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I soldered two wires on female USB type A pins,one wire to ground and the other to the VCC pin. I was trying to check the battery voltage of the li-ion battery in my flashlight,

I connected the a voltmeter to the VCC and the ground wires of the female usb-A connector. The female USB-A connector I connected to the the flashlight cable: one side has a male USB-A connector and other side has a male USB-C connector which is connected to the flashlight.

I haven't gotten any result (0000.)

Why?

I tried it on a cellular phone and I got only 0.4V,Vwhile on the phone it says 80%, so the 0.4V is probably a result for something else.

Is there any simple way to improve the connector to be able to read the voltage of the battery?

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    \$\begingroup\$ In a word, no. The battery is not connected directly to the USB pins. When either charging or discharging, there is a power conversion circuit of some sort involved. \$\endgroup\$
    – Dave Tweed
    Jun 14, 2023 at 11:18

2 Answers 2

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The battery of a device goes through a protection circuit, then usually a boost converter, then powers the device. For charging, the input connection connects to a charging circuit, which is not directly connected to the battery, as this raises safety issues if a metal object is inserted into the charging port, or if there is a fault with the charger.

The reason you detect 0.4V on your phone is because your phone is likely trying to negociate under the USB Power Delivery protocol, so your voltmeter is able to see that there is ocassionally some activity on the power pins to turn on any connected device for a time long enough for it to send power delivery data if any. However this explaination only works if you see short blips of voltage, without an osciliscope it hard to tell.

You could probably unscrew your torch and measure the voltage of the Li-ion battery that way.

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    \$\begingroup\$ USB power delivery is not negotiated on the VBUS supply wire and Type A connector can't support power delivery anyway, so your explanation why there is 0.4V on VBUS can't be correct. \$\endgroup\$
    – Justme
    Jun 14, 2023 at 17:57
  • \$\begingroup\$ @justme,do you have other explanation,why i see this 0.4V? \$\endgroup\$
    – xchcui
    Jun 16, 2023 at 8:28
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    \$\begingroup\$ Yes. You are seeing 0.4V because you are measuring from charging circuit input with a high impedance multimeter, so even picoamps of leakage current through diode or chip can show up as 0.4V. There is nothing unusual about it. \$\endgroup\$
    – Justme
    Jun 16, 2023 at 8:35
  • \$\begingroup\$ Yes,it seems more reasonable,as i didn't solder the two data pins(2,3)only the VBUS and GND ones.Thanks. \$\endgroup\$
    – xchcui
    Jun 16, 2023 at 12:11
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The USB supply wires do not connect to the battery so you can't measure battery voltage from USB connector.

The USB supply wire connects to a battery charger which takes power from the supply and uses it to charge the battery.

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