Battery Short-Circuit Current — Method
Battery short-circuit current is limited mainly by the cell internal resistance. From Ohm\'s law, Isc = V ÷ R. For a bank of cells, internal resistance scales with the series count and divides by the parallel count: R_int = (R_cell × series) ÷ parallel. Add external conductor resistance for the fault current at the load end. Always compare against the manufacturer\'s published short-circuit rating and size protection for the higher value.
Frequently Asked Questions
Use Isc = V ÷ R_total. V is the battery/bank voltage; R_total is the internal resistance plus any external conductor resistance (in ohms). Example: 12 V ÷ 0.005 Ω = 2,400 A. For a bank, R_int = (cell resistance × cells in series) ÷ strings in parallel.
It is on the manufacturer datasheet (often in mΩ per cell/block), or measured with a battery resistance tester. VRLA blocks are typically a few mΩ; large 2 V cells can be well under 1 mΩ, which is why their fault currents reach many kA.
The prospective fault current sets the minimum interrupting/breaking rating of DC protective devices (fuses, breakers) and the bracing needed for busbars and links. Under-rated protection can fail to clear a battery fault safely.
Series increases both voltage and total internal resistance proportionally, so Isc stays similar but at higher voltage. Parallel strings lower total resistance (R ÷ N), increasing the available short-circuit current roughly N times.
Yes. Adding the external conductor resistance to R_total lowers the fault current at the load end versus the terminals. For device rating, use the worst case (at the terminals); for cable withstand, use the value at that point in the circuit.