Relays are an alternative to motor-controllers.
They do consume a little power from your battery when in either on or off position (turned on) - but they also do deliver the full power that you want - where a motor-controller often takes some of the power (voltage-drop).
They can be found to be able to handle much more current than a motor controller matched in size (and price).
They are "slow". That is; You can not do PWM with them (Slow the motor down by just giving it pulses) - but then again you can, if your robot has a little inertia (is moving forward) - then the on/off speed of the relays is way fast enough to slow down / drive half speed etc.
They can be found in a million configurations.
One very common and useful is a standard 5V DPDT relay that can be found anywhere. 2 of those can make a motor reverse / turn on / off.