British Antarctic Survey: lithium batteries for -30°C field sites

Brief

British Antarctic Survey needed batteries for remote mountain survey equipment: kit that sits alone at field sites for months at a time and charges from a small solar panel.

Requirements

1. The 7Ah unit must stay under 100Wh, so it can travel as airline hand luggage.
2. Ultra-low self-discharge, so the equipment can sit unattended for months.
3. Charge from a 20W solar panel at -30°C.
4. Extra thermal insulation inside the case.

Engineering response

We designed a new 7Ah pack size alongside the 20Ah and wrote a bespoke BMS programme for both.

The critical decision was the heater logic. Lithium cells can't take charge below freezing, so a cold-site battery has to warm itself first, and a heater that draws from the pack can quietly flatten it overnight. We set the heater to run only from the solar input, never from stored energy. When the sun is up, the cells are warmed and then charged. When it isn't, the pack rests.

High-density insulation was fitted inside the case during assembly to hold that heat.