After the two minutes are the motors warm to the touch?
I ask because if you are draining an 18 amp hour battery in two minutes, you're probably averaging a current flow well in excess of 100A. Even if you have four CIMs in your drivetrain, that will definitely warm your motors after two minutes!
To find out more precisely what current you are drawing, try hooking up a voltmeter across your 120A circuit breaker... you'll find in
this thread that Bill_B from 2170 has estimated that there is a voltage drop across the main breaker of roughly 1mV per amp.
If the motors are warming up, or you are observing high current flow through the main breaker, then that is because for some reason your drive train is working the motors paricularly hard... perhaps the gear ratio is off, perhaps there is some binding... perhaps it is because if you don't have at least a little bit of rocker (middle wheels dropped a bit) that the sidways forces when you turn are really, really high and require a lot of torque (and thus current) to overcome.
If your motors aren't warming up, then perhaps you have either a battery issue, or a charger issue. Tools such as the
"Battery Beak" can help you determine your battery's state of charge and internal resistance quite easily. If you read the related documentation you'll see what tests the beak performs and may be able to replicate some of those tests using materials you have on hand.
In my experience a 150 robot powered by four CIMs should get 8-10 minutes of continuous operation with good batteries before showing a real drop off in voltages. You'll get more time ifyou drive gently, less if you flog it hard. After a second battery, or about 15 minutes of driving you may find the motors getting warm enough that you want to rest them for a bit.
Jason