C172 Flight Instructor reported losing electrical power in flight; and all electrical systems were turned off except for the transponder. The flight then diverted to a suitable airport.

Narrative

A CFII; myself; and my IFR student were on a long cross-country from ZZZ1 to ZZZ2 under IFR flight plan. After noticing ammeter discharging during the flight just after passing ZZZ [NAVAID] at 11;000 ft.; we cycled electrical systems and turned off all unnecessary equipment. We immediately tried to notify the issue to the ZZZ1 Approach Control which we were talking to at that time; yet no transmission was successful since the battery was not supplying enough electricity. At this point a complete electrical loss was apparent. We decided to terminate the flight and land as soon as practical. All electrical systems were turned off except transponder squawking 7600.After considering airport options to divert; we decided to divert into ZZZ because of following reasons: it was close enough to make it safely [and a] decent sized airport where we could expect some mechanical help on the ground. Right before entering the ZZZ Class D airspace; we turned on the master and radio once again and attempted to contact ZZZ Tower. One transmission in which we reported our position was successful [and] then the radio was completely gone again. We kept the squawk 7600 and entered the airspace at 1;000 ft. above traffic pattern altitude for safety; and waited for the light gun signal. After one complete circling; the Tower gave us a clear to land" signal; and we made a landing safely."