SnapEnergyLab
Batteries

How temperature changes usable battery capacity

A battery in an unheated garage in January is not the same battery the datasheet describes.

Published 23 August 2026 · Published by SnapEnergyLab. Methodology and default assumptions are documented separately.

A home battery and inverter mounted in an unheated garage with snow visible through the open door

Quick answer

Battery capacity is quoted at a laboratory temperature of about 25 °C. A pack in an unheated garage in January is colder, slower and holds less energy that it can actually deliver — typically 15–25% less around −10 °C for lithium chemistries, and considerably worse for lead-acid.

The loss is mostly reversible: warm the pack and the capacity returns. Charging a cold lithium battery is a different matter, and that damage is not reversible.

How to read the numbersSourced valueIllustrative modelCalculated result
Rated at
25 °C
Sourced value
Typical loss at −10 °C
15–25%
Illustrative model
Charging below 0 °C
Usually blocked
Sourced value

Why cold reduces deliverable energy

A battery’s energy is stored chemically and released by ions moving through an electrolyte. Cold electrolyte is more viscous and ion movement slows, which raises the cell’s internal resistance.

Higher internal resistance has two consequences. Voltage sags further under the same load, so the pack hits its low-voltage cut-off earlier and reports itself empty with charge still inside. And more of the energy that does come out is dissipated as heat inside the cell rather than delivered to your load.

This is why the effect is load-dependent: a cold battery discharged very gently may lose almost nothing, while the same battery under a heavy surge load may cut out at what it thinks is 30% state of charge.

Illustrative available capacity of a lithium iron phosphate pack against cell temperature0%25%50%75%100%-20°0°20°40°Cell temperature (°C)Available capacity (% of rated)
Illustrative model of typical LFP behaviour at a moderate discharge rate, not a measurement of any specific product. Manufacturer derating curves differ, and the loss is larger at higher discharge currents. Values are marked as an illustrative model deliberately: no single curve fits all cells.
Show the plotted values
Cell temperatureAvailable capacity
-20°70%
-10°80%
0°88%
10°95%
20°100%
25°100%
35°100%
45°98%

Charging in the cold is the real hazard

Discharging a cold lithium battery costs you capacity temporarily. Charging one below roughly 0 °C can plate metallic lithium onto the anode, which permanently reduces capacity and, over time, creates an internal short-circuit risk.

For this reason most quality battery management systems simply refuse to accept charge below a threshold, and better home and vehicle batteries include self-heating that warms the pack before charging. If your system reports “not charging” on a cold morning while the sun is up, this is usually the reason — and it is the BMS doing its job.

Lead-acid tolerates cold charging, but its charge voltage should be temperature-compensated: charging a cold lead-acid battery at a warm-weather voltage undercharges it and accelerates sulphation.

  1. cold-weather usable energy (kWh) = nominal capacity × DoD × inverter efficiency × temperature factor
  3. example at −10 °C:
  4. 10 kWh × 0.90 × 0.92 × 0.82 = 6.79 kWh

The temperature factor is a derating you take from your own battery's datasheet. The 0.82 above is an illustrative value, not a specification.

Worked example: a garage battery in winter

  1. Nominal capacity10 kWhYour input
  2. Depth of discharge90%Site default
  3. Inverter efficiency92%Site default
  4. Usable at 25 °C8.28 kWhCalculated result
  5. Temperature factor at −10 °C0.82Illustrative model
  6. Usable at −10 °C8.28 × 0.82 = 6.79 kWhCalculated result
  7. At a 300 W essentials load6.79 ÷ 0.30 = 22.6 hCalculated result
Runtime lost to cold≈ 5 hours of 27.6

Practical steps

Site the pack indoors where possible. A utility room or heated basement removes the problem entirely and costs nothing.

Insulate, do not seal. Battery enclosures need ventilation; an insulated but ventilated cabinet keeps self-heating from normal operation inside.

Check for a heating function. Many outdoor-rated home batteries and EVs draw a small amount of energy to pre-warm before charging. That energy is real and appears in your consumption.

Do not fight the BMS. A refusal to charge cold is protection, not a fault.

Apply your own derating

Reduce the capacity input by your datasheet’s cold-weather factor and compare the runtime against the same system at room temperature. The difference is what a winter outage actually costs you.

Open the battery runtime calculator

Assumptions and limitations

  • The capacity curve is an illustrative model of typical LFP behaviour, not measured data for a specific battery. Use your manufacturer's derating table where one exists.
  • Capacity loss depends strongly on discharge current: gentle loads lose far less than heavy or surge loads at the same temperature.
  • Cell temperature is not air temperature. A pack that has been working generates its own heat and may be well above ambient; a pack that has sat idle overnight will not be.
  • Lead-acid chemistries lose considerably more capacity in cold than the lithium figures shown here.

Sources

  1. IEC 62620 — Secondary lithium cells and batteries for industrial applications — International Electrotechnical Commission

    Supports: Rated capacity is declared at a specified test temperature, conventionally around 25 °C.

  2. Battery test manual for stationary energy storage — U.S. Department of Energy / Idaho National Laboratory

    Supports: Deliverable capacity of lithium-ion cells decreases at low temperature and recovers on warming.

Related guides