A solar node that runs without a hiccup from October to April can still fall over in June. Winter rarely brings one big failure. It brings several small problems that all land in the same few hours around dawn: the battery is at its lowest and coldest, the panel is frosted, the enclosure is damp, and the sun, when it arrives, is low and brief. Sizing a node properly in the first place is a separate subject. This article is about what winter does to a design that looked fine on paper.
Short days and a low sun
Around the winter solstice, near 21 June, the days are at their shortest and the sun sits low in the northern sky. The energy a panel collects in a day is a fraction of the summer figure, which is why sizing should always be done for the worst month. The Bureau of Meteorology publishes solar exposure data that gives a realistic picture for a given location, and it is worth checking a design against it rather than against a generic figure.
The low sun also changes shading. A tree, a shed or a ridge that never touches the panel in summer can shade it for hours in winter, because the sun is both lower and further north. Check the site at winter sun angles, on the day or with a sun-path tool, not by eye during a summer installation.
For a node whose limit is winter, tilt the panel more steeply than the site’s latitude. Latitude plus 10 to 15 degrees is a common starting point. It favours the low winter sun at the expense of some summer output, which is usually surplus anyway, and a steeper panel sheds dew, frost and dust sooner.
Cold batteries
Lithium iron phosphate is the usual choice for field nodes, and it has one winter rule that matters more than any other: most manufacturers specify that the cells must not be charged below about 0°C. Charging a frozen cell can plate lithium metal inside it and permanently reduce its capacity. Discharging in the cold is generally allowed, with reduced capacity. The exact limits vary, so read the datasheet for the battery you actually have.
This creates a trap at dawn. The battery is coldest at sunrise, which is exactly when charging would begin. A battery management system or charge controller with low-temperature protection will, correctly, refuse to charge until the cells warm up, and on a frosty morning that can be mid-morning. The node loses the start of an already short solar day. Without that protection the battery charges on schedule and is damaged quietly instead.
A few measures help:
- Make sure something in the system can see the battery temperature. A controller can only protect what it can measure, and a sensor on the controller’s own circuit board is not a sensor on the battery.
- Insulate the battery compartment. It will not generate heat, but it slows the overnight fall and shortens the wait for charging in the morning.
- Consider batteries with built-in heaters, which use incoming solar power to warm the cells before charging starts.
- Log battery temperature alongside state of charge, so you can see how long the morning wait actually is.
Burying a battery box moderates its temperature, and some off-grid systems do it. On a biosecurity-managed site, though, digging is a cost in itself, and the trade-off needs to be weighed. Lead-acid batteries have their own winter issue: they lose usable capacity in the cold, and a deeply discharged one can freeze at temperatures a charged one would tolerate.
Frost on the panel
A frosted panel produces very little until the frost melts, so the first hour or two of sun on a frosty morning can be lost. A steeper tilt helps frost and dew clear sooner, and there is little else to do about it except include those lost hours in the winter energy budget.
Cold has a second, less obvious effect. A solar panel’s voltage rises as its temperature falls, and on a cold, clear morning the open-circuit voltage of a panel or string can exceed its figure at standard test conditions. Check the charge controller’s maximum input voltage against the coldest temperature the site will see, not against the datasheet figure at 25°C. This one is easy to miss when a node is designed in summer.
If the node has a Starlink dish, its heating behaviour is another winter load. The dish can heat itself to melt snow, which draws extra power. Snow is uncommon at most Australian sites, so check the setting in the app and decide whether you need it.
Damp enclosures
Long, cold, still nights make winter the condensation season. Enclosures breathe as they cycle between day and night, and moisture accumulates inside. The measures are the same as at any time of year, but winter is when they get tested: log the humidity inside the box, replace desiccant before the cold months rather than during them, and check gaskets and cable glands, since materials that seal well when warm can seal less well when cold.
Duty cycling interacts with this. A node that sleeps overnight lets its enclosure fall to ambient temperature, then wakes its electronics into air that may be at dew point. Sometimes a small overnight load that keeps the boards a little above the enclosure wall is worth the energy. On a tight winter budget it often is not. Measure before deciding.
Plan for less, not for failure
The aim in winter is for the node to do less when energy is short, in a controlled way, rather than run normally until the battery management system cuts everything off.
- Shed load in stages by state of charge. Full operation above one threshold, reduced camera inference and less frequent uploads below it, logging only below a lower one, and a clean shutdown before the battery protection acts. Give each stage hysteresis so it does not flap at the boundary.
- Keep the always-on controller that manages the power rails running, so it can bring everything back when the sun returns.
- Shut down cleanly. A board that loses power mid-write can corrupt its storage, and a controlled shutdown at a set threshold is far kinder than a hard cut.
- Watch for dawn brownouts. When the battery is nearly flat, a surge such as a modem transmitting or a dish booting can pull the voltage low enough to reset the compute, over and over, consuming the little energy left. A low-voltage disconnect with a reconnect threshold well above the cut-out breaks that loop.
- Move heavy uploads to the middle of the day, when solar input can pay for them, and let the node buffer readings locally in the meantime.
Before winter, not during it
Autumn is the time to prepare. Check shading at winter sun angles. Confirm that battery temperature sensing works and the low-temperature charge settings are right. Compare current consumption with the original budget, because loads creep as cameras are added and firmware changes. Replace desiccant and check seals. Test a staged shutdown and remote restart while there is energy to spare. And if a site visit is needed, do it in autumn: winter tracks are wet, and wet tracks carry more soil, which makes clean entry and exit harder on a managed property.
Where Bizix Agritech fits
We engineer solar-powered field nodes and low-power ARM edge clusters for years of deployment in harsh conditions, and winter is where that design is tested: the energy budget for short days, battery temperature, moisture inside the enclosure and a clean way to recover remotely are the questions we work through before a node goes in the ground. Designed and supported in Australia.