BQ24780S: Charging related questions

Hi Tim,

1.  Can the chip automatically switch CC charging mode to CV charging mode? How can i know which mode the chip is in？

>>  Yes, the BQ24780s automatically switches from CC to CV charging.  There is not a status indicator (i.e. pin or register bit) on this device to indicate which mode the device is in, but you can see on a scope when the transition occurs because the current will begin to drop from the fastcharge current specified in the ChargeCurrent() register and the battery voltage (measured at SRN pin) will be constant and at the value set in the ChargeVoltage() register. 

2.  How can i know the battery is in charging or full? I want to add a charging indicator LED.

>>  In most cases for 2-4 cell battery packs, there will be a gas gauge IC on the battery pack, and that can provide this information.  You can reference BQ40Z60 as an example. The BQ24780s does not have a charging/not charging status indicator.  It was primarily developed to be used with battery packs that have a gas gauge IC.

3.  Can I get the charging current from the chip without adding additional circuit?

>>  BQ24780s does not have an integrated ADC, so it does not provide a means for directly measuring charging current.  The BQ25710 and BQ25713 devices do have an integrated ADC that could be used for this purpose, so you might wish to take a look at them. 

You may want to check out the BQ25710/BQ25713.  It has a status bit (REG0x20[10]) that indicates when you are in fast charge (i.e. CC mode) and it has an integrated ADC that can read the charge current as well as a number of other values.

Regards,

Steve

Hi Tim,

1.Once the battery switches to CV charging mode, will it be in CV mode all the time? Or will the chip terminate the charging as the current begins to decrease to a certain value(such as 0.02C)?

Some of our devices feature self termination, but not the BQ24780s.  It will remain in CV charging mode until a gauge or uC terminates charging by setting the charge inhibit bit or writing 0x0 to either the ChargeCurrent() or ChargeVoltage() registers.  Most battery packs have a gauge IC integrated that has very accurate sensing to determine optimal point for termination, and some battery packs even follow the SBS (smart battery specification) which allows them to terminate the BQ24780s charging directly without uC intervention.  For more see:  http://sbs-forum.org/

2.Refer to the datasheet: IPMON = KPMON (VIN x IIN - VBAT x IBAT). In charging mode , If i can get the values of VIN,IADP,IPMON,VBAT from a MCU, may i get the approximate charging current by this formula:ICHG=IBAT=(VIN*IIN-IPMON/KPMON)/VBAT=(VIN*IADP-IPMON/KPMON)/VBAT?

Theoretically this would work, but I don't think it will produce very precise results, particularly if your system load is varying.  I am not sure that the practical implementation will give you enough precision to accurately terminate the charging.

Regards,

Steve