13v when its on steady but when its running it goes up 2-3. When its off its 12vNo. He definitely did not need to upgrade the battery for 850 watts (59 amps). He did it thinking it would improve the voltage. The voltage regulator controls the voltage. On older vehicles the alt controls the voltage with it's internal voltage regulator. On newer vehicles the voltage is externally regulated by an ECU. Depends on the make/model/year. Op has not said what vehicle it is on, and so we don't know how the voltage is regulated yet.
OP has also NOT SAID whether or not that 13v is a constant steady, whether or not the stereo is even on, or whether they were talking about when the bass hits.
All we know is they upgraded the battery. That wasn't going to effect the voltage, and it wasn't needed for 850 watts.