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BQ24753A: Battery Charging Voltage Issue

Pradeep N52
Prodigy 190 points
Part Number: BQ24753A

Dear,

We are using the attached circuit to charge the battery(2 Cell Li-ion).

Now we have some concerns regarding charging voltage measured at TP49(BATTERY_PACK+ net). For eg. consider 2 cell battery voltage is 7.4V(which is also nominal voltage) now if we connect the External adapter the voltage at TP49 suddenly jumps 7.9V if we remove adapter immediately it again come back 7.4V only.This +0.50V difference observed for all other voltages also. Because of this we are unable measure the battery capacity i.e, Once the actual battery voltage reaches 7.9V the BATTERY_PACK+ terminal will show 8.4V  which is max. capacity of battery. We also worried that this may also cause some damage to battery. We didn't understand why this sudden jump occurring at the BATTERY_PACK+ net. 

Please reply to us if you require more data.

Regards,

  • 0
    TI__Guru**** 155586 points
    Pradeep,

    There voltage measured at the battery connector includes the cell voltage plus the voltage drop created by the charge current times the connector and trace resistor and the internal battery pack protector. That is why it is best to measure the battery pack voltage, in order to determine capacity, only when little to no current is flowing into or out of the battery. The battery cell also has a certain impedance that is a function of heat and age. Therefore, it also a good idea to let the battery relax from current flow for a several seconds before taking a measurement. The charger will not allow the voltage to increase above the CV voltage = 8.4V. Once the voltage at BAT reaches this voltage, the charge current naturally tapers down as the cell voltage slowly charges to 8.4V.