Battery Paralleling — Inrush Current Waveform

What happens when a second pack is connected in parallel to a live bus — and why pre-charge is critical

Bus voltage (V) — live pack 1 400 V
Pack 2 voltage (V) 380 V
Cable + contact resistance (mΩ) 5 mΩ
Pre-charge resistance (Ω) 50 Ω
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Peak inrush without pre-charge (A)
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Peak inrush with pre-charge (A)
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ΔV between packs (V)
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Energy dissipated in cable (J)
Why inrush is dangerous ΔV × capacitance creates impulse current.
I_peak = ΔV / R_circuit — with mΩ R, kA possible.
Welds contactors, trips protection, arcs.
Dangerous even at ΔV = 5V if R is very low.
Pre-charge solves this Insert R_pre in series with Pack 2 contactor.
Limits I = ΔV / (R_pre + R_cable) → safe <20A.
After voltage equalization, close main contactor.
BMS monitors V_pack2 to decide when ΔV < 2V.
BMS sequencing Monitor V_bus and V_pack2 continuously.
Close pre-charge relay only when ΔV < 20V.
Wait for equalization (RC time constant).
When ΔV < 2V: close main, open pre-charge.