I want to test if the mosfet of the H-bridge that controls a 12v x 100A DC motor gets hot, for this I am thinking of putting weights so that the motor lifts it, but for it to consume 100A I really need to put a lot of weights, it is possible to increase consumption without increasing the weights? I was thinking of reducing the voltage from 12v to 8v but I'm not sure it will increase the current consumption
you could block the rotor, that maximizes the current as there would be no counter emf on the winding, however, you may want to be careful as it may very well overheat even if the motor is rated for that current as, the fan that motors typically have won't be spinning.
an other option is using rheostats to simulate the load.
Adding weights won't really help, the current will be high until it reaches it's operational speed (the period where you are applying most energy) what you would like there is a brake, that dissipates all the energy continuously, instead of accumulating it in the form of kinetic energy
DC Motors have a dynamic resistance that spans typically 100:1 (+/-20%) .
This means locked rotor or start current at full voltage is about 10x rated current and no load is about 10% of rated load current.
So to test your Bridge, it must have the ability to absorb the \$I^2R*t\$ during initial start well within operating temperature without melting the FET junction. This means the FET RdsOn needs to be lower than the motor DCR. How much lower depends on the thermal resistance of each as the current declines during startup so that neither overheat from rapid start/stops.
The last statement hints how you may test the bridge without adding any load. Do this by regulating the speed range ant the toggle frequency of acceleration and braking to maintain the avg power of rated load. This requires current sensing and +/- acceleration control. Consider it like a SMPS continuous current mode (CCM) with low difference between +/- currents at some lower speed that matches full load rated current .
Can you do that?