3 other airplanes and the alternators are never turned off.This airplane POH is to turn alternators off at shutdown and back on after cranking.I flew the plane that morning and came back to the radio shop to have programming changed to the way I have it in another plane I own.Alternators were undoubtably turned on and I turned them off; as I thought I was turning them on. Indicator lights are just to the right of panel and I obviously did not notice. Batteries were weak; as I had to be jumped off. I did notice the 3 green for gear were dim when I disengaged the gear lock for take off; but thought the alternator would bring the batteries up.I had iPad; cell phone and ForeFlight for back-up with me.
Pilot reported depleting the aircraft batteries in flight after failing to  turn the alternator switches to On after engine start.