A degradation mechanism in lithium-ion batteries where lithium metal deposits on the anode surface instead of intercalating normally into the graphite. This occurs during charging at low temperatures (<5°C) and permanently reduces battery capacity. In severe cases, lithium dendrites can grow through the separator and cause an internal short circuit.
Why Winter Storage Matters More Than You Think
A robot mower battery typically costs $150–$400 to replace and has a useful life of 2–5 years. Proper winter storage can extend that life to 4–6 years; improper storage can destroy it in 2 or fewer winters. The difference is worth $300–$800 over the mower's lifetime.
Lithium-ion batteries are electrochemical systems — their health depends on temperature, voltage, and time. During the 4–6 months of winter dormancy in northern climates, the battery is vulnerable to three specific failure modes:
Failure Mode 1: Deep Discharge (Voltage Drop)
Lithium-ion batteries self-discharge at 2–5% per month. A battery stored at 30% in October may reach 0% by February. When cell voltage drops below approximately 2.5V (versus the nominal 3.7V), the copper current collector on the anode begins to dissolve into the electrolyte. This is irreversible. The cell becomes a permanent dead weight — it cannot be safely recharged.
Failure Mode 2: Cold Temperature Degradation
Below 0°C (32°F), the electrolyte viscosity increases, slowing lithium-ion movement. If the mower's built-in charge management attempts to charge the battery in these conditions (e.g., an unexpected warm spell triggers a charging cycle on an outdoor-stored mower), lithium plating occurs. The 5°C (41°F) threshold is the minimum safe storage temperature for lithium-ion cells.
Failure Mode 3: High-SOC Calendar Aging
A battery stored at 100% state of charge (SOC) experiences accelerated electrolyte decomposition at the cathode-electrolyte interface. This "calendar aging" occurs even when the battery is not being used. Over a 5-month winter, a battery stored at 100% SOC will lose 3–5% of its total capacity permanently — compared to less than 1% capacity loss at 50% SOC.
The Optimal Winter Storage Protocol
- Clean the mower thoroughly. Remove all grass clippings, debris, and moisture. Clean the blade disc and charging contacts.
- Charge the battery to 50–60%. If the app does not show exact percentage, charge until the indicator shows approximately half-full.
- Disconnect from the charger. Do not leave the mower on the dock all winter — continuous trickle charging at 100% SOC accelerates calendar aging.
- Store indoors above 5°C (41°F). A garage, basement, or heated shed is ideal. An unheated garage in a northern climate may drop below 0°C during cold snaps — monitor with a thermometer.
- Check monthly. If battery level drops below 30%, charge to 50–60% and disconnect. Do not fully charge.
- Store the base station separately. The base station can stay outdoors (it is weather-rated), but removing it prevents unnecessary power consumption. Store it in a dry location.
Storage Temperature Impact on Battery Lifespan
| Storage Temperature | SOC at 50% | SOC at 100% | Risk Level |
|---|---|---|---|
| 20°C (68°F) — ideal | ~1% capacity loss per year | ~4% capacity loss per year | Minimal |
| 10°C (50°F) — good | <1% capacity loss per year | ~2% capacity loss per year | Low |
| 0°C (32°F) — marginal | ~1% capacity loss per year | ~3% + lithium plating risk if charged | Moderate |
| -10°C (14°F) — dangerous | Electrolyte viscosity increase | High lithium plating risk | High — bring indoors |
| -20°C (-4°F) — destructive | Potential cell damage | Severe degradation | Critical — battery may be destroyed |
Spring Reactivation Checklist
- Charge the battery to 100%.
- Install fresh blades (recommended at the start of each season).
- Place the base station and power it on. Wait for satellite lock (solid green LED).
- Check for firmware updates in the manufacturer app.
- Run a short test mow in the primary zone to verify boundary accuracy.
- Expect slightly reduced runtime on the first 3–5 cycles — this is normal "wake-up" behavior as the battery management system recalibrates.
Frequently Asked Questions
Store at 50–60% state of charge (SOC). This is the voltage range where lithium-ion cells experience the least chemical stress. Storing at 100% accelerates electrolyte decomposition; storing below 20% risks voltage drop below the safe threshold during extended storage.
Not recommended. Temperatures below 0°C (32°F) can damage the battery's electrolyte and separator. Even at 0–5°C, the battery degrades faster than at room temperature. Store the mower indoors (garage, basement, heated shed) where temperatures stay above 5°C (41°F).
Check the battery charge level once per month. Lithium-ion batteries self-discharge at 2–5% per month. If the charge drops below 20%, plug in the charger until it reaches 50–60%, then disconnect. Never leave the charger connected continuously during storage.
A single freeze event is unlikely to cause permanent damage to a partially charged battery. However, repeated freeze-thaw cycles and extended storage below 0°C cause cumulative degradation — reduced capacity, increased internal resistance, and potentially dangerous lithium plating on the anode.
When consistent daytime temperatures exceed 10°C (50°F) and the last frost date has passed. Before the first mow: charge to 100%, update firmware, inspect blades, and run a short test mow. The battery may show reduced capacity on the first few cycles — this is normal and will recover after 3–5 full charge cycles.