About the Charge/Discharge Current
Charge-discharge current calculation determines the maximum current flowing through BESS cables, busbars, and protection devices during charging and discharging — essential for sizing conductors, fuses, and circuit breakers, and for verifying that the battery's C-rate limits are not exceeded.
Formula
Discharge current (A) = PCS rated power (W) ÷ DC bus voltage (V)
- PCS rated power (W)
- Battery inverter rated discharge power
- DC bus voltage (V)
- Nominal DC bus voltage of the battery string (V)
How to calculate it
- 1Determine the DC bus voltage from the battery string configuration (series cells × nominal cell voltage).
- 2Calculate discharge current = PCS power ÷ DC bus voltage.
- 3Verify against battery C-rate: current ÷ battery Ah capacity = C-rate. Must be ≤ manufacturer's maximum C-rate.
- 4Size DC cables for 1.25× calculated current (NEC/IE Rules continuous current derating).
- 5Size fuses and circuit breakers for 1.25–1.5× calculated current.
Worked example
A 500 kWh LFP BESS at 768 V DC nominal (16S × 48 V racks). PCS rated power: 250 kW. Discharge current = 250,000 ÷ 768 = 325.5 A. Battery capacity in Ah = 500,000 Wh ÷ 768 V = 651 Ah. C-rate = 325.5 ÷ 651 = 0.5C — within LFP's 1C continuous rating. Cable sizing: 325.5 × 1.25 = 407 A continuous → 300 mm² copper cable or equivalent busbars.
What you can calculate
- Max current A
- C-rate
- DC bus voltage
- BMS limits
Frequently asked questions
What is a safe maximum C-rate for LFP batteries?
LFP cells from major manufacturers (CATL, BYD, Eve, Gotion) support 1C continuous discharge and 0.5C continuous charge as standard. Some high-power LFP cells support 2C discharge for short periods. Operating above the warranted C-rate causes excessive heat generation, accelerates SEI layer growth, and reduces cycle life — the BMS should enforce C-rate limits by cutting the discharge current if they are reached.
Why does charge current differ from discharge current for the same power?
Charge and discharge occur at different battery SoC points, so the DC bus voltage varies across the cycle. At the start of charging (low SoC), voltage is lower and current is higher for the same power. At the end of discharge (low SoC), voltage is lower and current is higher. The PCS manages this by operating in constant-current mode at the start and transitioning to constant-voltage mode at the end of each charge/discharge phase — this is the standard CC-CV charging protocol.
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