A cordless window cleaning robot in 2026 realistically runs 55–70 minutes per charge on a 20–35 m² glass load, against a quoted 90. The gap is pump load, brush drag and low winter temperatures, and it decides whether you need a second battery.
Cordless sounds like the obvious upgrade until you time your actual windows. A conservatory with twelve panes takes longer in total cycle time than one large office glazed wall, and the battery does not care about your assumptions.
Why is real runtime lower than the spec?
Three loads stack up. The vacuum pump is the largest draw, then the drive motors on inclined glass, then the pump that feeds the pad. Add a pad that is dragging over unwashed surface and the current climbs further.
Cold weather makes it worse. A lithium pack at 2 °C delivers roughly 15% less capacity than at 25 °C. Winter window cleaning in northern Europe is exactly when you need runtime and exactly when you get the least.
Battery age compounds it again. After 300 charge cycles — roughly a year of weekly use — expect 80% of the original capacity. Owners rarely re-test, so the decline feels like the robot getting lazy.
| Condition | Claimed runtime | Real runtime | Notes |
|---|---|---|---|
| New battery, warm, smooth glass | 90 min | 70–80 min | Best case, indoor |
| 6 months old, dusty pad | 90 min | 60–70 min | Filter half-loaded |
| Winter, 2 °C, external | 90 min | 50–58 min | Lithium capacity drop |
| Inclined skylight, 35° | 90 min | 45–55 min | Gravity plus seal drag |
When does corded still win?
Corded units are lighter because the pack is outside the chassis. On a tall glazed wall where the machine is on a tether anyway, a cord costs nothing extra — you already ran a line up. The weight saving means less stress on the vacuum seal and fewer slip alarms.
Corded also wins on fixed installations. An atrium roof with a permanent power drop can run a corded machine for six hours straight. A cordless unit in that role spends half its life on a charger.
Cordless wins where there is no socket within reach and where setup time dominates. A single conservatory panel is the classic case. If you can carry the machine up a ladder, you can carry a spare battery and finish in one trip.
How many batteries do you actually need?
Count glass area, divide by 25 m², round up. That is the number of 70-minute sessions. Two sessions means two batteries or one battery plus a 90-minute lunch break.
Buy the second battery at purchase, not later. Packs from three-year-old models are often discontinued, and a machine without a pack is scrap.
Who should avoid cordless entirely?
Anyone cleaning more than 120 m² of glass in one shift should look at corded or a fleet approach. Cordless is for intermittent use, not production cleaning.
Also avoid it on self-cleaning coated glass, where cycle time stretches because you cannot use aggressive brush pressure. The reduced pressure means more passes and more battery.
For a side-by-side of machines that handle the corded-versus-cordless trade-off differently, the Lingyun Y3 review covers power routing on multi-pane runs, and the review archive covers the rest of the field.
What about charging time in the field?
Most packs charge in 90–150 minutes. That is longer than a cycle, so field swapping only works if you own two packs. Charging from a car inverter adds 10–20% because of conversion loss, and cheap inverters cut out under load.
A small inverter plus a second pack costs less than a mid-range machine and doubles daily output. It is the cheapest capacity upgrade available.
Storage temperature matters more than most buyers think. A pack left in an unheated van over winter loses capacity permanently, and no warranty covers that. If the machine lives on a van shelf between jobs, bring the pack inside at night. It is the least glamorous maintenance advice available and the one that saves the most money.
One practical note on numbers: measure your own glass before buying anything. A single conservatory roof at 12 m² becomes 30 m² of cleaned surface once you count the up-and-back passes a cordless machine makes. Multiply your floor area by 2.5 and you have the real load, which is the figure that should drive your battery count.
Plan the job order around the battery, not the rooms. Start with the largest glass area while the pack is fresh and suction is strongest, then work down to small panes. Crews that clean conservatory panels first and big walls last usually end up with a half-cleaned wall and a dying machine, which is the exact opposite of what the day needed.
Keep a written runtime log. Three lines per session — date, glass area, minutes consumed. After ten sessions you have a real capacity curve and can stop guessing when to replace the pack. Very few owners do this, and the ones who do never get surprised by a machine that quits halfway through a job.
One last thing about the charger: use the one in the box. Third-party packs and fast chargers with the wrong cut-off voltage are the fastest way to shorten a lithium pack’s life, and the failure mode is a slow decline nobody notices until a busy week. A branded replacement costs less than one ruined job, and the original charger will outlast the machine if you keep it dry.
Key Takeaways
- Expect 55–70 minutes of real runtime against a 90-minute claim.
- Cold weather costs about 15% capacity; inclined glass costs far more.
- After 300 cycles, batteries hold roughly 80% — replace before performance complaints start.
- Buy the second battery at purchase; packs get discontinued.
- Above 120 m² per shift, corded or fleet beats cordless every time.