It's quite common that the actual output voltage, 15V, goes higher than the marked one 12V. What we have done is ensure the successful rate of a jump start.
How can it make that:
1) The more the battery power the jump starter has remaining, the higher the output voltage.
2) Although the designed output voltage is rated at a value, it will fluctuate within a specific range.
Please don’t worry about the difference, we have taken multiple safety protections to ensure your safety.