Building bike speaker, having SLIGHT power/charging issue...

Discussion in 'The Projects Forum' started by five4three2, May 11, 2013.

  1. five4three2

    Thread Starter New Member

    May 11, 2013
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    0
    Hello,

    My project design is in its final stages, but alas, I am new at this to say the least, and I'm looking for a quick fix, or any fix really. I need to find a way to charge SLA's at their ideal current, while powering an amplifier at a higher current.

    Basically, I am modding a passive speaker into an active one, then fixing to my bike, but I need to get it working first.

    The amp take a range of input voltage, (see specs in link); I'm aiming for 24V. To power it, I have 2X12v SLA, 10aH each, in series to get 24V DC. On battery mode, its pretty straight forward, no issues there.

    The thing is, I want to be able to charge it and run it at the same time. My initial reaction was to buy a 24V power transformer AND a charger, and use a switch to shift the amp to run on the transformer, and then charge on a separate circuit. I now understand that most SLA chargers are regulated, with charging while consuming not really an issue.

    This WOULD solve my problem, but at 10 aH, I would like a charger that gives around 1A at 24 V. The amplifier, on the other hand, draws a maximum power of around 120 Watts (connected to an 8 ohm speaker), with an average estimate around 90 at full volume, so I'm looking at (roughly) 3-6 amp draw at full volume (which would be a nice option to sustain while plugged in).

    What I want:
    When plugged in:

    1. 28V DC (13.8V x 2 for SLA charge voltage) at 1A going to the batteries. I need all the perks of a regulated charge, so that I can just leave it plugged in indefinitely without damaging the batteries.

    2. At least 6A at 24V (give or take a lot, can be 10-36V, higher is better actually) available for the amp.

    3. Can just charge the batteries.

    Here are my ideas/issues:
    1. Hooking up a 4A regulated charger to the batteries, and the amp to the batteries. This seems like it will be too much current for the batteries, and not enough for the amp. These charges are also expensive/bulky.

    2. Using a 1A-2A regulated charger as in 1. I (think) this will not really power the amp as much as it needs, or the amp will just suck up all the current, leaving none to charge the batteries.

    3. Using an unregulated transformer, i.e. the one that comes with the amp (linked). This is fine for the amp, but I understand hooking the batteries into this circuit would be disastrous.

    SPECS:
    The thing is, I already have the transformer and amp listed here.
    http://www.sureelectronics.net/goods.php?id=1762

    So, I am thinking I can have a ciruit like this:
    [​IMG]

    Note that the two switches here are the same switch (i.e. 3 way with those as two of the options)

    The "thing" regulates the charge to be 1A, constant voltage, and tapers the current when the batteries are at full charge (to account for unloaded discharge). I thought maybe a solar regulator could do the trick, or something like that?

    MY QUESTION:
    Does anyone know of such a thing? I know this chip (http://en.wikipedia.org/wiki/LM317) has a lot of the desired qualities, but will it protect it from overcharge?

    Otherwise, I could just make the "thing" in the diagram the 24V 1A charger itself (http://www.rapidonline.com/Electric...-18-0742/?source=googleps&utm_source=googleps), and hook it up to the AC source, but that would be heavier and more expensive then finding the "thing" I want.

    Thoughts on the best way to do this? Any advice welcome really. Thanks a bunch!!
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,965
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    What sort of bike is it going on, push bike or motor?
     
  3. five4three2

    Thread Starter New Member

    May 11, 2013
    3
    0
  4. wayneh

    Expert

    Sep 9, 2010
    12,086
    3,024
    It would be great to know a bit more about that power supply. It is likely regulated to nearly 24V and therefore would not be great for keeping your batteries at full charge. It could charge them a bit if they've been drawn down below 24V, but it won't take them up to 26-27 for a full charge. Maybe you can plug it in and see what it does?

    The LM317 would work fine for a charger if you have, say, 29V or so at the power supply but my bet is that you don't. You need a few extra volts to cover the dropout of the regulator.

    If you add a separate charger, it can be "charging" while the amp is running, no worries. It will fall behind, the battery voltage will slowly drop, but the charger won't be damaged. A dead short might not even damage it if has built-in protections, and anyway you'll always have at least the battery voltage on the charger's output. Battery voltage should never be less than 20V or so, for the sake of the batteries.
     
    Last edited: May 11, 2013
  5. five4three2

    Thread Starter New Member

    May 11, 2013
    3
    0
    thanks for your help!

    it has an adjustable voltage on, so there might be a little wiggle room there. I will give it a test and see if I can't push it up to 27V.

    but would the chip/circuit protect the batteries from overcharge, i.e. drop current as full charge is reached?


    I would really just like to run the whole thing of a charger, and return this transformer or sell it on ebay. Occam's Razor is a great principle.

    But like I said before, what kind of current would I want on the charger to make sure the amp gets enough power, but the batteries don't get too much current?
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,086
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    Yes, it works fine. The current just keeps dropping as the setpoint voltage is approached. I charge my boat battery during the winter using a LM317-based charger. I can leave it connected for weeks on end but the current is nearly zero after the first few hours.

    The biggest negative with a simple LM317 charger is that it cannot provide high current to quickly charge a low or dead battery. It will try, and may get hot and put itself into protection mode. So if you know it will be often used at full blast, you need to design for that. (Heat sink, maybe a higher-current version of the chip, use a power transistor, etc.). In my little device, the transformer is limiting and the LM317 never even gets warm.
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,086
    3,024
    To answer that you need to know the maximum charge current recommended for your batteries.
     
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