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SLA charging for Lepai 2020 boombox

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  • SLA charging for Lepai 2020 boombox

    Question about a 12V SLA-powered boombox. I've seen recommendations on this forum to use an inexpensive "Battery Tender" trickle charger/float charger (not sure I know the difference), typically at 0.75 Amps, to keep these SLA batteries charged. If, however, battery power runs out, these chargers would not be able to power the Lepai 2020 (2 amp recommended, 5-6 amps preferred from what I've read).

    The higher-amp SLA chargers are big, expensive, and designed for car batteries. They sound like overkill for something like this.

    My thought was to combine an inexpensive 12V 5Amp DC power adapter (less than $10 on Amazon) with an inexpensive 10Amp solar charge controller (about $10 on Amazon). Substituting the 12V 5Amp DC power for the "solar" inputs, it seems like this would be a great way to supply a full 5 Amps of power to the amp when the battery is low, safely charge the battery, and allow the SLA to power the amp when unplugged.

    Does this make sense? Am I misunderstanding how these devices work?

  • #2
    Re: SLA charging for Lepai 2020 boombox

    Tenderfoot,

    You don't mention a critical parameter; the amp-hour rating (ah) of your battery.

    In the absence of that ... first the voltage overview.

    SLA-AGM batteries are nominally 12 V as are the flooded lead acid batteries likely in your car. However, they have different operating voltages. See the State-of-Charge charts for both types, below. The AGM batteries are ~$5% higher voltage.

    Click image for larger version

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    The solution to your problem is this $21 - $25 3-stage smart charger: http://www.batteryspace.com/Lead-Aci...ldwide-UL.aspx

    As in the description, it starts with a 3 amp charge, then continues to 14.7v (mine measured 14.9 v), then drops to maintenance mode. Bringing the battery up to 14.7 v removes much of the deposits that build up on the internal structure maximizing service life. I've used a 12v 18ah battery on the beach. Almost everytime there, the battery was deeply discharged to where the power inverter would shut down (10.5v near 100% discharge). It lasted 9 years before it gave out. The 3 amp charger was a perfect size for the 18ah battery.

    The problem is the ah rating of the battery you plan to use. The 3 amp charger is too big for a 7ah battery (and smallish for a 35ah). With an 18ah battery and the 3 amp charger, you could put 2 amps into the amplifier and the 3rd amp into recharging the battery. The unit may cycle each time the battery tops out at 14.7v but I don't see that as a problem. A 1 amp charger would be ideal for a 7ah battery. The battery having some charge would help with short term heavy demand from the amp. Theoretically, you couldn't run continuously given your estimated current need. But if you started with a decent charge on the battery, I'd bet you run for a very long time. The vendor in the above link has a chart of the various size chargers it offers.

    The Lepai will only put out 13 w rms per channel at 4 ohms unless your into distortion (half that into 8 ohms). Running a sine wave on both channels without clipping, you'd be using 2 amps. In music, peaks come but as mentioned before, the battery would supply the needed amps during the peak. Overall I'd be very surprised at 2 amps averaged over long periods. That would drive your battery size: how many hours battery life you desire. That requires another calculation and you simply can't divide the ah rating by 2 amps to get your useable time.

    Lastly, never leave the battery discharged. If you use it mobile. Put it back on charge as soon as you get home. Leaving a lead acid battery discharged will kill its service life faster than anything else - short of a short (pun?). If you go on extended road trips, get a small power inverter for the car to run the smart charger if you won't be near an outlet for hours.

    Regards,

    Millstonemike
    Attached Files
    Last edited by Millstonemike; 12-17-2015, 07:59 PM.

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    • #3
      Re: SLA charging for Lepai 2020 boombox

      The trick is not to let the battery get badly discharged before putting it on a charger. And use the right kind of charger, discussed above.

      Music is highly dynamic so power draw varies over time. Most of the time, the amp is pulling very little current, but when it gets loud, current draw rises exponentially.

      The idea is to size the charger so it has enough capacity to cover the average draw. Use the battery to level out the voltage; charge when current draw is low, discharge when the draw is high.

      I like your solar cell idea, and the cost of such a system is coming down, but the critical part is the control circuitry in the smart charger.

      HAve fun,
      Frank
      Take the battery out of the circuit,

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      • #4
        Re: SLA charging for Lepai 2020 boombox

        Originally posted by tenderfoot View Post

        My thought was to combine an inexpensive 12V 5Amp DC power adapter (less than $10 on Amazon) with an inexpensive 10Amp solar charge controller (about $10 on Amazon). Substituting the 12V 5Amp DC power for the "solar" inputs, it seems like this would be a great way to supply a full 5 Amps of power to the amp when the battery is low, safely charge the battery, and allow the SLA to power the amp when unplugged.

        Does this make sense? Am I misunderstanding how these devices work?
        You are not far off, but here's a couple of things to consider. Your power supply needs to be higher than 12V. The voltage of a lead-acid battery during "bulk" charge will rise to 14+ volts. You need at least that. The cheap solar charge controllers are simple on-off "shunt" units. They take forever to bring a battery up to full charge. You would need to spend a bit more to get a "PWM" type.

        The advice you got about not discharging the battery and leaving it is spot on. Best practice is not to discharge below 50%.

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