Cold Weather Charging of Lithium-Ion Batteries: Lessons for Meshtastic Deployments

Ask almost any battery expert and they'll tell you: don't charge lithium-ion batteries below freezing. Cold charging causes lithium plating, dendrite formation, internal resistance buildup, and eventually cell failure. The conventional wisdom is clear.

But here in the Midwest, where winter temperatures routinely drop well below freezing, deploying solar-powered Meshtastic nodes requires either ignoring that advice or giving up on year-round operation. The Meshtastic community has been running this experiment at scale, and the results challenge the conventional wisdom—with important caveats.

Real-World Data from Harsh Climates

The YYCMesh community in Calgary, Alberta has deployed dozens of Meshtastic LoRa nodes over the past two years in conditions that would make most battery engineers wince. Winter temperatures in the Canadian Rockies regularly approach -40°C (-40°F). Their deployments span mountaintop locations above 7,000 feet, rural Alberta installations, and urban nodes throughout Calgary.

Their hardware choices are straightforward: nRF52-based microcontrollers (primarily RAK WisBlock series), standard unprotected 18650 Li-ion cells (3000-3500mAh), and small solar panels ranging from 1W to 6W. Average charging currents stay below 200mA, occasionally peaking around 300mA on optimal days.

The results after two full winters: zero battery failures.

One node's battery—an eFest 3500mAh 18650 cell—was tested after two winters of continuous operation. Remaining capacity measured approximately 3100mAh against a factory rating of ~3300mAh. Internal resistance measured 26 milliohms against a factory spec of 30 milliohms. No signs of degradation or dendrite-related issues.

This isn't isolated luck. Multiple operators across the community report similar experiences with standard lithium-ion cells in sub-freezing conditions.

Why These Batteries Aren't Failing

Several factors appear to work in favor of low-power solar charging:

Low Charge Currents

Most solar-powered Meshtastic setups charge at less than 0.1C, often below 0.05C. Research suggests theoretical "safe" cold charge rates hover around 0.02C. Small solar panels paired with low-power microcontrollers naturally stay within these bounds.

For context: a 3000mAh battery charging at 0.02C receives only 60mA. Even a modest 6W solar panel rarely achieves this in winter conditions—short days, low sun angle, and cloud cover keep actual charge currents well below theoretical maximums.

Thermal Gain from Charging

Even small charging currents generate heat. In enclosed weatherproof cases, this raises battery temperature a few degrees above ambient. It's not much, but every degree helps.

Favorable Charge/Discharge Timing

Solar charging happens during daylight—typically the warmest part of the day. Heavy discharge (radio transmission, MCU activity) happens around the clock but especially during evening and night communications. This natural cycle means charging tends to occur at slightly warmer temperatures than the daily minimum.

Passive Solar Heating

Dark enclosures in direct sunlight can reach significantly higher temperatures than ambient air. A black plastic box on a sunny winter day might be 10-20°C warmer inside than outside, even with freezing air temperatures.

Critical Hardware Choice: nRF52 vs ESP32

All successful cold-weather deployments I'm aware of use nRF52-based boards (RAK4631, RAK19007, etc.). These microcontrollers consume very little power—often under 10mA during normal operation—making them well-suited to small batteries and limited solar input.

ESP32-based Meshtastic devices consume 5-10× more power. This requires larger batteries, higher charge currents, and creates different thermal dynamics that may not be as forgiving in cold conditions.

If you're planning cold-weather deployments, choose nRF52 hardware. The power efficiency alone makes winter operation practical, and the community's positive cold-charging experience is specific to these low-power platforms.

Battery Sizing for Cold Weather

Cold temperatures temporarily reduce battery capacity. A cell that's 30% charged at -30°C may hit its undervoltage protection limit early, shutting down your node even though there's charge remaining. As temperatures rise, that capacity returns.

For reliable cold-weather operation, size your battery with significant reserve capacity. The community consensus suggests 3000mAh as a realistic minimum for harsh winter conditions. Larger is better—more capacity means more buffer against temporary cold-induced capacity loss.

When Standard Lithium-Ion Isn't Enough

For mission-critical nodes in remote locations that can't be serviced for months, some operators use Lithium Titanate (LTO) cells instead of standard lithium-ion.

