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BQ24650: Recommendation for charger and BMS ICs for 2S1P LiFePO4 and solar

Brad Smith
Prodigy 30 points
Part Number: BQ24650
Other Parts Discussed in Thread: BQ25672, , BQ25820, BQ25750, BQ25756E

Tool/software:

Currently in development and looking for recommendations on TI ICs for a lean or reduced BOM design.  Device runs on a wifi connected MCU, and powered via LIFePO4 2S1P battery, primarily from solar.  In my brief studies it would seem I may need a 'charger IC' that support MPPT algorithm.  Then a BMS IC to handle things like temp monitoring, cell balance, etc?  

Parameters are as such, battery to be 30Wh, solar of 5-10W, daily power consumption estimated at 2Wh/day, max charging current 1-2A, solar panel voltage 22Voc.  

Ideal strategy - receive temp input to MCU and allow MCU to turn on/off charging.  Would like to be able to adjust charge voltage.  We typically would not need a true 100% SOC, generally 90-95%.  Need a strategy to monitor battery capacity.  LiFe has a very flat SOC curve.  Not yet sure what features come built into BMSs yet to know what circuits will needed added but will need reverse polarity protections, over current, and will need to evaluate the failure mode for battery Vmin disconnect and how we can re-initiate charging.  

As sort of an aside, I am curious on these charger ICs, if they are setup for solar input, we would like the ability to disconnect the solar plug and plug in a separate 12VDC source if the panel becomes damaged.  I am hoping the charger will still accept this as a charge source?  

My apology for the wordy post.  Please ask any questions if my rambling is hard to comprehend.  

  • 0
    TI__Mastermind 35326 points

    Hello Brad,

    Brad Smith said:
    n my brief studies it would seem I may need a 'charger IC' that support MPPT algorithm.  Then a BMS IC to handle things like temp monitoring, cell balance, etc?  

    Yes, This is correct, for the charger I would recommend looking into BQ25672.

    BQ25672:

    • Has integrated FET for reduced BOM size.
    • Has MPPT capability
    • able to adjust charge voltage with MCU, not able to do on the BQ24650 since it requires hardware changes to do.
    • Supports Dual input for solar panel and 12VDC source

    If you have any specfic questions related to BQ25672, please let me know.

    If you need information related to a battery monitor/fuel gauge for cell balacing /temp monitoring, I recommend posting a new question. I unfortunately, only support the charger IC.

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Prodigy 30 points

    Christian,

    I really appreciate your accelerated response, despite my rambles.  Could I ask the best section to post the other questions on cell balance, fuel gauge, etc? Or same section, just need to separate tasks?  

    I will evaluate your suggestion for the charger.  However, question for you though, the intent with the aux 12V charge source was 'not' to have two different sources, but rather unplug the solar panel, and plug in a constant 12V source for charging.  I can't say I've ever tried that with any solar charger to know.  the intent is "if your solar panel fails, just unplug,and connect to 12V charger".  We would rather not have to provide two connection ports as this is an outdoor device with water tight considerations.  

  • 0 in reply to Brad Smith
    TI__Mastermind 35326 points

    Hello Brad,

    Brad Smith said:
    really appreciate your accelerated response, despite my rambles.  Could I ask the best section to post the other questions on cell balance, fuel gauge, etc? Or same section, just need to separate tasks?  

    Same section, just a different task.

    Brad Smith said:
      However, question for you though, the intent with the aux 12V charge source was 'not' to have two different sources, but rather unplug the solar panel, and plug in a constant 12V source for charging.  I can't say I've ever tried that with any solar charger to know.  the intent is "if your solar panel fails, just unplug,and connect to 12V charger".  We would rather not have to provide two connection ports as this is an outdoor device with water tight considerations.  

    The BQ25672 can be used with a single input as well. I still recommend this charger, based on your other requirements:

    • Has integrated FET for reduced BOM size.
    • Has MPPT capability
    • able to adjust charge voltage with MCU, not able to do on the BQ24650 since it requires hardware changes to do.

