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BQ24133: Why no system battery tap option for 2 cells?

Mike Ciholas
Prodigy 170 points
Other Parts Discussed in Thread: BQ24133, BQ24161

Hi,

BQ24133 datasheet figure 2 shows system being powered from battery through sense resistor from driven side.  Datasheet also says:

"For 1 cell applications, if the battery is not removable, the system can be directly connected to the battery to simplify the power path design and lower the cost. With this configuration, the battery can automatically supplement the system load if the adapter is overloaded."

Why won't this work for 2 cells?

In looking at the operating logic of the BQ24133, I don't find anything that would prevent 2 cells from having system voltage tapped at the battery as shown in figure 2.  The AC input would be current limited as usual, all that power goes through the regulator, so that limit has to be within the safe range for the chip.

Why won't this work for removable battery?

I presume this is because the battery detection system would be confused by the system load?

The reason tapping system load from the battery is desired is that my system would prefer to have a charging voltage that is higher than the system can tolerate.  If I do figure 1 (charger voltage goes to system), then I have a system voltage that is too high.  If I do figure 2 (system tapped on battery), then I don't have this problem.  Figure 2 also solves the problem of an occasional system load that exceeds the AC input capability, battery handles the deficit.

Basically I want a 2 cell charger that works more like the BQ24161 (1 cell) which allows battery to supplement momentary high system loads under charge.

Thanks for any help...

  • 0
    TI__Mastermind 31120 points

    I agree with you. I really see no differences between 1 cell and two cells.

    The basic difference between using the battery FET and not is the isolation.  If the battery FET is not used and the battery is fully discharged, the current going to the battery/system will be the precharge current which may not be enought to power the system and recover the battery.  With the battery FET the system can get all the power from the source.  Battery detection is also an issue for the system if connected directly to the battery.  The system  most likely would not run during the discharge portion of the battery detection.