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BQ24170: circuit for automatic detection and charging of 2-cell or 3-cell LiPo / Li-ion battery?

Thomas Bewley
Expert 1030 points
Part Number: BQ24170
Other Parts Discussed in Thread: BQ24133, BQ29209, BQ40Z60, BQ40Z50-R1, BQ40Z50-R2

I'm interested in using the BQ24170 (or,  BQ24133, or similar) on a board that can accept either a 2-cell or 3-cell LiPo / Li-ion battery (I'll put both connectors on the board - let's give the user the choice of which to use, and assume the user is informed / intelligent enough to not plug in both at the same time...).  I am looking for a circuit that will automatically detect if a 2-cell or 3-cell battery has been plugged in, and charge it appropriately.  Can the BQ24170 (or similar) handle this?  Any pointers would be greatly appreciated.

  • 0
    TI__Expert 8690 points
    Hi Thomas,

    The device does not automatically detect if it is a 2 cell or a 3 cell.

    You can place a mechanical switch where you connect the 3 cell battery. Make sure that the molex connectors for both 2 cell and 3 cell are different from each other.

    Set default cell count at the CELL pin to 2 cell (float), with the mechanical switch OFF. If the 3 cell battery is connected, have the mechanical switch turn ON and short CELL pin to HIGH for 3 cell setting.

    Regards,
    Steven
  • 0 in reply to Steven Bangdiwala
    Expert 1030 points
    Hi Steven - thank you for this explanation!

    Regarding the molex connectors, I reckon a 2-cell LiPo/Li-ion would have a 3-pin molex, and a 3-cell LiPo/Li-ion would have a 4-pin molex, so that seems like mostly a non-issue?

    Based on the remainder of your hint, it seems that a little bit of intelligent circuitry (without a mechanical switch) attached to the 3-cell molex could solve this problem, pulling the CELL pin high if someone plugs in a battery (that is at least not deader than a doornail) into the 3-cell molex, and letting it float if they don't. Does that sound reasonable? Has anyone thought of this? Seems pretty natural...

    take care,
    - Tom
  • 0 in reply to Thomas Bewley
    TI__Expert 8690 points
    Hi Thomas,

    Of course, that also works as well. I was just thinking something simple like a mechanical switch, but it can wear out, so you can also control CELL pin to HIGH doing the following:

    1. Have a resistor divider from 3 cell battery input to the gate of NMOS
    2. Connect drain of NMOS to gate of PMOS
    3. Use PMOS to short CELL to HIGH

    You should also make sure that the 3 cell input and 2 cell input are isolated from one another, and should close connection to the charger only when you detect voltage from the molex connector. Better yet, you can also prevent connection of both 2 cell and 3 cell at the same time by adding FET logic such that none of the batteries will connect if both are present.

    Regards,
    Steven
  • 0 in reply to Steven Bangdiwala
    Expert 1030 points
    Cool, yes, your 2-MOSFET proposal for controlling the bq24170 is exactly along the lines of what I was thinking for charging.

    While cogitating this, I've also been worried precisely about the isolation of the 2-cell and 3-cell input, because of course I'll also need to apply balancing to the battery used. I've been looking at the bq2945xx ICs, the example circuits for which are again shown as 2-cell OR 3-cell arrangements. When providing inputs on my board for both 2-cell AND 3-cell battery, and trying to automatically switch from one to the other in the balance circuit, I again have a logic problem for isolation that should hopefully be solvable with FETs. Any suggestions here? This one is not as obvious to me...
  • 0 in reply to Thomas Bewley
    TI__Expert 8690 points
    Hi Thomas,

    Can you provide a block diagram of your application? Include logic that you would like to add. Then from there, we can plug in analog logic to where it needs to be so we can get closer to your final application schematic.

    I don't think the bq2945xx ICs have cell balancing though. You can ask in the Gauges forum, they will help you with the selection.

    Regards,
    Steven
  • 0 in reply to Steven Bangdiwala
    Expert 1030 points

    Hi Steven -

    I'll put together a block diagram / logic description today.  

    Before I do, I need to locate the correct balancing chip.  We currently (in our 2-cell design) use the bq29209, which is a 2-cell balancer.  I'm having a hard time poking around finding the corresponding TI 3-cell balancer.  Help?  I'll see if I can repost this question in the Gauges forum per your request.

    cheers,

    - Tom

  • 0 in reply to Thomas Bewley
    Expert 1030 points

    So, after much looking, the bq40z50-r1, bq40z50-r2, or bq40z60 is probably the way to go (or, the forthcoming z80??).  They are integrated devices that handle 2-4 cells.  I don't know how to select between them, nor how I can wire it up in the manner requested here, to recognize how many cells are being used, and respond appropriately.

  • 0 in reply to Thomas Bewley
    TI__Expert 8690 points

    Hi Tom,

    These devices are definitely from the Gauges forum. You can get more detailed help over there on how to configure them. The signals that you use for the charger configuration based on 2 cell or 3 cell may also be used for the gauge, but to modify the gauge accordingly I cannot help you there.

    Regards,
    Steven

  • 0 in reply to Steven Bangdiwala
    Expert 1030 points
    Hi Steven - Ok, It looks like I should move this discussion to the Gauges forum then. I'll close off the discussion here and repost it over there... cheers - Tom