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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).


    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.


    (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.

    Click image for larger version  Name:	LI-Ion Battery.jpg Views:	1 Size:	97.2 KB ID:	1381797
    Last edited by Millstonemike; 07-11-2018, 04:55 PM.