It’s cold, snowing hard, and the battery is discharging with no solar input.

The scenario. No charge source is available: cold weather, no sun, no generator running. The bank discharges until it is empty or the battery management system stops it.

Uptime 24V 160Ah Sodium-ion Battery, solar drop-in configuration. In this configuration the heater runs on incoming charge current and never on stored charge, so with no charge source it does not run at all. The battery discharges at ambient temperature. Capacity retention is interpolated between the two specified points, more than 96% at 32°F (0°C) and more than 85% at -38°F (-39°C). That interpolation is a model assumption. Discharge floor: -38°F (-39°C).

Lithium iron phosphate (LFP) heater. This one is the operator\'s call. Leave it on and the pack stays online below its low-temp cutoff, but the heater draws stored energy alongside the load. Turn it off and the pack shuts down once ambient reaches the cutoff. LFP capacity also falls in the cold, modeled here as roughly 95% at 32°F (0°C) down to roughly 70% at -4°F (-20°C). Heater size is not user-adjustable: it is auto-sized from the battery bank's own case, sized so it can hold 32°F down to -38°F at full output. Case surface area is scaled from a real reference battery (Victron 12.8V/200Ah Smart LiFePO4, 237x321x152mm) using the standard surface-area-to-volume relationship, and heat loss uses 5.5W/(m²·K), the industry-default coefficient for a painted-steel outdoor enclosure. Bigger bank, bigger case, bigger heater, automatically.

Lead-acid. Capacity retention: 100% at 77°F (Victron's rated-capacity test condition), 80% at 32°F, 50% at -20°F — industry-standard reference points, cross-checked against Victron's own AGM/GEL datasheet, which confirms the same sharp-drop shape but does not publish exact figures. Manufacturers do not publish lead-acid performance below -20°F, so figures below that point are extrapolated by continuing the same observed rate of decline down to -38°F, shown here for educational illustration only, not as a spec. Actual loss varies by brand, age, and discharge rate. Discharge stops at 50% state of charge to avoid damaging the battery, so only half of whatever capacity is available at that temperature counts toward usable runtime — the same 50% depth-of-discharge basis used in every Uptime lead-acid comparison. Below -13°F, a warning notes electrolyte freeze risk at that 50% SOC point, per the Lifeline/Concorde AGM Technical Manual (premium AGM; typical AGM would freeze at the same temperature or milder).

Fixed assumptions. Both batteries start at 100% state of charge. The LFP low-temp cutoff is -4°F: below that, without a heater, the lithium battery management system won't allow discharge.

Basis of comparison. One Uptime 24V 160Ah Sodium-ion Battery against a lithium or lead-acid bank of the same usable energy, at the same load, with no charge source on either side.