Quantify discharge behavior with precision. Calculate DoD, remaining state of charge, usable capacity, and energy consumption for any battery bank configuration.
State of Charge (SOC) describes how much energy remains in a battery as a percentage of its total capacity. A battery at 100% SOC is fully charged; at 0% SOC, it is fully discharged.
Depth of Discharge (DoD) is the inverse: the percentage of capacity that has been consumed. A battery discharged from 100% to 30% SOC has experienced a 70% DoD. The two always sum to 100%.
For a bank with multiple batteries in series or parallel, the total energy scales with the number of batteries while the SOC and DoD percentages remain the same for the string as a whole (assuming balanced batteries).
Not all batteries tolerate deep discharge equally. The maximum recommended DoD varies significantly by chemistry, and exceeding it can accelerate capacity loss or cause permanent damage.
| Chemistry | Typical Max DoD | Cycle Life at Max DoD | Notes |
|---|---|---|---|
| Flooded Lead-Acid | 50% | 500–800 cycles | Deep cycling below 50% causes sulfation and plate damage |
| AGM / Gel | 50–60% | 400–600 cycles | Sealed design limits electrolyte recovery from deep discharge |
| LiFePO₄ (LFP) | 80–100% | 3,000–6,000+ cycles | Modern BMS allows deep cycling with minimal degradation |
| NMC Lithium-Ion | 80–90% | 1,000–2,500 cycles | Higher energy density but more sensitive to high DoD than LFP |
Important: Manufacturer datasheets often state maximum DoD for warranty purposes. Regular operation at the extreme edge of the rated window may reduce cycle life even if it does not immediately void the warranty. For daily-cycling applications, operating at 80–90% of the rated maximum DoD often delivers the lowest cost per kilowatt-hour over the battery's life.
Usable capacity is the energy you can actually extract without exceeding the manufacturer's recommended DoD. A 100 Ah battery with a 50% maximum DoD provides only 50 Ah of usable capacity per cycle.
Cycle life follows an inverse relationship with DoD. Shallow discharges yield more total cycles; deep discharges yield fewer. However, the total energy throughput over the battery's life can vary. In some cases, moderate DoD (80% for LFP) produces a lower levelized cost of storage than very shallow cycling because it avoids "stranded capacity" — paying for capacity you never use.
Lead-acid batteries exhibit a steep cycle-life penalty for deep discharge. Moving from 50% to 80% DoD can reduce cycle count by more than half. Lithium iron phosphate (LiFePO₄) batteries degrade far more gradually across the 50–90% DoD range, which is why they are often rated for daily deep cycling in solar storage systems.
The underlying mechanism differs by chemistry. In lead-acid batteries, deep discharge causes lead sulfate crystals to form on the plates. If the battery is not immediately recharged, these crystals harden and reduce the active surface area permanently — a process called sulfation. This is why lead-acid batteries should be recharged promptly after discharge and rarely cycled below 50%.
Lithium-ion batteries do not suffer from sulfation. Their degradation at high DoD is driven by mechanical stress on the electrode materials as lithium ions move in and out. Modern LFP cells are engineered to withstand this stress for thousands of cycles. However, even lithium batteries experience accelerated capacity fade if held at very low SOC (below 10%) for extended periods, as the anode can become unstable.
Temperature compounds these effects. High ambient temperatures accelerate degradation at any DoD level, and the combination of high heat and deep discharge is particularly damaging to lead-acid batteries.
SOC (State of Charge) measures how much energy remains in the battery as a percentage of full capacity. DoD (Depth of Discharge) measures how much energy has been consumed. They are complementary: SOC + DoD = 100%. A battery at 70% SOC has a 30% DoD.
Usable capacity = Nominal capacity × Maximum allowed DoD. For example, a 200 Ah battery with a 50% DoD limit provides 100 Ah of usable capacity per cycle. Always check the manufacturer's recommended DoD for your specific chemistry.
It depends on the specific lithium chemistry and BMS design. Many modern LiFePO₄ batteries are rated for 100% DoD and can handle it routinely with minimal impact. However, NMC lithium-ion batteries and some LFP cells may experience faster capacity fade at 100% DoD. Check your battery's datasheet for the recommended maximum DoD.
Deep discharge in lead-acid batteries causes lead sulfate crystals to form on the plates. If left uncharged, these crystals harden and reduce the active material available for future reactions — a process called sulfation. Keeping lead-acid batteries above 50% SOC and recharging promptly after discharge minimizes this damage.
Voltage correlates with SOC, but the relationship varies by chemistry and load conditions. A 12V LiFePO₄ battery at 20% SOC rests around 12.8V, while a 12V lead-acid battery at 20% SOC rests around 11.6V. Voltage-based SOC estimation is most accurate when the battery has been at rest for several hours with no load or charging current.
For LiFePO₄ residential solar storage, research suggests that 80% DoD often produces the lowest levelized cost of storage (LCOS) because it balances cycle life against per-cycle capacity. Very shallow cycling (e.g., 30% DoD) can leave capacity "stranded" and increase effective cost per kWh delivered. For lead-acid solar banks, 50% DoD remains the standard recommendation.