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I want to measure output of a charger (MODEL: GB28-25126) with below picture:

Charger specification

When I connect my HIOKI multi-meter (3280-10F) to output of my charger in order to measure the resistance, it shows OF (OverFlow) at first and then starts decreasing until 5 mega Ohm. At 4 mega ohm it suddenly shows 4.5 kilo Ohm and starts increasing until 9.67 kilo ohm and then will remain constant. I have some questions:

  1. What causes that behavior?
  2. At last what is the output resistance of charger?
  3. Above all, if I connect a 12V battery to the charger, the battery will be depleted by the charger when it is not connected to home electricity?
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  • \$\begingroup\$ What is your end goal if you had a value for the power supplies output resistance? \$\endgroup\$
    – winny
    Jan 20, 2021 at 13:51
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    \$\begingroup\$ Connecting a multimeter set to resistance measurement mode to the output of a power supply that's plugged in may destroy the meter. If you did this you may be fortunate that HIOKI have decent protection on the meter. \$\endgroup\$
    – Transistor
    Jan 20, 2021 at 14:01
  • \$\begingroup\$ @winny My aim was to find if my battery would be depleted when there was no electricity at input of the charger. 3. Above all, if I connect a 12V battery to the charger, the battery will be depleted by the charger when it is not connected to home electricity? \$\endgroup\$
    – Pana
    Jan 20, 2021 at 14:24
  • \$\begingroup\$ @Transistor While I measured output resistance the charger was being disconnected from home electricity plug. \$\endgroup\$
    – Pana
    Jan 20, 2021 at 14:27
  • \$\begingroup\$ Ah! That makes more sense but I suspect it may be quite voltage dependent and your multimeter will only test near zero. Have you considered a Schottky diode? \$\endgroup\$
    – winny
    Jan 20, 2021 at 17:14

2 Answers 2

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1) The output of the charger is not a simple resistor

2) It doesn't have a defined resistance. A multimeter measures the voltage and current at its terminals, and reports the ratio as resistance. If you have it connected to a simple resistor, that's fine. If you have it connected to a capacitor, the reading will vary over time. If you have it connected to semiconductors, then the ratio will change as the voltage changes. If you have it connected to an active circuit, then things get really complicated.

3) Maybe, maybe not. Because the behaviour is voltage sensitive, you'll need to do the experiment. Connect the battery, and measure the current.

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  • \$\begingroup\$ I tested it and the charger draws 37 mA. Since my battery is 5200 mAh it will be depleted in 5.85 days! not good at all! Thank you. \$\endgroup\$
    – Pana
    Mar 19, 2019 at 10:45
  • \$\begingroup\$ Can you please provide a common prototype circuit of a charger? I wanna get a better look at its output stage. \$\endgroup\$
    – Pana
    Mar 19, 2019 at 11:25
  • \$\begingroup\$ There are ways to use a FET and a comparator to switch the charger to the battery when it's on, and disconnect when off. \$\endgroup\$
    – Neil_UK
    Mar 19, 2019 at 11:26
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    \$\begingroup\$ There are no 'common' charger circuits. That's why I suggested you measure that particular one. You may find other chargers behave differently. I could certainly design a charger that didn't load the battery when off, but it would be more expensive than one that did. \$\endgroup\$
    – Neil_UK
    Mar 19, 2019 at 11:27
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If you subtract one voltage from the other and knowing the current, using Ohm's law this gives you the impedance.

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