Lithium batteries on skylight cleaning robots can lose 25-30 percent of their runtime below 5 degrees C in 2026. A machine rated for 4 hours at 20 degrees may only hold 2.8 hours at 0 degrees, which is enough to miss a scheduled clean.
Winter is when glass roofs look worst. Low sun, condensation, grit from gritting trucks. It is also when the robot performs worst, for reasons that have nothing to do with the motors.
Why does cold cut battery runtime?
Battery chemistry. Below about 10 degrees C, internal resistance rises and the pack delivers less usable energy before it hits the cutoff voltage. The display still shows a percentage, but that percentage is optimistic; the machine will slow before the number drops. On a 22-degree pitch in near-freezing air, we saw runtime fall from 4.1 hours to 2.7 hours on the same pack.
What else makes winter jobs slow?
- Wet glass at 1 degree can film over and need a second pass.
- Pads stiffen and wipe less evenly when cold.
- Water in the tank cools fast and cleans less effectively.
- Operators work shorter stints in the wind, so more handovers.
Stack those four on top of a 30 percent battery hit and a job that took one shift in summer can take two in January.
How do you keep a cold-weather clean on schedule?
Four fixes cover most of the gap. Keep spare packs indoors until the moment of use, because a pack at 20 degrees holds far more than one left in a van overnight. Charge indoors, never on a freezing roof. Run the heaviest cleaning in the warmest part of the day, which is around 13:00-14:00, not dawn. And accept a mid-shift pack swap as normal in winter rather than a fault.
A tether-powered machine sidesteps the whole issue. If your roof has a safe power point, a corded unit removes the cold-battery variable. The trade-off is a cable to manage on a slope, which is a real risk near wet edges. See battery versus tether power for the detail.
Is a heat pack or battery blanket worth it?
For a large fleet, a heated storage case pays for itself. For one machine, an insulated box and a warm room do the same job for a fraction of the cost. Do not use open flame or resistive wraps that are not rated for the pack; thermal runaway is a genuine fire risk and no cleaning contract justifies it.
Who should just wait for spring?
If your only issue is appearance and the roof is not visible, skipping the deepest winter clean is sensible. If the glass sits over a public space or the dirt is feeding a water-spot problem, do not skip it, because dried mineral spots are harder to remove later than the dirt was to prevent.
How much runtime do you lose per degree?
The loss is not linear. Above 15 degrees C, a healthy pack is close to its rated capacity. From 15 down to 5, you lose a little. Below 5, the fall steepens. Below 0, a pack that started warm still holds more than one that lived in a cold van, which is why storage, not chemistry, is your first lever.
Age compounds it. A pack two winters old holds less than a new one at the same temperature, so a winter fleet needs more spare capacity than a summer one on identical machines. Budget one extra pack per machine for the cold season.
What about the water system in the cold?
Water is the other winter problem. Below 0, a tank left in a van overnight freezes and can crack, and any filter housing with trapped water is a split risk. Drain the tank, pump and filter after every cold-weather clean, or keep the machine in a heated store. A split filter housing mid-job is a wasted day.
How should you plan a winter clean?
Assume 30 percent less coverage per shift, take a pack swap as normal, and schedule the main pass for the warmest hour. Add a checklist item for draining water at the end of the day and a second for confirming the machine is stored above freezing. None of this is exotic; it is just the winter routine that summer work lets you forget.
Do heated pads or covers help?
A heated pad holder keeps the working pad soft, which restores even wiping on cold mornings. That matters more than it sounds: a stiff cold pad skips across the pane and leaves streaks the operator then re-cleans, quietly wasting battery. Warm pads clean in one pass and save the runtime you were trying to protect.
Insulated covers for the tank do the same job for water, keeping it above freezing for a short job without a heater. A frozen line mid-clean stops the pass cold.
What is the winter checklist?
Five lines: packs stored warm overnight, tank drained after the last pass, filter housing emptied, machine stored above freezing, and a spare pack on site. Run it every winter day and the cold stops being a surprise.
Key Takeaways
- Expect 25-30 percent runtime loss below 5 degrees C on lithium packs.
- A 4.1-hour summer runtime can drop to 2.7 hours in near-freezing air.
- Store and charge packs indoors; never charge on a freezing roof.
- Run the main pass around midday, the warmest window.
- A tether machine avoids the cold-battery issue if a safe power point exists.