C172S pilot reported an electrical malfunction that forced the pilot to reduced the aircraft's electrical load in-flight.

Narrative

I just wanted to describe what had occured at ZZZ this morning between XA-XD am with Aircraft X. Normal procedure; following the checklist to start the engine and perform the necessary run up. Voltage was reading perfectly fine at 28-28.1 Volts. Amps were positive and all systems were green. Started my taxi towards Runway XX with everything still performing normal. During takeoff and full throttle I noticed the voltage started to drop as low as 23.6 Volts. I requested ZZZ Tower for a turn back around and got cleared to land at Runway XY with no necessary declaration of emergency. Landed safely and got set up to try and take off again at Runway XX as the voltage came back to 28.1 Volts at the Hold Short Line. However; as I pulled power to 1000 RPMs while holding short of Runway XX; it immediately dropped to 24 Volts. That indicated to me some sort of abnormality in the system; so I aborted the takeoff all together; and requested Tower to return me back to the FBO at ZZZ and I contacted 2 CFIs at ZZZ1. Via Person A's instructions; I did a run up again and sent him a video of the voltages present at 1800 and 2200 RPMs and based on those results was told to continue the flight because this is fairly normal to see with Aircraft X; especially. I was also informed that if the voltage was to drop even lower then to turn off all Avionics; Master Switch and Unnecessary Lights to preserve battery life enough to get back to ZZZ1. I took off and got up to altitude (3500 MSL). I contacted ZZZ Departure and kept my avionics on until I got transferred to ZZZ approach. Simultaneously to getting transferred to ZZZ Approach; the voltage started to break below 24 Volts and amps were still showing a slight negative indication. This is where I had to inform ZZZ Approach that I will have to turn my Avionics and Lights off until I reach ZZZ2 airport in which I will contact them again and request to transfer to ZZZ3 Tower. When everything was turned off voltage stayed constant between 23.8-24 volts. Once I was near ZZZ2 and within range of ZZZ1; I turned my avionics back on and the voltage dropped to 22.9 volts. I re-established and terminated communication with ZZZ Approach and established communication with ZZZ3 Tower and touched down with the voltage reading 22.7 volts on Runway XZ. I taxiied to parking and shut down the aircraft as normal and reported my flight to all necessary CFIs and other proper authorities.