Depth of Discharge (DoD)
Depth of Discharge (DoD) is the percentage of a battery's nominal amp-hour or kilowatt-hour capacity that is discharged during an operating cycle relative to its full capacity—the inverse of State of Charge (SoC)—which directly determines both usable daily energy throughput and the mechanical and electrochemical stress per cycle.
How it is measured and specified
Depth of discharge is expressed either as a percentage of rated nameplate capacity (for example, discharging 90 kWh from a 100 kWh nominal rack equals 90% DoD) or as a bounded State-of-Charge operating window enforced by the Battery Management System (BMS), such as 95% SoC upper charge cutoff down to 5% SoC lower discharge cutoff. At the cell level, the lower DoD boundary corresponds to an end-of-discharge (EOD) terminal voltage threshold—typically 2.50 V per cell for lithium iron phosphate (LFP) chemistry—below which copper current-collector dissolution and irreversible capacity loss accelerate.
Because deeper discharge excursions increase lattice volume change and electrode stress during each cycle, battery manufacturers index their warranted cycle life directly to the configured DoD ceiling. Reducing operational DoD from 100% to 85% or 90% preserves an upper and lower safety buffer while extending total cumulative megawatt-hours delivered over the asset's life.
How it matters when sourcing from China
When comparing quotations from Chinese LFP cabinet and rack assemblers, commercial buyers must distinguish between gross nameplate kilowatt-hours and usable kilowatt-hours at the BMS-enforced DoD limit. A supplier quote calculated per nominal kWh understates the true capital cost per usable kWh if the BMS restricts discharge to 85% or 90% to keep the warranty valid. As outlined in how battery and storage TCO is modelled, usable beginning-of-life capacity and the DoD condition attached to the warranty must be confirmed in writing before purchase.
Across Tradvolt's corridor studies and pre-shipment inspection protocols, DoD directly governs revenue sizing and factory grading:
- In the 100 kWh South African museum battery storage TCO study (referencing 51.2 V 100 Ah rack-mount LFP modules such as BYD Battery-Box LV Flex and Pylontech US5000), a 90% usable DoD limit caps gross daily discharge at 90 kWh/cycle before applying 92% round-trip efficiency, yielding 82.8 kWh of net delivered AC energy per cycle.
- In the 750 kWh EU bottling plant ESS TCO study, the model bounds daily operation at 90% DoD (675 kWh/day gross throughput) while referencing commercial LFP cabinets (such as Sungrow PowerStack ST500CP and BYD Cube Pro) that specify a 90%–95% BMS DoD ceiling.
- During pre-shipment quality control under the battery module EOL grading SOP for the China-to-Spain corridor, 280 Ah LFP modules undergo a full 100% DoD bench test—charging to 3.65 V/cell and discharging at 1C down to the 2.50 V/cell EOD cutoff—to confirm nominal amp-hour capacity before the BMS is reset to shipping state of charge (≤30% SoC).
What to confirm before procurement
- Require the supplier quotation to state both gross nominal capacity (kWh) and usable capacity (kWh) at the recommended BMS DoD setting.
- Check the BMS parameter sheet to confirm the exact upper and lower cell voltage cutoffs (for example, 3.65 V charge cutoff and 2.50 V–2.80 V discharge cutoff for LFP).
- Confirm that the factory ships modules at the reduced transport State of Charge required for international dangerous-goods carriage rather than full operational charge.