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Today when I was going to measure charging current in BQ2057WSN charging circuit (Input Supply: 12VDC, Battery: 7.4V,6600mAH) by putting a multi meter in series with Q1 (shown in figure) and measured the current at the battery end is 300mA and at the same time the current drawn from the power source is also 300mA(though the charging current is set at 600mA). I have also observed today that before inserting the multi meter, the battery terminal voltage was 7.39V and the circuit was drawing 600mA from the power source. And after inserting the multi meter in series with Q1, the battery terminal voltage become 7.44V and the circuit was drawing 300mA from the power source. Why so?

Also,When I was doing the same experiment ( putting the multi meter in series with Q1) with a battery (7.4V,6600mAH Li-ion battery)having voltage 6V, I have found that both the current drawn from the power source as well as current in the battery terminal was 600mA. But, when this was doing with a battery having voltage 7.39V, the current reduces to 300mA with insertions of multi meter. Why the current is different in these two case as in both the cases the batteries are not fully charged?enter image description here

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  • \$\begingroup\$ The BQ2057WSN is an 'intelligent' charger, which charges at different currents and voltages depending on the sensed battery voltage. Readup on charging of LiPo batteries and chargers. \$\endgroup\$ – Kripacharya Jul 24 at 11:53
  • \$\begingroup\$ @Kripacharya, As I am Charging 7.4V Li-ion battery in both the cases and according to the datasheet, BQ2057WSN should charge a battery at constant current rate when the battery voltage is between 6-8.2V. As in both the above cases the battery voltage is in this range, it should charge the battery at constant current rate (the rate where i set the charge current ie 600mA) in both the case. \$\endgroup\$ – Aakash Dey Jul 24 at 12:23
  • \$\begingroup\$ I don’t think that Q1 has high enough beta for this application. \$\endgroup\$ – Andy aka Jul 24 at 13:19
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And after inserting the multi meter in series with Q1, the battery terminal voltage [increased from 7.39V to 7.44V] and the circuit was drawing 300mA from the power source [instead of 600mA]. Why so?

Because you inserted the multimeter between Q1 and the regulator's BAT sense pin.

For any typical hand-held multimeter to measure current, it must drop a little voltage, called the burden voltage. This is what the meter is measuring and scaling to provide a current value output. Dropping voltage slightly here fools the charger into thinking the battery is more charged than it really is.

Solution: place the meter at the 12v source, before the charger.

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  • \$\begingroup\$ @ rdtsc, After the including the burden voltage with the battery voltage (7.39V), I have found the total voltage as 7.44V( battery voltage + burden voltage). As BQ2057WSN should charge a battery at constant current rate when the battery voltage is between 6-8.2V for a 7.4V Li-ion battery. As in this case the battery voltage is in this range, it should charge the battery at constant current rate (the rate where i set the charge current ie 600mA). \$\endgroup\$ – Aakash Dey Jul 24 at 12:41
  • \$\begingroup\$ I disagree with this answer. If the meter were placed between battery and BAT pin it would be correct but it’s placed in the collector, a high compliance current source with a good emitter feed of 12 volts. \$\endgroup\$ – Andy aka Jul 24 at 12:55
  • \$\begingroup\$ @Andyaka, I have also observed by putting the meter between battery and BAT pin, but the results are same as mentioned in the question. \$\endgroup\$ – Aakash Dey Jul 24 at 13:27

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