Calculate battery temperature in winter
Couples two statements that look harmless on their own: how much capacity the cold costs — and on how many days a year the battery at your location is not ALLOWED to charge. Winter use rarely fails on capacity and often on the charge lockout, exactly when the sun finally shines. The comparison works through both ways out: heating versus a bigger battery.
- LiFePO4 must not be charged at -7.6 °C. Lithium plating damages the cells irreversibly. A battery heater or a BMS with low-temperature charge cut-off is required.
LFP_CHARGE_BELOW_FREEZING - On about 41 days a year charging is locked (battery below 0 °C; ambient below 0 °C on 45 days). Exactly then solar yield is at its lowest — that is the real winter problem, not capacity.
CHARGE_LOCK_DAYS
The formulas behind the calculator
Every number above can be recomputed: the full calculation path, all assumptions and the data source with retrieval date — plus cross-validation against independent references. Disclosed, not claimed.
An estimate based on the stated assumptions. The final design must be checked by a qualified professional against the rules that apply where you are.
Data as of: 2026-07-30
| Step | Formula | Value | Provenance |
|---|---|---|---|
| Battery daily minimum (damped) | T_mittel + k_Einbau*(T_min - T_mittel) | -7.5779 °C | assumed |
| Capacity factor at the minimum | f_temp(T_min) aus Kennlinie | 0.73633 | assumed |
| Heater holding energy | Summe UA*(T_ziel - T_min)*24h | 35.14 kWh/a | assumed |
| One-time warm-up | m*c_p*Delta_T/3600 | 52.408 Wh | assumed |
- Formula
T_batt = T_mean + k_install·(T_min − T_mean) · f_temp(T) from the chemistry curve · heating: Σ UA·(T_target − T_batt)·24 h + m·c_p·ΔT/3600- Valid for
- Annual profile from the site TMY temperature series, thermally damped per installation scenario (ranges per Annex B12 — order of magnitude, not a design); curves and charge-lock limits from chemistries.json, pack mass from the comparison dataset’s class figure (switch for datasheet values). Reference case Berlin, outdoors, 100 Ah LiFePO4: 41 charge-lock days (ambient 45 — inertia saves four), battery minimum −7.6 °C. The heater-versus-oversizing comparison marks oversizing as a non-solution to the lock.
- Not covered
- The installation’s specific heat transfer (three scenarios with ranges instead of point values), self-heating from charge/discharge current, solar gain on the case, heater control and efficiency in detail, extreme years below the typical year.
- Data sources
- Manufacturer datasheets (Victron, Fronius, BYD) and IEC 61427-1, aggregated · retrieved 2026-06-15
- JRC Photovoltaic Geographical Information System (PVGIS), European Commission — endpoints tmy, MRcalc, printhorizon · PVGIS API v5_3, solar radiation database PVGIS-SARAH3 · retrieved 2026-07-30
Frequently asked questions
Why is the charge lockout worse than the capacity loss?
Because it is absolute: below the lockout limit (LiFePO4 and NMC: 0 °C cell temperature; many manufacturers lock at +5 °C already) the BMS must not charge at all — lithium plating would damage the cells irreversibly. The capacity loss is gradual by contrast: at −10 °C about 70% remains per the curve. The fatal part is the coupling: the lockout days fall exactly into the period of lowest solar yield — when the winter sun finally shines, the battery may not accept it.
How does the calculator derive battery temperature from air temperature?
Via the thermal inertia of the installation: the battery does not follow the air’s daily minimum but a damped daily cycle — T_batt = daily mean + k·(daily min − daily mean). In an uninsulated outside compartment k ≈ 0.9 (nearly full cool-down), in an insulated box ≈ 0.5, in living space it stays frost-free. This is explicitly an order-of-magnitude estimate per annex B12 of the specification — heat transfer depends entirely on the actual installation.
What does a battery heater achieve, and what does it cost in energy?
It is the only fix that truly solves the charging problem. The energy bill stays modest: holding +5 °C in the Berlin example (uninsulated installation) costs some tens of kWh per year depending on heat transfer — concentrated in the winter months. Many LiFePO4 models ship with heating foils built in, self-controlled from the charge line. Important: the heating energy increases winter demand and thus the solar sizing — the hand-off link passes it to the panel calculator.
Does a bigger battery help against the winter problem?
Only against half of it: more capacity compensates the cold factor (at −10 °C you need about 43% more nameplate capacity for the same usable energy), but it changes nothing about the charge lockout — even the biggest bank may not charge below the limit. Exactly this distinction is the calculator’s core: way B (buy bigger) only acts on the discharge side, way A (heat or install frost-free) solves both problems.
Does this apply to lead-acid batteries too?
Differently: lead has no hard charge lockout — AGM, gel and flooded batteries may be charged in frost (with temperature compensation of the charge voltage). In return the cold factor hits them harder day to day, and deeply discharged flooded batteries can freeze. For an unheated cabin lead is therefore often the more robust choice — the cabin calculator covers exactly that case; for a heatable camper LiFePO4 stays superior.
How reliable are the curve and the thermal model?
Two grades of certainty, both declared: the capacity curves and lockout limits come from the versioned battery dataset (manufacturer aggregate, marked as guide values). The thermal model is deliberately an order-of-magnitude estimate with bands (annexes B12/D2: heat transfer installation-dependent, pack heat capacity 800–1,200 J/(kg·K)) — the calculator says so in a standing notice. For the actual design: put a temperature sensor in the battery compartment.
Where does the pack mass in the thermal calculation come from?
From the class figure of the comparison dataset: mass per amp-hour times capacity — 12.5 kg for 100 Ah LiFePO4. Previously a guessed default of 12 kg sat here. Mass drives thermal inertia: a heavier pack cools more slowly and bridges short frosts better. Your block's datasheet value remains the better number via the switch.
How many lock days does Berlin have exactly — and why is that the core figure?
On about 41 days a year charging is locked in the outdoor scenario (battery below 0 °C; ambient falls below on 45 days — thermal inertia saves four). The figure is central because it coincides with the lowest-yield period: winter operation rarely fails on capacity and often on the battery not being ALLOWED to charge when the sun finally shines.
Heater or bigger battery — what does the comparison say?
Both paths are computed, but only one solves the problem: the heater lifts the charge lock (a per-scenario range in kWh per year), the bigger battery does not — it only stretches how long stored energy lasts while charging remains forbidden. The calculator therefore explicitly marks oversizing as a non-solution to the lock.
How reliable are the installation scenarios?
As an order of magnitude, not as a design — and they are labelled exactly so. Heat transfer depends entirely on the specific installation; the three scenarios (exposed, insulated, indoor) therefore carry ranges from the specification instead of point values. If you need better, log one winter of battery temperature — any Bluetooth BMS records it.
Does the charge lock also apply to lead batteries?
No — lead has no hard lock threshold, but slowed charging and capacity loss in the cold; the curve reflects that. In return, lead tolerates sustained low states of charge worse. The chemistry choice thus changes not only the lock days but the KIND of winter problem — the chemistry comparison puts both side by side.