Battery Fundamentals¶
Core Battery Concepts¶
Voltage (V)¶
Battery voltage determines compatibility with the inverter system. Common configurations: - 48V nominal — most residential battery systems (LFP: ~51.2V nominal) - Higher voltage — commercial systems, some EG4 configurations
Capacity (Ah and kWh)¶
- Amp-hours (Ah): Charge capacity at a given voltage
- Kilowatt-hours (kWh): Energy capacity = Ah × Voltage / 1000
- Example: 100Ah at 48V = 4.8 kWh total capacity
State of Charge (SOC)¶
- Percentage of available charge remaining (0–100%)
- BMS tracks SOC in real time
- Displayed on inverter, battery monitor, or app
Depth of Discharge (DoD)¶
- How much of the capacity is allowed to be used
- LFP: typically 80–95% DoD allowed (full depth cycling is acceptable)
- Lead-acid: typically 50% DoD maximum (deep discharge causes damage)
C-Rate¶
- Rate of charge/discharge relative to capacity
- 1C = full charge/discharge in 1 hour
- 0.5C = full charge/discharge in 2 hours
- Higher C-rates generate more heat and stress
Battery Pack Architecture¶
- Cell: Individual electrochemical unit (e.g., LFP prismatic cell ~3.2V)
- Module: Multiple cells in series/parallel inside a housing
- String: Multiple modules in series to achieve system voltage
- Cabinet: Full battery system, includes BMS
Charge and Discharge Curves¶
LFP batteries have a very flat discharge curve — voltage stays relatively constant across 20–80% SOC, then drops sharply below 20%. This makes SOC estimation more complex (no simple voltage-SOC correlation across most of the range).
Cycle Life¶
| Chemistry | Typical Cycle Life |
|---|---|
| LFP | 3,000–6,000+ cycles to 80% capacity |
| NMC | 1,000–2,000 cycles |
| Lead-acid | 300–500 cycles |
LFP cycle life at typical residential use (1 cycle/day) = 8–16+ years of useful life.