Check out this document in the SI. Seems 15.5 V is acceptable depending on Charge Mode and SoC of the battery. Applying the brakes may change the state.

Document ID: 2809645 Page 1 of 6 2014 Chevrolet Sonic (US/Canada) [1G1JC5SH1E4203760] | Sonic U.S./Canada... Login to read more.