I have at car usb charger. It has 2 ports and is rated 2.1A when one port is used and 1A when both ports are used. I attached samsung tablet (N8000) witch requires 2A output. So, tablet isn't charging. Then i plugged my phone (S5 neo). I drive around with waze turned on (about one hour). My battery does not charged. It even discharge by about 3 or 5 percents. I tried both ports.

Then i found my old 1A charger with single port. That charger charged my phone, with waze turned on. From 80% it goes to 100% within one hour. In both cases i used the same cable.

So - how i can test charger output amperage? Do i need special equipment or it can be done with multimeter?

Edit: How I can test if charger really can supply rated amperage? So, how to test its limits ?

Edit2: Found this article: http://www.deeptronic.com/variable-dummy-load-for-power-supply-testing/ and I try to build that.


closed as off-topic by uint128_t, Asmyldof, Adam Haun, PeterJ, Daniel Grillo Apr 19 '16 at 11:18

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    \$\begingroup\$ The problem may also be insufficient voltage out of the "charger". \$\endgroup\$ – DoxyLover Apr 18 '16 at 18:56

The issue may also be that the "charger" (actually power supply) is putting out insufficient voltage, instead of insufficient current.

If you don't want to build something like @transistor has described, there are many commercial products out there that will connect between a USB host port or power supply and the cable and display both voltage and current drawn. Search for "USB ammeter" to find them.



simulate this circuit – Schematic created using CircuitLab

Figure 1. Monitoring USB current.

You can use a USB extension lead or cut into one of your existing leads. Add the multimeter set to 'DC A' in series with the red lead.

The D+ lead is actually white. Yellow shown in Figure 1.

  • \$\begingroup\$ With this I can test how much phone is taking. Right? \$\endgroup\$ – Guntis Apr 18 '16 at 18:08
  • \$\begingroup\$ All power to the device connected will be drawn through the red wire, so yes. \$\endgroup\$ – Transistor Apr 18 '16 at 18:18
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    \$\begingroup\$ There are also many commercial products available to do the same thing, plus show the voltage at the same time. Search for "usb ammeter" to find them. \$\endgroup\$ – DoxyLover Apr 18 '16 at 18:55
  • \$\begingroup\$ @DoxyLover, that is what I am looking for. Can You write it as answer? \$\endgroup\$ – Guntis Apr 19 '16 at 4:26

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