C172 Flight instructor reported during departure experiencing a burning smell resulting in the flight crew returning to the departure airport where they landed safely.

Narrative

Set out to do my students 2 hour day; 2 hour night for the commercial requirement on Tuesday. We landed at ZZZ without any issues. We taxied to FBO for fuel; flight planning; etc. We picked up flight following and taxied to Runway XX. On takeoff between 500-800 ft agl we started to smell an electrical burning smell. I looked at our EIS and noticed a discharge off our main battery (-10.5) and the battery was showing 24.5 volts. - Should have been at 28 volts. Immediately we opened our windows; turned off the cabin heat; and told tower we were going to come back in and land. I communicated to tower about what we experienced; they cleared us to land on XX. The voltage continued to drop on the landing and taxi back to the FBO.