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BQ24172: 3A charger for 24.5Ah 1S Lithium Ion Battery

Alex Gribbin
Prodigy 60 points
Part Number: BQ24172
Other Parts Discussed in Thread: BQ24192, BQ21040, BQ24040

Hi,

I'm in the final stages of developing a test meter with lithium ion (LG MJ1 3500mAh) batteries that will require charging-

Main Battery - 24.5Ah (7 x 3500mAh cells in parallel)

Reference Battery 1 - Single cell

Reference Battery 2 - Single cell

Each of these batteries must be isolated from each other, and will not share a common ground.

Supply voltage is maximum 14.7V (car alternator)

The main battery needs to be charged at a minimum rate of 3A

The reference batteries require charging at a minimum rate of 400mA

Due to the large capacity of the main battery, its charger must have adjustable end of charge / termination current (to be set at 700mA)

Reference battery termination current can be set to 100mA

My original plan was to use a BQ24172, but layout is proving to be problematic, as I am using a 2 layer PCB (not 4, as recommended in the datasheet).

I'd like to keep the supply design simple (no need for Powerpath or USB features) with a small footprint.

Are you able to give any recommendation for a charging solution?

  • 0
    TI__Genius 10385 points

    Alex,

    I am currently directing your question to someone that can help you make a selection.

    Thanks,

    Ricardo

  • 0 in reply to Ricardo
    TI__Genius 10385 points

    Alex,

    I recommend a multi charger solution. Use the BQ24192 switching charger to charge your main battery as it can meet your 3A charge current and 15V input voltage.

    Then use two BQ21040 linear chargers feeding from the SYS output of the switching charger to charge the reference batteries. Please be aware that the SYS voltage can be as low as VSYSMIN and as high as VBATREG, so it may not always be possible to charge the reference batteries.

    Thanks,

    Ricardo

  • 0 in reply to Ricardo
    Prodigy 60 points

    Thanks Ricardo,

    I'll have a deeper look into the datasheet over the next few days.

    Appreciate the help.

    Alex.

  • 0 in reply to Alex Gribbin
    Prodigy 60 points

    I've gone through the datasheet and come up with the attached schematic based on the bq24192 and BQ21040.

    Does this look correct?

    PSEL - Low, as no USB power source will be used.

    PG, STAT, and OTG - Not required. Left floating.

    INT - May be used, depending on MCU I/O avaliability

    CE - Low. Charging will begin only when 12V in supplied to VBUS. Possibly connecting to MCU output for temperature triggered shut down.

    ILIM - Set to 3A. Can be changed via I2C to lower value if required.

    TS1 and TS2 - We'll most likely let the MCU monitor battery temps.

    PMID - Bulk of input capacitance placed at this pin.

    Triple Threat Charger.pdf

  • 0 in reply to Alex Gribbin
    TI__Genius 10385 points

    Alex,

    For the BQ24192 I have a few comments 

    • I suggest adding 2 more 10 uF caps on VSYS for optimal performance
    • Your TS network (R8 and R7) will not work. You will need to have a 30.1k going from TS1 to GND, a 10k going from TS1 to GND, a 5.23k going from TS1 to REGN, and finally short TS1 and TS2 together

    My colleague will be reviewing you BQ21040 schematic shortly.

    Thanks,

    Ricardo

  • 0 in reply to Alex Gribbin
    TI__Mastermind 19195 points

    Hi Alex,

    The schematic for bq21040 looks ok, issue I see it that the input voltage is limited to bq24192 Vbatreg, 4.2V. The bq21040 has DPM feature that kicks in around 4.3V, device will not charge at all with 4.2V input.

    We recommend using 5V input for BQ21040.

    Best,

  • 0 in reply to Nick Brylski
    TI__Genius 10385 points

    Alex,

    Nick brings up a good point. You would have to have boost converter between the BQ24192 and the BQ21040s to step up to 5V or a buck converter off of the alternator output powering the BQ21040s.

    Thanks,

    Ricardo

  • 0 in reply to Nick Brylski
    Prodigy 60 points

    Hi Nick,

    The simplest solution might be using an external DC converter to feed 5v to all three chargers.

    The BQ24040 will do just fine for charging the single cells.

    Do you still recommend the BQ24192? It has a lot of features that won't be used in our application.

  • 0 in reply to Alex Gribbin
    TI__Genius 10385 points

    Alex,

    I still recommend the BQ24192 as it will simplify your layout to 2 layers.

    Thanks,

    Ricardo