BQ77PL900, reduce Cell Balance Time, Do I have to input external power for charging at first time?

1. System parameters must be adjusted and trade-offs made to get a minimum balance time. The figure refers to standalone balancing. Actual balancing occurs in the time between ta and tb, tc and td, ....  The OVH level must be set high enough to allow balancing to occur.  If OVH = 0, the part recovers very quickly and basically cycles the CHG on and off as it recovers and evaluates the new OV condition, until the cell remains above OV long enough to allow some balancing.  This will extend overall balance time. If the charge current is high, the cell resistance and the V = I x R drop of the cell may be a large part of the OVH range, so the time available for balance may be reduced and the total balance time extended.  A higher balance current will help reduce time until it creates a V = I x R drop which causes recovery early.  So a moderate balance current may be better than a high one to keep overall balancing time low.  The differences in the cells is also important.  A cell with a high resistance will have more voltage drop during charge and balance and alter balancing as described above.  The balancing will be more effective at maintaining balance on a pack that is slowly varying over time than adjusting a pack assembled with cell mis-match. This should be considered when designing a balancing test.

Host mode balancing does not have the limitations of the standalone algorithm pictures above.  The host can recognize the current and voltages, and could adjust for I x R drops of the cells as well as start and end balance before the OV level is reached.. The host could also balance after charge current has stopped, or stop charge current to check voltages and balance if the charger allows it. Host mode balancing of course requires the extra effort and cost of the host, calibration, algorithm and code development.

2. Yes, the device requires a voltage on PACK to wake up the first time or after shutdown.  Please see the "Initialization" section, page 29 of the bq77PL900 datasheet, the "VI(STARTUP)" in the recommended operating conditions, and the right side of figure 15, page 27 to help explain this part of the device operation. This is the reason for step 3 of section 4.5 of the EVM user's guide.