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Original question:

Will TI's solutions for "Lead-Acid" chemistry also work with AGM?

Battery Management Components for Solar Application (REV 1)

Seamus de Mora
Prodigy 250 points
Other Parts Discussed in Thread: BQ24450, BQ24650, BQ25798, BQ24610

I am revising this question in hope of getting an informed & straightforward answer. After a few hours searching TI's website, and reading App Notes and Data Sheets, I may be more confused than when I began!

Let me begin by stating my requirements:

A "portable" power system consisting of a) solar cell, b) a 12V AGM or Gel battery, c) a "Controller"

12 VDC nominal output from the power system

It is the "Controller" that I would like some help with. I would prefer an MPPT-type controller over the older PWM-type controller.

I was encouraged by my findings here initially. It appeared there were two parts here that could potentially meet my needs very well: BQ24450 and the BQ24650; a PWM controller, and a MPPT controller. It was immediately clear that the BQ24450 supported Lead-Acid batteries, but the BQ24650 was far more ambiguous: On the first page of the datasheet, it is claimed, "Cell chemistry: Lead Acid, Li-Ion/Li-Polymer, Lithium Phosphate/LiFePO4". However in para. 8.1 of that same spec sheet it says "The BQ24650 is a highly integrated solar input Li-ion or Li-polymer battery charge controller." And then there is an AppNote titled, "Using the bq24650 to Charge a Sealed, Lead-Acid Battery (Rev. A)".  However - based on the extensive external circuitry required to support the Lead-Acid chemistry in this AppNote, it seems that the BQ24650 designed to support Lead-Acid chemistry only as an afterthought. If I've overlooked or mis-stated something in my summary, please correct me.

I feel confused after reading these documents, and so I am asking for some clarification: Is the BQ24650 TI's Best Recommendation for implementing an MPPT Controller for Lead-Acid batteries? If so, which of the two AppNotes (SLVA437A, or SLUA992) does TI recommend to realize a MPPT Controller for lead-acid batteries?

Thank you.

-------------------------------------

ORIGINAL

I need a battery management solution (IC??) for an AGM/Lead-Acid battery (6 volt or 12 volt) that can perform the following functions: 

1. Regulate the charging cycle: Signal when the battery is fully charged, and when charging should be discontinued - or determine the proper amount of low-level charging.

2. Signal when the battery has reached a "low voltage shutoff" point - a point at which the battery should no longer be discharged. 

The system which the battery powers is a portable one. In some situations, the charging source will be solar-powered, in others it will be powered from a mains source (120/230 VAC, 50/60 Hz).

Thanks for your help!

  • 0
    Prodigy 250 points

    In at least a partial answer to my own question, I have found the BQ24650. The BQ24650 is designed for Li-Ion chemistry of course, but the App Note SLVA437A shows how to adapt it for use in charging Lead-Acid batteries. 

    However, I would still like to hear from anyone with advice, comments, suggestions - or simply "better ideas" on charging lead-acid batteries from solar cells.

  • 0 in reply to Seamus de Mora
    TI__Guru**** 157571 points

    Hi Seamus,

    BQ24650 is the best option for standalone (no hot processor/controller).  Please see the app note at the link below.

    BQ25798 is another option if you have a host processor and can change some of the default I2C registers from LiIon to Lead Acid.  BQ25798 also has dual input MUX control pins.

    You will need a front end ac/dc converter to step down to 20V range for either charger above.

    Regards,

    jeff

  • 0 in reply to Jeff F
    Prodigy 250 points

    Hello, Jeff -

    I was apparently editing my original question while you were submitting an answer. Apologies for that confusion.

    Yes, the BQ25798 sounds like a solution that is more flexible with fewer peripheral bits of hardware. I did scan through its data sheet quickly, but saw no mention of support for lead-acid batteries under any circumstance. I hope that the register changes needed for the Li-Ion-to-Lead-Acid conversion will be specified clearly at some point?  I can certainly manage an I2C exchange, and the addition of a small processor seems a reasonable tradeoff for a more flexible system.  How would you suggest I proceed - the Eval Kit?

    Next question: You mentioned the need for a "front end ac/dc converter"... Can you elaborate on that ?- I'm not seeing how that fits in.

    Thank you,

    ~S

  • 0 in reply to Seamus de Mora
    TI__Guru**** 157571 points

    Hi Seamus,

    The BQ25798 is configured for CC/CV charging for LiIon.  Lead acid typically prefers CC/CV/float/CV/float/CV etc.  The BQ25798 cannot do this on its own.  Host software will have to provide that functionality.

