1. First, Understand What Battery Charging Is
Charging is the process of breaking lithium atoms in the positive electrode into lithium ions, which drift like little boats into the graphite of the negative electrode to "settle in" and store electrical energy.
The electrolyte is like a river channel, through which lithium ions shuttle back and forth.
2. What Happens Below Zero
When the temperature drops, the electrolyte thickens, like frozen syrup or honey, creating huge resistance in the channel; lithium ions can hardly move.
If you still force a charge, the positive electrode keeps sending out lithium, but at the negative electrode the lithium piles up at the entrance and cannot get inside the graphite in time.
3. The Most Critical: Growth of Metallic Lithium Burrs (Lithium Plating)
Lithium that cannot enter the graphite can only accumulate on the negative electrode surface, turning into pure metallic lithium, and will also grow fine, sharp lithium dendrites.
Two disadvantages:
1) Capacity decreases over time This metallic lithium can never turn back into ions to shuttle, directly becoming "dead lithium", leading to permanent loss of battery storage capacity.
2) Risk of fire and explosion Lithium dendrites are like fine needles, piercing the separator that isolates the positive and negative electrodes inside the battery, causing an internal short circuit, instant heating, electrolyte combustion, swelling and fire.
4. Distinguish Between Discharging and Charging (Many people confuse them)
- Below zero discharging is fine: it is lithium in the negative electrode moving to the positive electrode; at most it reduces range, but does not produce metallic lithium;
- Below zero charging is fatal: lithium flows back into the negative electrode, clogs at the entrance and plates out as metallic lithium, causing irreversible damage.
5. Why Electric Vehicles Heat Up First Before Charging in Winter
When the vehicle detects the battery is below 0°C, it first turns on the heater to warm the battery above 5°C before charging, essentially "thawing" the electrolyte, allowing lithium ions to smoothly intercalate into the graphite and avoiding the growth of lithium needles.
One-sentence Summary
Low temperature thickens the electrolyte; during charging, lithium clogs at the negative electrode entrance and turns into metallic spikes, permanently destroying the battery and easily causing short circuits and fires, so never charge below 0°C.
Core Principle: Strictly Prohibited Charging of Lithium Batteries Below 0°C
1. Root Cause: Lithium Plating (Metallic Lithium Precipitation), Permanent Battery Damage
The essence of lithium battery charging: lithium deintercalates from the positive electrode → lithium ions pass through the electrolyte and intercalate into the graphite negative electrode.
The space for lithium in the graphite negative electrode is limited; under low temperature, the activity of lithium ions drops significantly:
- Electrolyte viscosity increases, drastically reducing the movement speed of lithium ions;
- During charging, a large amount of lithium cannot intercalate into the graphite lattice in time, and can only deposit as metallic lithium on the negative electrode surface (lithium plating).
Two Deadly Hazards of Lithium Plating
- Permanent capacity fading The plated metallic lithium can no longer participate in charging/discharging, becoming dead lithium, causing the battery capacity to shrink with each charge.
- Safety hazards (short circuit, swelling, fire) The plated lithium grows dendrites; the sharp dendrites pierce the separator, causing internal micro-short circuits; the heat from short circuits further decomposes the electrolyte, generating gas and swelling, and in severe cases, thermal runaway, fire and explosion.
2. Other Associated Problems at Low Temperature
- Internal resistance sharply increases Low-temperature electrolyte has poor ionic conductivity, causing heat concentration during charging, and local temperature differences exacerbate lithium plating.
- Voltage judgment distortion At low temperatures, open-circuit voltage and charge cut-off voltage shift, making it easy for the BMS to misjudge the state of charge, leading to overcharge risks.
- SEI film damage and thickening Low-temperature lithium plating damages the stable SEI protective film on the negative electrode; each low-temperature charge continuously thickens the SEI, persistently increasing internal resistance and reducing capacity.
3. Temperature Standards for Different Batteries (Industry Common)
- Lithium iron phosphate (EVs / energy storage): Charging recommended ≥0°C, optimal 10–25°C; fast charging prohibited below 0°C, and charging basically not allowed at -10°C.
- Ternary lithium (mobile phones / NEVs): Lower charging limit also 0°C; low-temperature lithium plating is more severe than LFP, and more temperature-sensitive.
- Discharge has no strict 0°C limit: Lithium batteries can discharge at tens of degrees below zero, only capacity is discounted and power decreases, without plating metallic lithium.
4. How Modern NEVs Solve Low-Temperature Charging
The on-board BMS has a built-in preheating system:
- When detecting battery temperature <0°C, first start PTC heating to warm the cells above 5°C before allowing charging;
- During the low-temperature preheating stage, only small current pre-charge is allowed, high-power fast charging is strictly prohibited;
- Old devices without preheating, outdoor energy storage, outdoor lithium power tools will quickly be ruined if directly charged at low temperatures in winter.
Additional Misconception
Many people think it is just "can't charge in", but in reality it is not that it won't charge, but that forcing a low-temperature charge will irreversibly damage the battery. It is not noticeable in the short term, but after a few times, range plummets, and in severe cases the battery is directly ruined.
Simple summary: Charging below zero, lithium ions cannot move, metallic lithium plates out, both ruining the battery and posing a fire risk.