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?

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.

  • \$\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 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 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 at 11:26
  • \$\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 at 11:27

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