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BQ25896: Reaching JEITA T3 level Starts Constant Voltage Mode

Part Number: BQ25896
Other Parts Discussed in Thread: BQSTUDIO

Register Settings


I want to use the BQ25896 in a hot ambient environment (40°C).  Our battery has a max charge temperature of 45°C and contains a 10KΩ Thermistor with a Beta of 3435.  Whenever the battery temperature reaches the JEITA T3 level of 45°C the VREG voltage is set to a value of 4.0V (VREG(4.2) - 200mv = 4.0v).  This is expected as we have the JEITA_VSET bit set to 0.  What is unexpected is this appears to put the IC into the constant voltage phase of the charge cycle.  The battery continues to charge until it reaches our charge termination value of 256mA when the BQ25896 terminates the charge cycle and indicates a completed charge.  The battery however is not charged to 100% and confuses the user into thinking it is. 

Am I correct that the BQ25896 is in constant voltage mode when it hits the JEITA T3 value with the JEITA_VSET bit set to 0?

I know I could gain a little more capacity by setting the termination current to a lower value or allow charging at higher temperatures, by setting the JEITA_VSET value to 1, but are there any other register settings (see attached from bqStudio) that I could change to allow for a complete charge of the battery at 40°C ambient?

Is there something I'm not considering that could help me?




  • Hi Rob, 

    Thank you for reaching out via E2E. You are correct that constant voltage mode and charge termination are still enabled when device is in JEITA warm temperature region (above T3 level). 

    When in JEITA warm region the device does not automatically switch to constant voltage phase, that still depends on the battery voltage. The only difference is that constant voltage phase starts at 4V compared to 4.2V when in normal temperature range.  

    The 2 changes you mention are options. Reducing the termination current will extend charge some, but you will still not reach 100% charge capacity since VREG is reduced from 4.2V to 4V. If you choose to set JEITA_VSET = 1 the device will charge the same in JEITA warm region as normal temperature region. 

    An additional option is to disable termination when in JEITA warm region by setting EN_TERM bit = 0b (REG07 bit 7). With this change the IC won't attempt to charge battery above 4V when temperature is above 45°C, but avoids the user confusion by indicating charge complete when battery is not in fact charged to 100%. To implement this change your I2C host would need to monitor NTC_FAULT status in REG0C and disable termination only when its status = TS_Warm. 

    Best Regards,