2022-06-01 USB Cable Internal Resistance

I had a “high quality” 1 meter long USB A male to type C charging cable go bad so I cut the bad type C connector off. Then I stripped and twisted the two 5V wires together and measured the total wire resistance on the A end with my HP 3478 DMM in the 4 wire mode. The connectors were causing a bit of variablility, but it measured about a quarter ohm.
So if I use this cable to charge my cellphone at 1 amp, the voltage drop would be about a quarter volt, giving about 4.75 volts to the phone.
When the phone’s charging circuit sees the voltage dropping below 5 volts, it thinks the charger is having a problem delivering current, so it backs off the charging current. This causes your phone to take longer to charge and is annoying when you’re not able to wait for it to charge.
The solution is to keep the cable short as conveniently possible. A half meter long cable would have half as much voltage drop. Heavier wire would help but the makers are too cheap to use decent quality wire. Some charging cord makers even use copper covered aluminum or steel wire.

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