Motor & battery requirements for a small cargo rolling bot

Hey

I'm a newbie, planning a hobby project for a small "RC car" like bot that needs to carry weights of up to ~15kg with around 20min operating time between charges.
The bot's work is outside where temperatures could get to 37C (100F)

I've started doing the physics, but I take it that I'd need multipliers for energy loss (friction etc) and maybe gradual battery power loss.

I'd really appreciate hands-on experience of how strong should my motors be, and how much battery capacity I'd need.

(BTW, except for quietness and longevity, is there any other advantage to using a brushless motor in this kind of project?)

Would really appreciate your help

One approach is to design for a maximum slope. Since you say outdoors I assume the ground will not
be perfectly level, and thay there will be gradients at times.

If you pick something like 10% gradient as your max slope, then tan(theta) = 0.1, so 15kG translates to
15N resistance purely due to gravity. Rolling resistance and motor friction are best measured if possible,
and gearing systems have an efficiency < 100%, which you'd have to compensate for. There's no big problem
with erring on the safe side and ending up with more torque than needed, other than motor size.

Given the torque requirement from the above data, and the maximum speed, and the wheel radius, you
can calculate the power needed, and a suitable gear ratio (motors are typically 3000 to 15000 rpm before
any gearing and dependent on size (larger motors rotate slower).

Sometimes acceleration rates are important, that might add to the torque requirement.

Thanks a lot Mark!

Could you please take a look at my newb math?

I'll be using a ~8cm radius wheel
Max velocity of around 1m/sec, so that means motor max RPM is 180(m/min) / 2PI0.08 ~= 130

Weight is ~20N and a typical friction factor is ~0.6(??)
So torque is Weight / 4 wheels * friction = 20 / 4 * 0.6 = 3Nm = 300 N cm = 30 kg cm

Looking at common motors specs the torque number seems high to me - am I doing this right?