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Internal Resistance Calculator — Battery Load Test, Two Points & Voltage Sag

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Known measurements Open-circuit voltage E (V) Loaded terminal voltage V (V) Load current I (A)

Calculated result

Internal resistance = 20 mΩ

Internal resistance: 0.02 Ω

Internal resistance: 20 mΩ

Measured voltage sag: 0.6 V

Internal I²R loss at test current: 18 W

Load power at test point: 366 W

Simple-model delivery efficiency: 95.3125%

Show the working

1. Voltage sag = 12.8 − 12.2 = 0.6 V.
2. r = sag ÷ current = 0.6 ÷ 30 = 0.02 Ω.
3. Convert to milliohms: 0.02 × 1000 = 20 mΩ.
4. Internal loss = I²r = 30² × 0.02 = 18 W.

A DC load-test estimate depends strongly on source state, temperature and test duration. Compare only measurements made with a consistent method and follow the battery/device manufacturer’s test guidance.

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