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Battery Power Management Circuit Design Help

Kuber Singh
Prodigy 30 points
Other Parts Discussed in Thread: BQ24765, BQ24617, BQ24618, BQ24616, BQ24610, BQ24600, LM5121, BQ34Z100-G1, LM5060, LM5050-1

Hi, 

Our company has an old PCB that uses a lot of the TI IC's for performing its functions. Its essentially a power management board that takes a DC input and charges 12 batteries connected to the circuit. It has a power on/off button and a status LED light to show the overall battery level. Its a really old design and the designer has left. Instead of reverse engineering this, we would now like to prepare a alternate design with similar or better functionality. Basically, we need to power networking devices over PoE. So the board must have at least 5 PoE ports and be able to power a load of 15W for up to 80 hours. This should be enough to help us narrow down on the battery capacity and requirements. Wondering if anyone has any tips on how to get started on a circuit like this? I did robotics and specialised on the software side and wish I paid more attention in my electronic classes as well :( but its never too late to learn and would appreciate all the help I can get to help make build a circuit that achieves or exceeds this. 

  • 0
    TI__Guru 71526 points

    Hi Kuber,

      Wondering if you could provide some additional detail for this design, please.  A block diagram and/or power tree would be very helpful in putting together some recommendations, as I want to be clear on what the exact requirements are before any recommendations.  For example, I'm not clear on where the 12 batteries are, and what their specifications are.  As well, are the batteries on the network devices, or are they on a hub, and the hub then powers the network devices either from the power mains or the 12 batteries?

    For example, here is a Ref Design for a POE supply with Battery Charger: http://www.ti.com/tool/pmp4745 .  How close might this be to what your design requirements are?

    Looking forward to working with you toward a suitable design.

    ~Leonard 

  • 0 in reply to LeonardEllis
    Prodigy 30 points
    Hi Leonard,

    Thank you for having a look into this and my apologies for the confusion. I'll try to be more clear with the design requirements. Have made a short block diagram of our existing PCB design on https://goo.gl/hEKuEe.

    In this you will notice that the power management board takes in Vin at 24VDC. The board charges the 12 batteries individually connected from B1 to B12. From what I have heard from the users who use the boards is that there is embedded logic on the micro controller on board to maintain the batteries at an adequate charge level which means that the batteries are discharged and charged as needed based on the battery life charts to keep them healthy. There is a power on/off button to power the whole unit and a status LED array to show the overall battery level. The objective of the board is to supply enough power on the 5 PoE regulated ports to power a wireless access point from Cisco for about 72 hours or more. Ours does 80 hours currently. The battery and board are then finally packaged with the access point in a IP67 industrialized box so that the access point can be used to provide WiFi in a mesh network.

    Looking at the board closely, it looks like each battery is regulated by a TI BQ7692003 battery monitoring IC. There are other IC's on the board as well but the serials are a little tough to read. It seems like there is a mux on board and a TI microcontroller for sure. But like I said, its probably much tougher to reverse engineer this than to come up with a fresh design that can meet or exceed this.

    Let me know what you think and apologies once again in case I am lacking in my explanation.

    Regards,
    Kuber
  • 0 in reply to Kuber Singh
    TI__Guru 71526 points

    Hi Kuber,

     Thanks very much for the additional information.  I am asking out BQ and PWR applications to review and respond, so you will be hearing from them soon.

    ~Leonard  

  • 0 in reply to LeonardEllis
    Prodigy 5 points
    Hello Kuber,
    What kind of battery is it exactly? Do you have a part number?
    Your system needs to provide power for 80 hours.
    This leads to a Watt Hour rating we can work with.
    Vout, Iout, Pout.
    This will tell us, with 24Vin, whether we buck or boost to charge the battery and supply the load.
    Once we get some detail we can define what is needed.

    Here is a thread that discusses POE battery backup.
    e2e.ti.com/.../1249590

    Industrial Battery Packs have some examples of battery monitoring.
    www.ti.com/.../industrial_power_packs_battery_packs
  • 0 in reply to Ed Walker
    Prodigy 30 points
    Hi Ed,

    Thanks for getting back to me!

    So each of the 12 batteries in the enclosure is a 4000mAh LiPo battery rated for 14.8V.

    Thank you for sharing the notes from the other thread. Its a good share as they seem to be charging from a PoE source as well and then discharging down another port. So seems like their application is a hub of sorts. Ours is also the same but uses a 24VDC adapter as the Input to charge the 12 batteries and provide power eventually down the 5 PoE ports rated for 802.3af.

    Thanks again,
    Kuber
  • 0 in reply to Kuber Singh
    Prodigy 5 points

    Hello Kuber, Thank you for the reply.

    Here are a few last comments:

     

    You have a 4S, 4 series, LiPo pack.

    Maximum charge voltage per cell is ~4.2V, Minimum is ~3V.

    So the pack voltage range is 16.8-12V.

     

    Charging from your 24V source would only require a buck, stepdown, charger.

    Recreating the 24V Aux rail for the PoE requires a boost switcher.

     

    Typically you can charge up to the C rating of the battery.

    4000mA hour means you can charge up to 4 Amps.

    BQ24765 is a charger with SMBus and Integrated 10Amp FET’s.

    SMBus Controlled Battery Charger with Integrated FETs

    http://www.ti.com/product/bq24765?qgpn=bq24765

     

    TI has several “stand alone” chargers that would work here.

    These use external FET’s

    BQ24600, BQ24618, BQ24616, BQ24617, BQ24610

     

    For the 24V boost I would recommend a Synchronous Boost.

    This would provide highest efficiency, lowest heat dissipation, for your small enclosure.

    LM5121 is what I would recommend.

    3-65V Wide Vin, Current Mode Synchronous Boost with Disconnection Switch Control
         http://www.ti.com/product/lm5121?qgpn=lm5121

    It has an “input disconnect switch” that will remove the boost circuit from the battery when not in use.

     

    Consider a battery pack Fuel Gauge IC to keep track of each packs capacity.

    Here are 2 I recommend.

    SBS 1.1 Compliant Gas Gauge with LED/LCD Drive feature and Impedance Track™ Technology

    http://www.ti.com/product/bq20z655-r1

     

    BQ34Z100-G1 Multi-Chemistry Impedance Track Standalone Fuel Gauge | Battery Gas Gauge

    http://www.ti.com/product/bq34z100-g1

     

     

     

    For ORing the batteries consider:

    LM5060 High Side controller.

    See figures 39 and 41 in the datasheet.

    This ‘back to back’ fet arrangement allows you to select the battery you desire to use regardless of its pack voltage. With other typical ORing circuits ( LM5050-1 )the mosfet body diode will conduct with the highest voltage battery pack, even if it’s not selected.

    http://www.ti.com/product/lm5060

     

    It would be easier to try and use a larger capacity pack and fewer of them.

    ORing the batteries, selecting which one to charge and discharge, is very complicated.

    Presently you have 12  4000mAhr 4S packs.

    You could make a single 4S12P pack, or two 4S6P.

     

    Good luck with the project.