RC Time Constant & Pre-charge Calculator
Every EV and energy-storage DC bus needs a pre-charge resistor to limit the inrush current when the contactor closes. Pick the resistance value and this calculator gives you τ, charge time to 95 % and 99 %, peak current and the instantaneous power the resistor must survive at t = 0. Or pick a target charge time and it back-solves R for you.
Inputs
Calculated values
| Time constant τ | 47.0 ms |
| Charge time to 95 % | 141.0 ms |
| Charge time to 99 % | 216.4 ms |
| Peak inrush current | 17.0 A |
| Peak instantaneous power | 13,617 W |
| Energy stored in C | 320.0 J |
- τ = R × C
- t_95 = 3 × τ
- t_99 = 4.6 × τ
- I_peak = V_bus / R
- P_peak = V_bus² / R
- E = ½ × C × V_bus²
Pre-charge wirewound resistors for EV / energy-storage DC bus
View resistor rangeThis calculator provides preliminary sizing only. Always validate with the drive manufacturer's worksheet and prototype testing.
How to use this calculator
- Choose calculation mode: enter a fixed R, or enter a target charge-time and we back-solve R.
- Enter bus voltage — typically 400–800 V for EV traction, 1000–1500 V for grid-scale storage.
- Enter total bus capacitance in microfarads (sum of all DC link capacitors).
- Read τ and confirm t_95 lands inside your contactor's allowable pre-charge window (often 0.5–2 s).
- Confirm the resistor's peak-power rating covers V²/R for the first millisecond of charge.
- Verify the resistor's energy rating (½ × C × V²) exceeds the energy delivered per pre-charge event.
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