    Best Regards,

    Christian

  • 0 in reply to Christian Moyer
    Prodigy 30 points

    Christian, 

    I appreciate your response and recommendation regarding the BQ25672.  I do agree that it likely fits the requirement, but quite a bit over the budget for this project, but also quite a bit over on features and amperage capacity too.  I am curious if you might have any other recommendations, or possibly a good selection guide of the TI products to help make a decision?  

  • 0 in reply to Brad Smith
    TI__Mastermind 35326 points

    Hello Brad,

    Yes, You can refer to our catalog of TI chargers:https://www.ti.com/product-category/battery-management-ics/battery-charger-ics/overview.html

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Prodigy 30 points

    Christian, 

    Quick question, on these batter charger ICs, will they auto throttle the current to stay within the current rating of the IC?  Or can that value be otherwise set or limited?  As an example, we will have to allow a 10W solar panel to be connected in low Sun conditions.  Technically that would exceed a 1A in the right conditions, but we really don't need more than 1A of charging.  

  • 0 in reply to Brad Smith
    TI__Mastermind 35326 points

    Hello Brad,

    Yes, You will need to program the charge current using the charge current registers, this will allow the charger to limit the amount of current provided to the battery. 

    The Solar panel will be able to source the full 10W, but the battery charger can be programmed to prevent it from sinking the full 10W.

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Prodigy 30 points

    Christian,

    Another quick question.  When I review your website, it appears if I select "solar input" as a product feature, there are only 7 offerings.  Would this be correct?  And within the 1-5A output range, only 2 offerings?  I guess I figured there might be more options with solar input?  Just wanting to make sure.  

  • 0 in reply to Brad Smith
    TI__Mastermind 35326 points

    Hello Brad,

    Yes, unfortunately we only have 7 options for multicell chargers  with the MPPT feature.

    • BQ25756E,BQ25750,BQ25820, BQ25756- Same product family, but may be overkill for what you are looking for.
    • BQ25798- a buck-boost version of BQ25672.
    • BQ25672- already talked about
    • BQ24650-Standalone option, will need to use hardward to modify charge voltage/

    Best Regards,

    Christian

  • 0 in reply to Christian Moyer
    Prodigy 30 points

    Christian, 

    We have been evaluating options and hovering around the BQ25672 as discussed.  However, in the data sheet, we are having issue determining if it can be properly setup for LiFePO4 chemistry.  Ideally we would like full I2C control of battery voltage and temp status via our MCU.  As we examine the register tables, it does not seem apparent this IC really wants anything other than pre-programming values?  

    Our application again is a simple 5W solar panel as primary, but if that fails, plug in a rigid voltage adapter to the same port.  We were going to use a simple switch to change input sources if the charge loop would otherwise collapse.  

    Can you give a basic explanation on setup if this IC can indeed work?  Our 2S config would ideally run between 5.8-7.2V, and would like a bit of control via I2C via our MCU.  

  • 0 in reply to Brad Smith
    TI__Mastermind 35326 points

    Hello Brad,

    Brad Smith said:
    As we examine the register tables, it does not seem apparent this IC really wants anything other than pre-programming values?  

    Yes, This device can work in standalone and I2C host controlled mode. In order to working standalone mode, the charger will need to start up in a default programmed status in order to start charging. After startup the user can adjust the charge voltage to whatever they need using the register.

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Prodigy 30 points

    Christian,

    That's what I'm asking.  The register and data sheet are not clear on how and what to adjust for this.  Reviewed by 3 engineered in this office, and none are clear. Please clarify what you wrote.  "start up in default mode", then "adjust voltage to whatever", I don't see any of that.  I also don;t see any default settings which could be an issue for the design.  

  • 0 in reply to Brad Smith
    TI__Mastermind 35326 points

    Hello Brad,

    Brad Smith said:
    start up in default mode"

    There is a PROG pin on the IC to program the cell count and switching frequency.

    This will program the charge voltage register. For example: If you connect a 3kohm resistor from PROG to GND, the charge voltage register will default to 4.2V. This is mainly used for li-ion. 1s=4.2V,2s=8.4V,3s=12V.6,4s=16.8V. 