    Regarding the front end converter, either of the chargers can only accept a dc input voltage in the 20V range, not be directly powered from 120/230V mains supply.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 250 points

    Jeff,

    You indicated the BQ25798 was a possibility, but then seemed to back away from that. If there's no TI support for the software development needed to patch the BQ25798 for use w/ Lead-Acid batteries, that doesn't sound feasible. I guess I should have included "feasibility" as one of my requirements?

    So - perhaps the BQ24650 is the best choice? The App Note "SLVA437A–October 2011, Using the bq24650 to Charge a Sealed, Lead-Acid Battery" provides some specific suggestions. I thought there might have been some improvements on that A.N. since its last rev in 2011. If that's still the "best thinking" from TI, I'll go with it, but please confirm that is the case.

    Finally, if we could revisit this business re the front-end converter again:  This is still unclear. The BQ24650 data sheet lists, "5-V to 28-V Input solar panel" as its 3rd feature. I'm not clear what 120/230V mains supplies have to do with this.

    Thanks,

    ~S

  • 0 in reply to Seamus de Mora
    TI__Guru**** 157571 points

    Hi Seamus,

    There will be a Linux driver available for the BQ25798 when the IC is released toward the end of 2Q.  It will not be specifically optimized for lead-acid but we can provide advice on how to modify the driver for lead-acid.  Unfortunately, TI doesn't provide coding services.

    BQ24650 app note is the latest version.

    The maximum dc input voltage to either charger is in the 20V range.  You stated you wanted to power from either the solar panel or a mains supply, which I assume is AC 120/230V.  If so, then you will need a wall plug in adapter to step down the AC voltage to DC voltage for the charger, or add an AC/DC converter in the device in front of the BQ device.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 250 points

    It's good to hear that there will be a Linux driver available for the BQ25798. Even better if the source code is released. Thank you for that information.

    "BQ24650 app note is the latest version." I can't guess what that means. AFAIK, all TI's app notes, data sheets, etc have numbers and usually titles. It would clarify things if you included those. Heck - I might even be able to find them!

    Ok - at last that's clear re the "20V" comment. But that business about being powered from the mains AC was edited out of my question shortly after it was first posted. I stated the question was being revised, and lined through the original. Apologies if that wasn't clear to you. 

  • 0 in reply to Seamus de Mora
    TI__Guru**** 157571 points

    HI Seamus,

    Thanks for the clarification.  Regarding the appnote in question, I have included the link below.  This explains how to convert the BQ24650 into a lead acid charger.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 250 points

    Hello Jeff,

    Thanks for that clarification. Just so I'm clear on the status of the two app notes that cover using the bq24650 for Lead-Acid battery charging:

    • App Note SLVA437a was last revised in 2011
    • App Note SLUA992 was released in April, 2020

    There are clearly some differences in the schematics, but beyond that, can you describe the fundamental differences between the two approaches, and when one would be preferred over the other?

    Thanks,

    ~S

  • 0 in reply to Seamus de Mora
    TI__Guru**** 157571 points

    Hi Seamus,

    SLVA437A is for the BQ24650, which the charger with MPP tracking for solar panel input.  SLUA992 uses the same circuitry to implement lead acid charging for BQ24610, a sister device to BQ24650, but that device does that have MPP tracking.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 250 points

    I'm afraid your last sentence was a bit muddled... did you mean to say that the "BQ24610, a sister device to BQ24650, but that device does NOT have MPP tracking" ?? In other words, the BQ24610 does not offer MPPT, and so the App Note SLVA437A is the one to use for MPPT applications?

    If the answer to both the above is "YES", this seems to finally get to the answer to my original question:

    "Is the BQ24650 TI's Best Recommendation for implementing an MPPT Controller for Lead-Acid batteries? If so, which of the two AppNotes (SLVA437A, or SLUA992) does TI recommend to realize a MPPT Controller for lead-acid batteries?"

    And so the answer seems to be, "Yes, the BQ24650 is TI's best recommendation for "processor-free" MPPT applications for Lead Acid batteries, and the App Note SLVA437A is TI's recommendation for implementation."

    If you can confirm this as the answer here, we can close this one.

    Thanks,

    ~S

  • 0 in reply to Seamus de Mora
    TI__Guru**** 157571 points

    Hi Seamus,

    Yes, the BQ24650 is TI's best recommendation for "processor-free" MPPT applications for Lead Acid batteries, and the App Note SLVA437A is TI's recommendation for implementation.

    Regards,

    Jeff