I am using two Pololu 100:1 gear ratio, 12 V dc gear motors to build my robot. I am now figuring out the best way to power the motors to keep the weight down. I tried the Pb Acid battery but discarded the idea due to its heavy weight. I have two spare 7.2V 3300mAh 6-Cell NiMH Battery that I am wondering whether I can use in series to power the motors. What I am unsure is the combined voltage will be 14.4 V in this case and I am afraid if that will burn the motors. Can anyone here share their experiences of providing higher voltage to 12 v motors. I am using the L298N controller for the motors.
It sounds like you are talking about our 100:1 Micro Metal Gearmotor HPCB 12V. We generally expect the 12V gearmotors to work fine in the range of 9V to 15V, so a battery that is 14.4V nominally should be okay. In general, supplying a higher voltage that is only a little higher like what you want to do will not outright destroy a motor. Instead, the motor will draw more current, which will wear down its brushes quicker, ultimately reducing the lifetime of the motor. If you are concerned about that, you can use your motor driver to limit your maximum duty cycle to effectively lower the maximum current draw of the motors.
Also, if you want high energy density, you might consider using LiPo batteries (a 3S LiPo would be appropriate for that 12V motor). We do not carry any, but you might try looking at places like HobbyKing.