With only one gear, I wonder how drivers will be able to control their speed on downgrades without burning up the brakes.
Ding ding ding! School is in. The answer is going to be counter electromotive force. When the motors are switched to be generators for braking, they are loaded by the battery charging system. This will be felt by the motor/generators as a reverse current which will create a magnetic field opposite the field of the motor/generator. That will act against the direction the motor/generator is spinning and slow the truck. Clear as mud?
Almost forgot, the amount of braking required would probably generate too much power and overload the charging system. To fix that there would have to be a large resistor bank that will take the excess current and convert it to heat. It will be an energy loss but regular brakes are a 100% energy loss so it’s whatever.
There are electric dump trucks that are capable of never recharging if they only move loads from high to low.
Their is no transmission and 4 electric motors. You're thinking in a traditional sense. It probably has a motor for each rear wheel. (I'm assuming each rear axle is using one extra wide tire per side instead of the traditional 2) There might be some kind of brake that assists in breaking but the kind of motors they are probably using can effectively act as their own breaks.
What makes electric motors so awesome is they provide maximum torque at all times no matter what rpm. Want to go faster, give it more juice. Need to slow down, reduce power. Back up reverse the flow of power. Want to stop, turn off the power. If you have a well designed controller you can get them to do some awesome things. Think things like near zero radius turns. (One set of wheels spins one way & the other in the opposite direction) It's some cool stuff.
Based on your description, I can see how the electric motors would act as a brake downhill.
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