Lithium Rate of Temperature Rise
How Long To Heat Up To Usable Temperature — Or Cool Down Past It
A lithium (LFP) battery's low-temperature charge protection typically activates around 32°F, but a pack that's already cold-soaked won't accept a charge again until it warms back up to about 41°F. Having a heater isn't the same as being ready to charge. Switch modes below to see either direction: how long a heater takes to bring a cold pack up to usable temperature, or, once warm, how long it takes to cool back down past its -4°F discharge cutoff if the heat source disappears.
Hours To Reach 41°F
Energy Used Getting There (to enable charging)
Battery & Heater
Battery Size
320Ah
= 3,840Wh
Heater Size
100W
Ambient Temperature
-20°F
Heater is fully manual, and assumes unlimited input power (tap to read why)
No manufacturer publishes a heater wattage spec, so there's no real value to mark on the slider — adjust freely to see how heater size changes heat-up time. Assumes unlimited input power: this model assumes your charge source (solar, alternator, shore power, generator) can supply the full heater wattage the whole time. It does not check whether your actual power source can deliver that much — a heater can be sized right for the battery and still starve if the source can't keep up. Note: some real heated LiFePO4 batteries use their internal heater only to enable charging in the cold, powered by incoming charge current — not as a general-purpose warmer while discharging. This tool models a generic heater for either purpose.
How this is calculated
- Treats the battery as a single thermal mass, heated to a 41°F target.
- Mass comes from the interpolated dimensions for your selected battery size, using an average packaged-battery density of 1.4 kg/L (from 5 real 12V LiFePO4 products' case dimensions and weights).
- Case dimensions are sourced from real 12V products and scaled by total energy capacity (Wh), not by voltage. Energy density is roughly the same across 12V, 24V, and 48V packs of the same chemistry, so a 24V or 48V pack of equal Wh is modeled at the same physical size. Real packs at those voltages vary in shape, so treat the 24V and 48V cases as estimates. Above ~5,900Wh (460Ah@12V / 230Ah@24V / 115Ah@48V) there are no measured reference points, and the last known case proportions are extrapolated — treat larger bank sizes as rough estimates.
- Specific heat is 1,050 J/(kg·K), the average of several independent published LiFePO4 cell measurements (1,066–1,130 J/(kg·K)).
- Heat loss uses all 6 exposed faces at 5.5 W/(m²·K), the standard default for a painted-steel outdoor enclosure — the same figure used in Uptime's runtime calculator. Insulation is intentionally excluded as a variable.
- The battery starts cold-soaked at ambient temperature.
- The heater runs continuously at its full rated wattage the whole time, assuming an unlimited power source (no thermostat cycling, no check on whether your actual charge source can keep up).
- Why 41°F, not 32°F: real LFP low-temperature charge protection typically activates around 32°F but doesn't release again until about 41°F (a real published example: 32°F activation / 41°F release for charging, -4°F activation / 1.4°F release for discharge). A pack that's cold-soaked starts in the "protection activated" state, so it needs to clear the higher release threshold, not just the activation point, before it will accept a charge again.
This is an illustrative model, not a spec — actual heat-up time varies by battery construction and installation.
Volume:
Battery mass:
Surface area:
Thermal conductance:
Based on real example products.
Solar Reality Check
Solar Panel Size
W
Winter Peak Sun Hours
"Peak sun hours" is the standard solar rule of thumb: the equivalent number of hours
at full panel output that would deliver the same total energy as an actual winter day.
This check treats that window as if the panel ran continuously at up to its rated
wattage (capped by the heater's own size) for that many hours, then compares against
the heat-up time above. It's a simplification, not a substitute for a real system design.
Lithium Rate of Temperature Rise
How Long To Heat Up To Usable Temperature — Or Cool Down Past It
A lithium (LFP) battery's low-temperature charge protection typically activates around 32°F, but a pack that's already cold-soaked won't accept a charge again until it warms back up to about 41°F. Having a heater isn't the same as being ready to charge. Switch modes below to see either direction: how long a heater takes to bring a cold pack up to usable temperature, or, once warm, how long it takes to cool back down past its -4°F discharge cutoff if the heat source disappears.
Hours To Reach 41°F
Energy Used Getting There (to enable charging)
Battery & Heater
Battery Size
320Ah
= 3,840Wh
Heater Size
100W
Ambient Temperature
-20°F
Heater is fully manual, and assumes unlimited input power (tap to read why)
No manufacturer publishes a heater wattage spec, so there's no real value to mark on the slider — adjust freely to see how heater size changes heat-up time. Assumes unlimited input power: this model assumes your charge source (solar, alternator, shore power, generator) can supply the full heater wattage the whole time. It does not check whether your actual power source can deliver that much — a heater can be sized right for the battery and still starve if the source can't keep up. Note: some real heated LiFePO4 batteries use their internal heater only to enable charging in the cold, powered by incoming charge current — not as a general-purpose warmer while discharging. This tool models a generic heater for either purpose.
How this is calculated
- Treats the battery as a single thermal mass, heated to a 41°F target.
- Mass comes from the interpolated dimensions for your selected battery size, using an average packaged-battery density of 1.4 kg/L (from 5 real 12V LiFePO4 products' case dimensions and weights).
- Case dimensions are sourced from real 12V products and scaled by total energy capacity (Wh), not by voltage. Energy density is roughly the same across 12V, 24V, and 48V packs of the same chemistry, so a 24V or 48V pack of equal Wh is modeled at the same physical size. Real packs at those voltages vary in shape, so treat the 24V and 48V cases as estimates. Above ~5,900Wh (460Ah@12V / 230Ah@24V / 115Ah@48V) there are no measured reference points, and the last known case proportions are extrapolated — treat larger bank sizes as rough estimates.
- Specific heat is 1,050 J/(kg·K), the average of several independent published LiFePO4 cell measurements (1,066–1,130 J/(kg·K)).
- Heat loss uses all 6 exposed faces at 5.5 W/(m²·K), the standard default for a painted-steel outdoor enclosure — the same figure used in Uptime's runtime calculator. Insulation is intentionally excluded as a variable.
- The battery starts cold-soaked at ambient temperature.
- The heater runs continuously at its full rated wattage the whole time, assuming an unlimited power source (no thermostat cycling, no check on whether your actual charge source can keep up).
- Why 41°F, not 32°F: real LFP low-temperature charge protection typically activates around 32°F but doesn't release again until about 41°F (a real published example: 32°F activation / 41°F release for charging, -4°F activation / 1.4°F release for discharge). A pack that's cold-soaked starts in the "protection activated" state, so it needs to clear the higher release threshold, not just the activation point, before it will accept a charge again.
This is an illustrative model, not a spec — actual heat-up time varies by battery construction and installation.
Volume:
Battery mass:
Surface area:
Thermal conductance:
Based on real example products.
Solar Reality Check
Solar Panel Size
W
Winter Peak Sun Hours
"Peak sun hours" is the standard solar rule of thumb: the equivalent number of hours
at full panel output that would deliver the same total energy as an actual winter day.
This check treats that window as if the panel ran continuously at up to its rated
wattage (capped by the heater's own size) for that many hours, then compares against
the heat-up time above. It's a simplification, not a substitute for a real system design.