LTO chemistry offers:

• Safe charging even at -30°C
• 10× longer cycle life than standard Li-ion
• Fast charge capability
• Exceptional durability

The tradeoffs are cost and availability. LTO cells are significantly more expensive and harder to source than standard 18650s. They also have lower energy density, meaning physically larger batteries for equivalent capacity.

For most deployments, standard lithium-ion with appropriate capacity sizing works well. Reserve LTO for locations where a battery failure would mean a difficult or impossible service trip.

The Voltaic Systems multi-chemistry MPPT charger supports LTO cells and is worth considering for high-reliability deployments.[1]

Solar Panel Considerations

Angle Matters

In winter, the sun sits low on the horizon. Steep panel angles (50-70° from horizontal) maximize energy capture during short winter days. In areas with wet or icy snow, steep angles also help snow slide off rather than accumulating.

In dry, powdery snow climates like the Rocky Mountains, snow buildup on panels is less problematic—wind clears it quickly. But panel angle still affects daily energy yield significantly.

Cold Improves Panel Efficiency

Solar panels actually perform better in cold weather. Lower temperatures reduce internal resistance and improve output. Combined with snow-covered ground reflecting additional sunlight onto panels, winter solar harvest can exceed summer in terms of efficiency (though shorter days still mean less total energy).

Practical Deployment Checklist

Hardware Selection:

• [ ] nRF52-based Meshtastic device (RAK4631, RAK19007, etc.)
• [ ] Quality 18650 cells, 3000mAh minimum (3500mAh preferred)
• [ ] Solar panel appropriately sized for your latitude and conditions
• [ ] Weatherproof enclosure (IP65 or better)

Configuration:

• [ ] Set device to Client Mute or Router role to minimize transmit power consumption
• [ ] Configure appropriate hop count for your network topology
• [ ] Enable power-saving features in Meshtastic firmware

Physical Installation:

• [ ] Orient enclosure for maximum solar exposure
• [ ] Angle panel steeply for winter sun and snow shedding
• [ ] Use dark-colored enclosure for passive solar heating
• [ ] Ensure adequate ventilation to prevent summer overheating

Monitoring:

• [ ] Check battery voltage telemetry regularly through the mesh
• [ ] Note seasonal patterns in charge/discharge cycles
• [ ] Plan service visits for early spring when roads are accessible but before summer overgrowth

What We Still Don't Know

This community experience is encouraging, but it's not controlled laboratory research. We don't have:

• Microscopic analysis of cells for dendrite formation
• Large-scale statistical data on failure rates
• Long-term data beyond 2-3 winters
• Controlled comparisons between cold-charged and room-temperature-charged cells

The best indicator available—internal resistance measurement—shows cells remain within spec. But subtle degradation might not appear in resistance measurements until it becomes severe.

The practical takeaway: standard lithium-ion cells appear to tolerate low-current cold charging better than theoretical models suggest, at least for the use case of solar-powered IoT nodes. But treat this as empirical observation, not engineering certainty.

Building a Local Mesh Network

If you're interested in deploying Meshtastic nodes in your area, winter doesn't have to stop you. Choose appropriate hardware, size batteries conservatively, and plan for the conditions.

The goal is a resilient mesh network that operates year-round without constant maintenance. Based on community experience, that's achievable even in harsh winter climates—as long as you work with the physics rather than against them.

Kalamazoo, Michigan sees winter temperatures that occasionally dip below -20°F (-29°C). That's well within the envelope where the YYCMesh community has demonstrated success. A local mesh network here is entirely practical with the right approach.

Footnotes:

[1] Voltaic Systems Multi-Chemistry MPPT Charger: https://www.etsy.com/listing/1609406536/
[2] YYCMesh Cold Weather Charging Research: https://yycmesh.com/blog/cold-weather-charging
[3] Meshtastic Power Configuration: https://meshtastic.org/docs/configuration/radio/power/
[4] Lithium deposition during cold charging (ScienceDirect): https://www.sciencedirect.com/science/article/pii/S2772671125000695
[5] Safe cold charging strategies (ScienceDirect): https://www.sciencedirect.com/science/article/pii/S0378775322015270