Charging below zero: how field stations survive the Mongolian winter

Our weather stations sit along 1,000 km of railway, and they matter most in exactly the months when reaching them is hardest. A station that dies in December may not see a technician until spring. That is why we treat the power system not as a line item on the parts list but as the core of the product: in Mongolia, a field device that cannot manage its own energy through January is not a product at all.

The physics is simple, and it is hard. A lithium battery can discharge below freezing, but it must not be charged there: charging below 0°C causes lithium plating — instead of intercalating into the anode, metallic lithium deposits on its surface. Capacity is lost permanently, and the risk of an internal short grows. Worst of all, the damage is silent: the battery looks fine for weeks, then capacity collapses. And an ordinary charge controller will happily push current into a frozen battery as if nothing were wrong.

The first line of defense is a low-temperature charge cutoff. Our stations measure the temperature of the battery itself and stop charging completely below the threshold. The device keeps running on the charge it has; when the sun warms the enclosure back above the limit, charging resumes on its own. One detail matters here: the sensor must read the battery body, not the ambient air — the pack warms and cools far more slowly than the air around it.

The second is chemistry rated for cold. Cells that safely accept charge below zero exist; they cost more per watt-hour, but on a remote site that premium is smaller than the cost of a single service trip. Batteries that charge below zero are now standard on every field station we build.

The third is sizing solar for December, not for the yearly average. December has the shortest days and the lowest sun. If the panel harvests enough in the worst month, the other eleven take care of themselves. A steep mounting angle helps twice: it faces the low winter sun more directly, and snow slides off instead of accumulating.

Firmware carries the rest of the energy budget. The cellular modem is the hungriest component in the system, so we log measurements locally and send them in batches, keeping transmit windows short. The same mechanism that saves energy also protects the data: if the network disappears, the device buffers up to 30 days locally and back-fills when the connection returns.

The result: 9+ railway stations running at 99.5% uptime through −40°C winters. There is no magic in any of this — a temperature-aware charger, the right chemistry, December-sized solar, and disciplined firmware. The price of getting it wrong is a maintenance run across the steppe in the middle of winter. Every one of these decisions exists to prevent that trip.