D-Cube Energy Storage System Charge/Discharge and SOH/SOC Standard Definitions

D-Cube Energy Storage System Charge/Discharge and SOH/SOC Standard Definitions

Scope of application: Three typical specifications of the Hoenergy D-Cube series: 261 / 418 / 5016 kWh

1 Typical Model Rated Parameters

Note: Nominal capacity is calculated by $S(rated) = E(rated) / U(avg,cell)$, with the average cell working voltage taken as the typical LFP value of 3.2 V. For detailed electrical configuration (number of packs, cells in series, series-parallel topology), refer to the technical specification of each model.

2 Lithium-ion Cell Charge/Discharge Cycle

Complete definition: First discharge to 2.9 V, then charge to 3.65 V by constant current, then discharge again to 2.9 V—this complete set counts as 1 cycle.

Key elements:

• Cut-off condition: cell voltage (not system SOC)
• Discharge cut-off voltage: 2.9 V (lower limit of the D-Cube working voltage window)
• Charge cut-off voltage: 3.65 V
• Charging method: Constant Current (CC)
• Cell DoD: 100% (relative to the cell voltage range)

Boundary with the cell specification: The 2.9 V–3.65 V given in this section is the working voltage window mandatorily enforced by the D-Cube system BMS; the safe working voltage of the cell marked in the cell specification is typically 2.5 V–3.65 V (manufacturer specification). The wider voltage range is an engineering margin reserved for limit testing and will not be reached during system operation.

3 D-Cube System-Level Cycle

Complete definition: The system discharges from 100% SOC to 5% SOC, then charges back to 100% SOC, counting as 1 system cycle.

Key elements:

• Cut-off condition: System SOC (not cell voltage)
• System discharge cut-off SOC: 5%
• System charge cut-off SOC: 100%
• Actual usable energy = 95% × nominal energy
• System DoD: 95% (relative to the usable energy range)

3.1 Cell Cycle vs. System Cycle Comparison

4 Calculating SOH by the Ampere-hour Integration Method

The ampere-hour integration method calculates the charged or discharged energy by integrating the battery charge/discharge current over time, thereby deriving the remaining capacity (SOC) or evaluating the battery health (SOH).

4.1 Core Formulas

Real-time SOC calculation:

$$SOC(t) = SOC(t_0) + \frac{1}{C_{rated}} \times \int_{t_0}^{t} I(\tau) \, d\tau$$

Where:

• I(τ) = instantaneous current (positive for charging, negative for discharging)
• C(rated) = rated battery capacity (Ah)
• SOC(t₀) = initial SOC value

SoH calculation:

$$SOH(t) = \frac{\text{actual usable capacity}(t)}{S_{rated}}$$

The actual usable capacity is estimated by the BMS through long-term accumulation of charged/discharged Ah—e.g., when cumulative discharged Ah reaches S(rated), it counts as one full discharge, corrected in combination with the full-charge auto-calibration points.

4.2 Working Principle

Charging process: The current sensor measures the charging current (positive value) in real time, and the BMS accumulates ∫I dt at a millisecond-level sampling rate to obtain the "charged Ah", from which the SOC rise is derived.

Discharging process: The current sensor measures the discharging current (negative value) in real time, and the BMS continues to accumulate in the opposite direction to obtain the "discharged Ah", from which the SOC drop is derived.

Example: 261 kWh cabinet (S(rated) = 81,640 Ah)

• Initial SOC = 50%, i.e., 40,820 Ah already charged in
• Discharging at 314 A constant current for 1 hour: 314 Ah discharged
• New SOC = (40,820 − 314) / 81,640 = 49.62%

6 SOC Calibration Method

SOC (State of Charge) is automatically calibrated by the BMS to 100% when the system is fully charged.

Working principle:

1. Each time the system is charged to the full state (meeting the charge termination condition), the BMS automatically resets SOC to 100%
2. Full-charge determination basis: cell voltage reaches 3.65 V and is sustained for a certain period, or the charging current drops below the set threshold (CC-CV standard termination logic)
3. During calibration, the BMS tracks the SOC value in real time through ampere-hour integration
4. Fully automatic execution, no manual intervention required

SOC Calibration vs. SoH Calibration Comparison

7 BMS Cycle Recording and EFC Counting Convention

Operation-phase cycle definition (1 EFC, Equivalent Full Cycle):

1. System charges to SoH × S(rated)
2. Then discharges to 0.95 × SoH × S(rated)

EFC counting rule: Each such cycle counts as 1 EFC. The contractual total throughput energy commitment is calculated as follows:

$$E(total) = EFC\ count \times S(rated) \times DoD$$

Data flow: BMS (local cycle counting) → local EMS (aggregation, storage) → upper-level customer EMS (cloud display, reports)

Factory SAT Cycle vs. Operation-Phase Cycle

8 Equivalent EFC Cycle

EFC means adding up all the charge and discharge of the battery to see how many "full charges" worth of energy has been discharged in total. It is like calculating how many "full cups of water" you have drunk in total—no matter whether you drink half a cup or one-third of a cup each time, it is converted into "full cups".

8.1 Why EFC Is Needed

• In actual use, the battery does not always discharge from 0% to 100%. It might discharge 30% this time, 50% next time, and 80% the time after.

• Counted by "number of times": 3 times.

• But the total actual work done by the battery = 30% + 50% + 80% = 160% = 1.6 EFC.

• EFC more truly reflects how much "fatigue" the battery has actually accumulated.

8.2 An Example

D-Cube-261D battery cabinet, capacity counted as 1 full cabinet:

As shown, although the third operating condition has many "charge/discharge events", each one is actually very shallow and causes little damage to the battery, so the equivalent EFC is reasonable.

8.3 How to Calculate Lifetime

The commitment of "8000 EFC or 10 years" means:

• The battery reaches end of life when it has discharged 8000 "full-cabinet equivalents"
• Or it reaches end of life after running for 10 years
• Whichever comes first

If the battery is used shallowly every day (0.5 EFC per day), it can theoretically run for 8000 ÷ 0.5 ÷ 365 ≈ 44 years. If the battery is used deeply every day (2 EFC per day), it can only run for 8000 ÷ 2 ÷ 365 ≈ 11 years. Actual operating conditions determine the real lifetime.

9 Key Parameter Quick Reference

10 Typical Model Cycle Cut-off Capacity Comparison (SoH = 100% Baseline)