I was reading the voltage on my truck because its overcharging. I cut it off about 16 volts, i come back to the front and see the battery is charging itself somehow. Whats happening? it went up to 18 volts withing a minute, thats when i disconnected to battery cables

 

Chances are the battery is toast as in fried !!! The voltage regulator circuit is not working It could possibly just be the regulator but they are usually pretty reliable.. To start change the battery and the voltage reg... Do not run it for any length of time just long enough to take voltage measurements