How Long Does a Small Mop Robot Run per Charge?

• Cleaning
• Trip to the dock
• Charging at the dock
• Battery-level line
• Return-reserve line

What you will learn

• Split a robot's power into loads and estimate runtime from usable energy.
• Explain why the capacity printed on a battery cannot all be used, and how a robot knows its charge level.
• Work out the reserve that lets the robot reach its dock, then check it with measurements.

Before you start

You only need to know how energy and power differ, for example from the article How Does Battery Capacity Affect Runtime?. The illustrative design uses a finished battery pack from a manufacturer with its own protection circuit. This article does not cover building or charging packs.

How do energy and power give the runtime?

Ideal runtime equals usable energy divided by average power. The U.S. Energy Information Administration defines one watt-hour as the energy of one watt used for one hour and says watts measure power at a specific moment. From those two definitions it follows that if usable energy is E and average power is P, the runtime is E divided by P.

The difficulty is that a mopping robot's P changes with what it is doing. Average power is the sum of each load's power times the fraction of time that load is on. The table below is an illustrative model, not a measurement.

The last two loads are small but never switch off, while the motors and pump are larger but run only part of the time. A robot waiting on the floor therefore still drains its battery, only more slowly than when cleaning.

• Wheel motors
• Water pump
• Microcontroller and radio
• Sensors

Can all the capacity printed on a battery be used?

No: usable energy is smaller than the nominal capacity, and the gap depends on three conditions. The first is depth of discharge. Battery University says the smaller the discharge, the longer the battery lasts. A sensible controller therefore stops cleaning before the battery is empty, and the part below that threshold does not count as usable energy.

The second is temperature. According to a review of lithium-ion batteries at low temperature, reduced conductivity and electrolyte solidification in the cold can make capacity degrade rapidly. The same review says polarization grows and the discharge voltage falls accordingly. For a robot, an early voltage drop can limit its ability to supply a load even while charge remains.

The third is age. The U.S. Department of Energy explains that charging and discharging do not reverse completely, so the chemistry and structure of the materials can change over time and reduce performance. Battery University also lists cycling, elevated temperature and ageing as factors that lower performance.

For an energy budget, take the nominal capacity as a starting point and then measure the usable part under the conditions of the actual home, rather than trusting the number on the label.

• New battery at room temperature
• Usable energy
• Band left unused to avoid deep discharge
• Battery in a cold house
• Battery that has aged

How does the robot know how much charge is left?

A robot estimates its state of charge (SoC) from voltage, by coulomb counting, or by combining both, and each has its own limits. State of charge estimates the charge state; it is not a fixed proportional measure of usable energy. In the illustrative budget, the controller relates it to voltage and usable energy measured under load.

Battery University names two problems with reading voltage. The flat discharge curve of lithium-based batteries makes a voltage-only method impracticable. Right after a charge or discharge the voltage is disturbed and no longer a correct reference. The same source says coulomb counting estimates the state of charge by measuring the current flowing in and out, but it needs periodic calibration. A dataset from CALCE at the University of Maryland adds that one widely used technique infers the battery's open circuit voltage.

Given these limits, the illustrative design counts charge while the robot runs and corrects the count only when the pack’s reference conditions are met and voltage has settled; charging or a brief stop alone is insufficient. This pairing is proposed by the article, not a tested result.

• Voltage line against charge level
• Ripple when the load changes
• Tank counting charge in and out
• Count drifting from the real battery
• Resetting the count at the dock

How much battery must the robot keep to get home?

The robot must keep enough energy for the trip home plus a safety margin, and that threshold changes with the robot's position. The formula in words:

Return reserve = remaining distance to the dock × power while driving home ÷ speed + waiting and docking energy + safety margin.

The distance is the path the robot really drives around furniture, not a straight line. Turning off the pump removes a load, but total return power is not necessarily lower when detouring, crossing thresholds or escaping a jam; the model uses return power measured under adverse conditions. Speed converts distance into time. The safety margin covers charge-estimate error, a cold or old battery and detours when something blocks the way.

The robot heads home when its remaining energy reaches the reserve. Because distances differ, a robot at the far corner must turn back earlier than one near the dock. The controller can provide the route and distance estimate, but the robot must also keep a local return route and reserve threshold so a low battery does not depend on a working network.

The trip home also sets the rhythm of the whole fleet. Useful cleaning time between two dock visits equals the usable energy minus the reserve, divided by the mopping power. The longer the charge takes compared with the cleaning, the more robots sit at the dock at once, and a dock with one bay creates a queue.

Before trusting any figures, measure: run a real robot under a real load until its cut-off threshold, record the time at several temperatures and battery ages. Until then, do not promise hours.

• Dock
• Robot
• Way home
• Energy left for cleaning
• Reserve for the trip home
• Safety margin

Real-world example

This is a design scenario, not a test result. A robot is given the bedroom zone one afternoon and leaves the dock with a full battery. It mops in rows, and the pump follows its speed. The controller compares the remaining energy with the reserve computed from the robot's current position.

When the robot reaches the far edge of the zone, the reserve is at its largest because the way home is longest. The remaining energy touches that threshold, so the robot switches the pump off and turns back even though the zone is not finished. Halfway home something blocks it under a table and it detours. The safety margin absorbs the longer path, and the robot reaches the dock with a little energy left.

The dock charges the robot and the controller hands back the part of the zone still missing. If a cold afternoon makes the voltage sag early, the return threshold comes sooner and the cleaning run gets shorter, but reaching the dock still needs real prototype testing and is not guaranteed by adding a reserve alone.

Common misunderstandings

A bigger battery mops exactly as long as its label says. Usable energy is smaller than nominal capacity because of depth of discharge, temperature and battery age.

The robot heads home when it reports a low battery. The return threshold depends on position: at the same charge, a robot near the dock can keep cleaning while one far away must leave at once.

Reading the voltage is enough to know the charge. The voltage curve is flat and gets disturbed when the load changes, so counting charge and recalibrating at the dock is needed.

Frequently asked questions

How long does a small mopping robot run per charge?

A figure cannot be given before measuring. Runtime is usable energy divided by average power, and both depend on the robot, the floor and the temperature. Measure on a real prototype and only then set a target.

Why does the robot go home while the battery still has charge?

Because it must keep enough energy for the trip home. In a far corner the way back is long, so the return threshold is higher.

Is a robot's battery gauge accurate?

Only as an estimate. Voltage reading and coulomb counting both have limits, so the design needs a safety margin and a recalibration whenever the robot rests at the dock.

Summary

• Runtime is usable energy divided by average power, and power is the sum of the loads times the fraction of time each is on.
• Usable energy is smaller than nominal capacity, and a robot estimates its charge from voltage, coulomb counting or both.
• The return reserve combines distance, power, speed and a safety margin; measure to confirm before promising hours.