Mobile Robot Fleets Are Sized by Charging, Not by Robots

The business case for a mobile robot fleet is usually written around the cost of a robot and the cost of the labour it offsets. The number that actually determines the outcome is neither: it is utilisation — the share of operating hours a machine spends doing the task rather than travelling to a charger, waiting for a free charger, charging, or sitting idle because its remaining charge cannot cover the next assignment. That share is a design output, not a specification, and it is routinely overestimated at procurement. The arithmetic is unforgiving in a way that surprises operators. A machine that runs for several hours and then needs a substantial recharge is unavailable for a meaningful fraction of every shift. If charging positions are scarce relative to fleet size, queueing time is added on top and grows non-linearly as the fleet expands. The fleet therefore has to be sized for the number of machines simultaneously available, not the number owned, and the difference between those two numbers is where optimistic business cases go wrong. The engineering responses each trade one constraint for another. Opportunity charging — short top-ups during natural pauses in the work — keeps machines in service but requires charging points distributed where the pauses occur and a scheduler smart enough to use them. Battery swapping restores a machine in minutes but requires a spare battery inventory, a swap station, and the floor space and handling procedures both imply. Faster charging reduces downtime directly but stresses cells, shortening service life and moving the cost into replacement cycles. There is no option that removes the constraint; there are only choices about where to absorb it. The consequence for anyone evaluating a deployment is that the questions worth asking are logistical rather than robotic. How many charging positions, placed where, on what electrical supply, with what scheduling policy under peak demand, and what happens when one station fails during the busiest shift of the year. Vendors quote runtime because runtime is a property of their product. Utilisation is a property of the facility, and it is the one the return depends on.