I'm using 12V (~40W) car wipers motors for small robotic project. Motors are driven over MOSFET H-bridge (IRF1405/IRF4905) with PWM speed/power regulation. Currently a voltage source for motors is a 3S 11.1V LiPo battery (full charge 12.6V and 9V discharged). So most of the time battery voltage is below 12V (nominal motor voltage).
Now I'm thinking about switching to a 4S 14.8V LiPo (full charge 16.8V and 12.8V discharged) and regulating a voltage on motor with duty cycle of PWM and targeting 13-14V as a 100% power of motors so I could get a little more power and speed in "short and rare" moments when I need it. For the most of the time of robot operation, motors are on 30% duty cycle because robot is typically moving slowly. I'm doubted is this type of voltage regulation safe for motors? Without load, motors can run on 18V without any problems and heating (tested with lab power supply). P.S: I'm not concerned with long-term effects like wearing of bearings because I'm planning to change motors with stronger in month or two.
EDIT: Also, what will be "safe" frequency for PWM in this case?