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AC / Li-ion Battery Power Supply: 24 V DC Output for DC Amplifiers

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  • AC / Li-ion Battery Power Supply: 24 V DC Output for DC Amplifiers

    Safe Harbor Statement

    The following is a theoretical design. It is untested. Use at your own risk.

    This is an updated design based on recent information from Sure (and Gordy, thanks).

    Overview

    The picture shows an AC / DC 24 V Li-Ion battery system for powering portable boomboxes using Sure's new Battery Management Module (BMM).

    The design shows: (1) Battery operation using two BMMs in series to run an amplifier at 24 V; and (2) AC operation using a 24 V AC PS to charge the batteries and simultaneously power the amplifier. A DPDT switch (or automatic relay) allows the user to switch between AC PS or battery operation.

    Each Sure BMM supports 3S Li-ion 18650 unprotected cells for a nominal 12 V output. As with the KAB battery module, the new Sure BMMs include cell protection (cell under/over voltage and max discharge current: 10 A). The new modules also include charge current management (1.5 A). The charge current management function is on the KAB amp modules for the KAB BMMs and cannot be used in this design.

    Battery Operation

    When the switch is in the BAT position, the BMMs are wired in series and provide a nominal 24 V to the amplifier. If the AC PS is plugged in, it will not charge the batteries nor will it power the amp as the fully charged batteries are at 25.2 V. The diode prevents any battery output from: (1) entering the AC PS; (2) entering the battery charge ports; and (3) tripping the auto relay if included. As the batteries drain, their voltage will drop below 23.5 volts. At that point, the 24 V PS (minus 0.5 Voltage drop in the diode) will power the amplifier and the batteries will not provide any energy to the system.

    AC Operation

    When the switch is in the AC position, the BMMs are wired in parallel. Each BMM sees 24V and will charge the batteries. The AC supply will also power the amplifier at 23.5 V. In this position, if the AC PS is removed, the amplifier will be powered at 12 V by the top BMM.

    Optional Relay - Automatic Operation

    A DPDT relay can be used in place of the switch. Without an AC PS, the relay's default position is battery operation with the BMMs in series. The batteries will power the amplifier at 24 V. When an AC PS is connected, the PS DC output will trip the relay to the AC position. The AC PS will charge the batteries and power the amplifier. The diode prevents the batteries from tripping the coil when the AC supply is not present. With the auto relay option, the unit cannot be set to run on batteries if the AC PS is connected.

    Notes:

    (1) We assume the amplifier has a on-off switch to turn it off, else it will always be on - powered by either the AC supply or the batteries.
    (2) Additional BMMs can be wired in parallel with each BMM for longer run times. Additional amps will be required from the AC PS during charging. You'll need at least a 6 A PS just to charge 4 BMMs. That's hard to come by in the convenient "Brick" format; easy for a "cage" type supply such as a Meanwell. If this was my go-to 2.1 system with four BMMs, I would use a 24 V, 8 - 10 A supply for home operation. If the batteries have a reasonable charge, then little current will be diverted to charging then. Then I'd use a 24 V 5 A "brick" PS on my friends patio or what not.
    (4) The main diode connected to the AC PS + (on top) should be minimum rated at 10 A. Two diodes in parallel can be used to achieve minimum the amp rating. The two diodes connected to the BMM's Vcc+ should be rated at 3 A or more.

    Click image for larger version  Name:	LI-Ion Battery.jpg Views:	1 Size:	99.7 KB ID:	1383311
    Attached Files
    Last edited by Millstonemike; 07-26-2018, 09:40 AM. Reason: Added protection diodes to BMM Vcc+ per recommendation from Sure Electronics (via PE Patrick's post # 8). Rearranged diagram to reduce complexity - note new positions of BMMs Vcc+ and Bat +.

  • #2
    I greatly appreciate you taking the time to write this up! Thank you...

    Comment


    • #3
      Originally posted by Millstonemike View Post
      ...Battery operation using two BMMs in series...
      Mike, have you verified with Sure that it is OK to wire the outputs of 2 of those boards in series?

      Comment


      • #4
        Originally posted by 1100xxben View Post

        Mike, have you verified with Sure that it is OK to wire the outputs of 2 of those boards in series?
        Not directly. But the basic Li-ion multi-cell structure uses cells in series (as each BMM does to achieve 12 V) and many applications put individual BMM packs in series.

