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BQ24133 - VBAT level when charging

Jason Traud
Prodigy 20 points
Other Parts Discussed in Thread: BQ24133

I'm noticing a different behaviour between two runs of a design based on the BQ24133 battery charger. In both situations the batteries seem to charge fine. I want to be sure that I know what the proper behaviour should be. I am using it to charge a 2cell 8.4V Li-ion battery.

On an earlier run, when the battery is connected and then the unit power on VBAT would be the 2cell voltage and it would charge normally. However, on this run the voltage level seems to be the current voltage of the battery + 0.5V until it reaches the 2cell voltage level. 

I noticed some charging information in the post below showing the typical charging profile. In the fast charging phase the charge current should be constant with the voltage of the battery increasing until it's charged. This seems to suggest that I should be seeing the battery voltage increase at a steady pace until it's reached a threshold where the charging current exponentially decreases. 

https://e2e.ti.com/support/power_management/battery_management/f/179/t/359600

For the current configuration, I have Ichg set to 2A by using a 100k-13.7k voltage divider using the VREF pin. For RSR I'm using 10m Ohm value used in the datasheet. For ACSET I'm using a 100k-57.6k voltage divider and a 20m Ohm sense resistor across ACN and ACP like the reference in the datasheet. 

What waveform should I expect VBAT to follow as it charges? Should it immediately be 8.4V while connected or should the voltage across the battery gradually increase over time? Is one waveform representative of precharge and the other fast charge?

  • 0
    TI__Expert 5565 points
    Jason,

    The charge profile waveform you are referring to in the link is correct. However, you have to be careful where you are measuring the "battery" voltage. For the charge, the "battery" voltage is the SRN pin. So if you have a long cable between the SRN pin and the BAT+ terminal of the battery pack, the SRN voltage will be higher than the actual BAT+ voltage due to cable impedance.

    In CC (constant current) mode, the charge current is constant and the battery voltage increases till you hit the target charge voltage (8.4V in your case). At this point, the charge operates in CV (constant voltage) mode and the current decreases until you hit the termination current threshold.