Depth of Discharge (DoD): The 80% vs 50% Rule
Deep-cycle AGM or Flooded Lead-Acid batteries should not be discharged beyond 50% of their rated capacity. Discharging lead-acid batteries to 80% or 100% causes rapid plate sulfation and slashes their cycle life from ~600 cycles down to fewer than 200 cycles.
In contrast, Lithium Iron Phosphate (LiFePO4) chemistry safely supports 80% to 100% Depth of Discharge without damage. A 100Ah LiFePO4 battery yields 80Ah to 100Ah of usable energy, whereas a 100Ah AGM battery yields only 50Ah of usable energy. You need twice the nominal lead-acid capacity to match a single lithium pack.
Cycle Longevity: 3,500–6,000 Cycles vs 500 Cycles
Quality LiFePO4 cells with a robust Battery Management System (BMS) achieve 3,500 to 6,000 cycles at 80% DoD before falling to 80% of original capacity. For daily off-grid living, 3,500 cycles equates to nearly 10 years of continuous daily deep cycling.
Standard AGM batteries typically last 400 to 600 cycles under gentle 50% cycling, requiring complete replacement every 2 to 3 years. Over a decade, you would replace lead-acid banks 3 to 4 times, multiplying initial battery costs, transport weight, and labor.
Peukert’s Law and Voltage Sag Under Heavy Load
Lead-acid batteries suffer severely from Peukert’s Law: the faster you discharge a lead battery, the lower its total delivered capacity becomes. Pulling 100 Amps to run an induction cooktop or microwave causes instant voltage sag below 11.5V, often tripping low-voltage inverter cutoffs even when the battery is 70% full.
LiFePO4 batteries feature an extraordinarily flat discharge voltage curve, holding 12.8V–13.2V across 90% of their discharge cycle with minimal Peukert degradation.
The Cardinal Rule: Never Charge LiFePO4 Below Freezing (0°C / 32°F)
While LiFePO4 batteries can safely discharge down to -20°C (-4°F), you must never charge LiFePO4 cells when their internal temperature is at or below 0°C (32°F). Forcing charge current into cold lithium cells causes irreversible lithium metal plating on the graphite anode, permanently destroying cell capacity and creating short-circuit fire risks.
Always verify your solar charge controller and battery BMS include a low-temperature charging cutoff sensor, or choose batteries with internal self-heating pads.