Laptop chargers measure current with small resistors of a few milliohms, placed in the adapter input path and the battery path. The charger reads the tiny voltage across each one. You can do the same with a multimeter on its millivolt range — no need to break the circuit to measure current. This calculator turns that millivolt reading into amps, or tells you what reading to expect.
How to use it
- Enter the sense resistor value from its marking or the schematic.
- Measure the voltage directly across its two pads, with the meter on millivolts, and enter it.
- Read the current and the power in the resistor.
Fill in any two of the three fields; the third is calculated.
The formulas
- I = V ÷ R — current from the voltage drop.
- P = I² × R — the resistor's own dissipation.
With milliohm resistors the numbers are friendly: 20 mV across 10 mΩ is 2 A. Make sure the meter probes touch the resistor's own pads — a few millimetres of copper trace add resistance comparable to the resistor itself and distort the reading.
Worked example: is the battery charging?
A laptop reports "plugged in, charging" but the percentage never rises. Across the 10 mΩ battery-path sense resistor the meter shows 0.3 mV, which is 30 mA — barely a trickle. Across the 10 mΩ input sense resistor it shows 18 mV, 1.8 A, which is the laptop running. So the adapter is fine and the board is powered, but the charger is not pushing current into the battery: check the charge settings from the EC, the battery's own state and the battery MOSFET. That routine is described in The Laptop Charging IC: How It Works, and the resistors themselves in Current Sense Resistors in Laptop Charging Circuits.
Measuring millivolts reliably
- Use the meter's millivolt range, not the volts range, for resolution.
- Probe the pads at the ends of the resistor, not the traces leading to it.
- Expect small readings to fluctuate as the load changes; watch for a stable average.
- Direction matters: the sign of the reading tells you which way current flows.
The component is described under current sense resistor.
FAQ
Can I measure a 10 mΩ resistor with my multimeter's ohm range?
Not accurately. The leads alone have more resistance than that. Measure the voltage drop in operation instead.
Why is my current reading negative?
The probes are reversed relative to the current direction — or the battery is discharging into the system.
What happens if a sense resistor cracks?
The charger misreads current: charging may stop, run at the wrong rate, or the laptop may refuse the adapter.