# BQ2057WSN Charging current issue

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?

• 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. – Kripacharya Jul 24 at 11:53
• @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. – Aakash Dey Jul 24 at 12:23
• I don’t think that Q1 has high enough beta for this application. – Andy aka Jul 24 at 13:19