Backup DC Power Box With Audio Amp - May Need to Run on AC

Discussion in 'The Projects Forum' started by juddco, Apr 9, 2015.

  1. juddco

    Thread Starter New Member

    Apr 9, 2015
    I'm building a combination backup power / audio amp.

    I have a 12V / 35Ah battery, a 400W inverter, numerous DC outlets, a cooling fan, and a 2 Channel, 600W amp is on the way. My housing is a 40MM ammo box. I'm going to run 6X9 speakers in two 50cal ammo boxes.

    I think that the battery will run out before the party does, so I want to be able to plug it into AC. I'm integrating a charger for that purpose.

    I understand that it's a bad idea to run a charger, the battery, and the load on the same circuit. I'm looking for an alternative. Can I use a cheap solar charge controller for this? Or should I use a manual 4PST switch to disconnect the battery and run on the DC power supply?

    Any ideas on how to do this safely?

    I'd love any other feedback or suggestions on the design / approach.

    Thanks very much for any response.

    P.S. How much should I worry about grounding the amp or any of the other components? Should I ground it on the box and have a spike protruding out?
  2. Søren

    Senior Member

    Sep 2, 2006
    Yeah, even if we pretend that you'd see no losses in the up-conversion, you'd have max. 420Wh to spend and that's with a new and fully charged battery. Add in the losses, say 80% efficiency and you're left with 336Wh.
    Then comes the fact that lead-acid batteries are usually specified from a 20h discharge where 35Ah means that it can give 1.75A for 20h.
    As current drain goes up, capacity goes down, so it's quickly going to be a sad party - perhaps change your parties to unplugged sing-a-longs :D

    A 400W amplifier doesn't take a constant 400W, that's only in the peaks, so you're looking at much less than 400W average. Before you ask me how much less... That depends on the music and it's impossible to guess, it has to be measured for each "song" that you're likely to play.

    To get a longer runtime, just turn the volume down a bit - it's barely discernible if you're playing 600W or 300W.

    And the ultimate way to save power... Get very efficient speakers. I've had some that needed 100W to give the same SPL as another pair using around 1W. the former was obviously better behaved, as the amplifier had a very good grip on the speakers, but at a party, who cares - it's just background noise for boozing, doping and something that starts with forni ;)

    Another way to increase runtime would be to convert the amplifier to run on low(er) voltage DC and avoid the 12V to mains converter.

    Whoever gave you that wrong idea?
    You just have to assure that the charge voltage never gets higher than your equipment can handle. The amp will run from the charger (assuming the latter has the power to do so) and the excess will go into charging the battery. The battery will work as a ginormus capacitor as long as it's coupled to the charger, so it's best to run the charge voltage at something the battery can handle continuos, like 13.5-13.8V.

    The charger should be grounded through the mains connector.
    The 0V of each component should all be connected to the 0V on the amplifier. If they are, you should stop worrying and enjoy the party.

    Only if vampires and/or zombies attend the party ;)
    juddco likes this.
  3. juddco

    Thread Starter New Member

    Apr 9, 2015
    Thanks for that, Soeren! You got me off the dime - I was stuck as I thought about this problem.

    What I think I'm going to do (my parts are arriving before this weekend):
    - Plug it all up as described in the diagram. (with the charger out of the loop)
    - Run the amp and speakers at what I think the actual volume will be. More of a cocktail party, not a rave. :)
    - Test actual consumption.
    - Order a charger based on that actual consumption. I have a 3A charger on the way...

    I'll take a shot at this over the weekend, and then I'll post an update, with a proposed new schematic.

    Thanks again!