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Battery Pack - Some basic questions

Discussion in 'Power Electronics' started by vitalious, Nov 23, 2014.

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  1. vitalious

    vitalious

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    Nov 22, 2014
    Hello everyone,

    I have recently purchased a non-refundable battery for my fish finder in error since I thought my older one is dead.
    Now I have two 12V 9A batteries.

    They both fit comfortably in my fish finder enclosure so I figured I'll keep them both in there and use one of them as a backup and a portable power source for my Phone/GPS/Music while out ice fishing.

    I guess I'm looking for someone to confirm that I have the right idea here.
    I want to order a 12V to 5V 6A converter and connect it to the battery and parallel connect 3 type A female USB cables to the output on the converter.

    Q: Where can I find short cables with a female USB Type-A plug on one end and 2 bare power wires on the other? Will this work well?
     
  2. Anon_LG

    Anon_LG

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    Jun 24, 2014
    You can just strip away the casing of a standard female usb type a to reveal the power wires. Look for an image of the configuration of usb power wires and this will tell you which wires to use.
     
  3. vitalious

    vitalious

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    0
    Nov 22, 2014
    Thanks for the reply. Is it safe to use devices that need 2A to a the converter that supplies 6A? I assume it's up to 6A and the devices will take as much current as needed.
     
  4. Anon_LG

    Anon_LG

    453
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    Jun 24, 2014
    The battery rating of 6A is stating the MAXIMUM current (of 6 amps) it can output, the devices will only draw as much current as they need. So yes, the devices and converter will be fine.
     
  5. vitalious

    vitalious

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    Nov 22, 2014
    Thanks again for the responses.
    I started looking for the components I need to build this and I'm struggling finding something.

    I'm looking for a part that will allow me to select where I want to direct the current, so that I can select whether to connect the battery to the voltage converter or the AC plug for charging. I tried looking for "switch selector" but no avail. Here's a (really) crude diagram of what I'm trying to do.

    Battery usb_0.jpg

    Any idea where I can get a switch like that or what it's called?
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    What you've drawn (or imagined) is probably a double pole double poll toggle switch. You'll want one described as "on-off-on" or "centre off".

    However you can get away with a single pole double throw switch (with the same additional description as above) because you only need to switch one side of the power supplies. Typically the negatives are left connected and you switch the positives.
     
    Last edited: Nov 26, 2014
  7. vitalious

    vitalious

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    Nov 22, 2014
    Your writing was hard to decipher but I understand what you mean now! Thanks! Looks like I need a single pole triple throw (in case I want to have an additional sink) switch. Thanks for your help!
     
  8. Gryd3

    Gryd3

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    Jun 25, 2014
    Would such a switch be required?
    I think any appropriately rated switch will do... you only need to place in on one of the wires that go to the 5V converter to prevent the charger from feeding into it. You could simply turn 'off' the 5V converter when you charge the batter, and turn it on after when you want to use it. It could be placed on the + or - wire for the converter, would be simpler to wire and less expensive.

    Of course.. if the charger behavior is known, would a switch even be required? The charger voltage will typically not exceed 14-15V, so if the converter would handle that would it be fine?
     
  9. vitalious

    vitalious

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    Nov 22, 2014
    Actually, the converter I ordered is rated up to 28V and the AC adapter produces 14.5V so I guess I don't need a switch for it.
    But having said that, I'd like to to able to connect another device potentially on a separate circuit (maybe an electrical heating pad) so I would need a switch for that then just in case my sink requires different voltage. So a one pole two throw switch.

    Here's what I'll be wiring, let me know if it looks right.

    2014-11-26 12.28.34.jpg
    My electrical knowledge is very rusty, but can I parallel wire that open circuit to the battery? How would it affect the power draw if I were to have say 3 USB devices charging and a heating pad consuming 12V? Both would get their 12V but the battery would deplete faster? If that's the case, I don't need a switch at all...
     
  10. Gryd3

    Gryd3

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    Jun 25, 2014
    You can certainly parallel those together, but there is a catch.
    Parallel items will receive the same voltage, but your battery is not a perfect voltage source... so when you add a parallel item, you also add it's current draw directly to the battery. If the battery cannot produce the required current, then it's voltage will droop, which will lower the voltage across the rest of the parts.
    This of course, would depend on the current you plan to draw.
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    A couple of points. Firstly the long bar on a battery is positive. Secondly are you sure your device can operate from both 12VDC and 28VAC?
     
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