        ​​​​​​​Also, the BMM's discharge current protection is set at 10 A. So the 24 V system can handle a 2.1 amp with a max theoretically peak draw of <6 A (4 ohm sub and 8 ohm L-R).

        Comment


        • #5
          Originally posted by Millstonemike View Post

          Not directly. But the basic Li-ion multi-cell structure uses cells in series (as each BMM does to achieve 12 V) and many applications put individual BMM packs in series.

          ​​​​​​​Also, the BMM's discharge current protection is set at 10 A. So the 24 V system can handle a 2.1 amp with a max theoretically peak draw of <6 A (4 ohm sub and 8 ohm L-R).
          First, I'm assuming when you say battery management module, you're referring to a circuit that handles protection/monitoring/balancing of the individual cells. I'm more familiar with the term BMS (battery management system). As long as we're on the same page here, then I'll continue. If we're talking about something entirely different, then correct me and ignore the rest of this.

          The reason I bring up the question is that a typical BMS will have a low-side switch to disconnect the cells in an OV, UV, or OC scenario. If you stack 2 battery packs, the low-side FET will see double pack voltage (if it opens to protect the pack), which can exceed the Vds if it was not designed to be wired in series with another pack. For example, I know specifically of a 6S pack with a BMS that has a low-side FET rated for 30V. If wired in series with another pack, the first pack to open the low-side FET will see >40V across it, potentially causing damage. Just something to be aware of in case someone tries this and starts blowing up boards.

          Comment


          • #6
            Originally posted by 1100xxben View Post

            First, I'm assuming when you say battery management module, you're referring to a circuit that handles protection/monitoring/balancing of the individual cells. I'm more familiar with the term BMS (battery management system). As long as we're on the same page here, then I'll continue. If we're talking about something entirely different, then correct me and ignore the rest of this.

            The reason I bring up the question is that a typical BMS will have a low-side switch to disconnect the cells in an OV, UV, or OC scenario. If you stack 2 battery packs, the low-side FET will see double pack voltage (if it opens to protect the pack), which can exceed the Vds if it was not designed to be wired in series with another pack. For example, I know specifically of a 6S pack with a BMS that has a low-side FET rated for 30V. If wired in series with another pack, the first pack to open the low-side FET will see >40V across it, potentially causing damage. Just something to be aware of in case someone tries this and starts blowing up boards.
            These packs are rated at a nominal 24 V and will never see more than that in either mode of operation. So I think tripping the protection will not cause an element failure. And typical Vds is 30 V, especially for FETs that can handle 10 A or more.

            Wish I had a need for these to experiment. But I already have a 6S, 24 V, 18 aH pack with a single positive terminal and a computer charger.

            Note, I've been thinking of adding another diode out of the the upper modules Bat + terminal to avoid the AC PS DC from entering that terminal. Though I believe The Bat + is a high impedance to positive voltage.

            Comment


            • #7
              Our PM reached out to Sure regarding the design. They mentioned the design may cause short circuit and sent over a revised schematic for reference. We are also bringing this in as a Dayton Audio product. Our version will have external LED's and external charge status indicators.

              Comment


              • #8
                Originally posted by Parts Express Patrick View Post
                Our PM reached out to Sure regarding the design. They mentioned the design may cause short circuit and sent over a revised schematic for reference. We are also bringing this in as a Dayton Audio product. Our version will have external LED's and external charge status indicators.
                Stop it. . I just removed the micro LED and soldered leads on for an external charge indicator of my Sure board. Oh well, I don’t mind being a trailblazer.

                Any ETA on the Dayton board? Lofty I know but I have to ask.
                "A dirty shop is an unsafe shop, if you injure yourself in a clean shop you are just stupid" - Coach Kupchinsky

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                • #9
                  Originally posted by Gordy View Post

                  Stop it. . I just removed the micro LED and soldered leads on for an external charge indicator of my Sure board. Oh well, I don’t mind being a trailblazer.

                  Any ETA on the Dayton board? Lofty I know but I have to ask.
                  Haha that's too funny. And, yes! ETA is set for early August.

                  Comment


                  • #10
                    Originally posted by Parts Express Patrick View Post
                    Our PM reached out to Sure regarding the design. They mentioned the design may cause short circuit and sent over a revised schematic for reference. We are also bringing this in as a Dayton Audio product. Our version will have external LED's and external charge status indicators.
                    Thanks for that. I have updated the diagram in the original post to include the protection diodes on the BMM's Vcc + terminals (and rearranged the diagram for simplicity).

                    Comment

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