    The charge current register has a default of 1A for 1S/2S and 2A for 3S/4S.

    So, if you set the Prog pin to 3kohm, it will charge at 2A, until it reaches 4.2V.

  • 0 in reply to Christian Moyer
    Prodigy 30 points

    Christian, 

    Thank for for that.  I have a few other questions as we have to work through several false safe conditions.  

    1.  Is it stated somewhere the memory type or if/when I2C register writes are forgotten and the IC reverts back to defaults?  Basically volatile or nonvolatile.   This is a huge concern as the default battery voltage for 2S is 8.4V, in which that would destroy our LiFe pack in the event of a glitch or MCU issues.  

    2.  Could you speak to the behavior of the two input sources we would have, one being solar, other being a rigid adapter.  Is the MPPT input loop good enough to figure this out?  Our solar would be approx 22Voc, 19Vmp, but if adapter is used, 12V.  

    3.  I'm also currently reviewing the data sheet for ADC performance and any stated accuracy or resolution of the battery.  Could you point to that?  We are probably not as concerned with 'accuracy' per se, but enough resolution that we can estimate the state of charge rather than a dedicated monitor ADC.  

  • 0 in reply to Brad Smith
    TI__Mastermind 35326 points

    Hello Brad,

    Brad Smith said:
    1.  Is it stated somewhere the memory type or if/when I2C register writes are forgotten and the IC reverts back to defaults?  Basically volatile or nonvolatile.   This is a huge concern as the default battery voltage for 2S is 8.4V, in which that would destroy our LiFe pack in the event of a glitch or MCU issues.  

    No, the BQ25672 does not have memory. Yes, unfortunately, this is a possibility. If this is a concern for you, then you may need to switching to a different charger such as the BQ25820.

    Brad Smith said:
    2.  Could you speak to the behavior of the two input sources we would have, one being solar, other being a rigid adapter.  Is the MPPT input loop good enough to figure this out?  Our solar would be approx 22Voc, 19Vmp, but if adapter is used, 12V.  

    I recommend disabling MPPT when you switch from solar input to DC supply.

    Brad Smith said:
    3.  I'm also currently reviewing the data sheet for ADC performance and any stated accuracy or resolution of the battery.  Could you point to that?  We are probably not as concerned with 'accuracy' per se, but enough resolution that we can estimate the state of charge rather than a dedicated monitor ADC.  

    We don't spec the accuracy of the ADC. The internal ADC is mainly used as a general indicator of charger input power/output power.

    We have a 15bit effective resolution.

    Best Regards,

    Christian

  • 0 in reply to Christian Moyer
    Prodigy 30 points

    Christian, 

    I am looking either for supporting documentation, or assistance in wiring the BQ25672 to our MCU.  We have decided to supervise all charging through the MCU and control that with the CE pin to enable/disable charging.  I am now looking for the best approach to receive a "charge adapter detected" from the charger to our MCU when the circuit is sleeping, which will trigger the MCU to wake up.  Our I2C buss will be offline during sleep so looking for a possible logic level up or down signal we can use.  I cannot make sense of the QON and INT functions to determine a best strategy.  

  • 0 in reply to Brad Smith
    TI__Mastermind 35326 points

    Hello Brad,

    Brad Smith said:
    QON

    We use this for a feature called shipmode, basically when you have a product that has a battery installed, it is recommend to put the battery charger into ship mode, to reduce the power that is being drawn from the battery when the battery is being shipped. To exit shipmode, you need to apply a signal on this pin to get it out of shipmode.

    Brad Smith said:
    INT

    I recommend using this pin, this is the interrupt pin. The charger has multiple status_flags that will produce a pulse on the INT pin: poorsource detection, PG(power good), Watch_dog timeout, ETC.

    If you have the battery connected first, you can write to the BQ25672 and maskout all of the status indicators that produce a pulse on the interrupt pin.

    Maskout every thing in these two registers, except PG, keep the PG_Status

    If you don't have a battery, you will need to wait for the interrupt pulse, then read the flag registers, to determine if the interrupt is from PG or poorsource.

    Best Regards,

    Christian.