Backpack Device Charger

Discussion in 'The Projects Forum' started by MajorTom525, Nov 28, 2012.

  1. MajorTom525

    Thread Starter New Member

    Nov 28, 2012
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    Hey everyone! I have this project that I want to get into. That is, I would like to build a backpack that charges devices from a few different power sources. IE solar, outlet, heck maybe even a hand crank. I'm not terribly great at electronics, but I know the basics and can solder. My plan for this thing is sorta like this:
    Multiple Power storage units, IE several USB charged usb device battery packs that provide ~5000mah of juice.
    Multiple power sources, and the ability to switch the battery packs they charge, and if there's enough current, charge both.
    Finally, a USB power source that any usb device can connect to to charge. The power source should be able to dynamically (and manually) be able to switch between what it draws current from, and when I say dynamically I mean that it can switch from single power source to both packs if the load is too much. This is a lot to ask, but I was wondering if someone may be able to point me in the direction I would need to go with this? Any help is appreciated!
     
  2. wayneh

    Expert

    Sep 9, 2010
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    The easiest way to "OR" power sources together is to make them all DC (ie., rectify any AC source) and then just put a blocking diode on each one as they come together. This prevents current flowing into any power source instead of out of it. The downside is the diode itself - they lose a bit of power. This would be an extra loss on top of the rectification loss for an AC source You wouldn't care about this for the wall outlet, but it could matter for a hand crank.

    I think to say much more, folks will need to know the details of your battery packs. Those details will dictate the charging strategy as well as the scheme for getting 5V regulated power coming out.

    But before you get much farther, I suggest you work out the details of how much solar panel or hand cranking you'll really need to get the power you want into your device(s). Figure that you might achieve 50% power efficiency between your source and your load. Allow for practical energy harvesting times. In other words, 14 hours of direct sun or hand cranking is not a practical plan.

    I think you'll be disappointed, frankly. We see a lot of these projects here and that's often how they end, with the cold slap of reality.
     
  3. MajorTom525

    Thread Starter New Member

    Nov 28, 2012
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    Yeah, I was wondering about that. Well, here's what I was thinking:
    x2 of these:
    http://www.ebay.com/itm/221157082840?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649
    And then use some sort of switch to send whatever amount of power that is coming off of this:
    http://www.ebay.com/itm/170900715065?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649
    to either power pack. The solar panel is specifically designed for use with charging devices, and I think that I could have the solar panel charging one battery pack, while the other is discharging into my devices; then a switcheroo when needed.
    So yeah, the solar panel has a 5v regulator built in, so that problem is solved, therefore I could just link the output of the solar-panel into the battery pack, via some sort of switch that can route the power coming IN from the panel to either battery pack, and another switch that can route the power going to the device from either battery pack.
    I think its feasible, but I'm not experienced in this matter.
     
  4. KJ6EAD

    Senior Member

    Apr 30, 2011
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  5. wayneh

    Expert

    Sep 9, 2010
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    The battery pack linked by the OP has solved some of the problems, for instance that you cannot normally charge a 5V battery with a 5V source, and then expect to supply 5V to a USB device, but you apparently can with that device. It must have internal step-up and step-down regulation.

    But here's the problem. 12Ah (12,000 mAH) at 5V is 60W of stored energy in that battery pack. The panel is a 5W nominal panel, meaning it will take 24 hours of ideal, direct sunlight to charge the pack using that panel. (Don't forget the rule-of-thumb 50% charging efficiency.) I consider that impractical by a factor of 10. Two to three hours of charge time might be doable.
     
  6. John P

    AAC Fanatic!

    Oct 14, 2008
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    Oh no. Journalists do this all the time--surely we know better.

    It's 60W-hrs or 216000 Joules. A Watt is a rate of energy delivery, not a unit of energy itself.
     
  7. wayneh

    Expert

    Sep 9, 2010
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    Sorry, sorry, dropped a unit, yes you're right. 60W-hr.

    The point remains; it's a big number compared to a 5W PV panel.
     
  8. MajorTom525

    Thread Starter New Member

    Nov 28, 2012
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    So the solar panel wont just provide current when there's sun that will charge the pack?
    The charger pack takes as an input a mini-usb plug, which still have the same 5v rails as a regular usb. So my reasoning is, and this is obviously not based on any real electrical knowledge, just logic, :D that if the solar panel can provide enough juice to output a regular usb current (5v @ 100mA I believe?) shouldn't the pack charge like it was plugged into any usb slot on say a computer?
    The pack doesn't have to provide a constant power source, the idea is that it just stores the energy that it can glean from the available power source and make them available to the devices when needed.
    So I guess my next question would be, milliamp hours means how many milliamps it can consistently sustain for an hour, not necessarily the amount of juice it takes to charge it?

    EDIT:
    Thanks, however these are really expensive and I don't really have that kind of cash. Thats why I figured I would try to DIY it.
     
  9. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Each individual "USB Power booster pack" has internal step-up (boost) and/or step-down (buck) converters, which run at anywhere from 70-90% efficiency.

    You would be MUCH better off having a single LiOn battery pack of large capacity, with one boost/buck converter for charging, and a buck converter for the output of 5V (if that is the only desired output). This would require better power regulators on both the charge and output ends, capable of handling the anticipated power load.

    At the same time, the money saved on not buying many pre-made smaller capacity packs with their own less capable regulators would get you most of the way to where you want to be.

    The link above with the 19V battery @ 60 W-h is a good example of using a single charging circuit and output circuit for a high capacity battery. The more regulators you add in, the lower your efficiency will be. Adding diodes/intelligent power switching to the mix will reduce efficiency more by wasting power in the form of heat.

    When dealing with PV solar power, which is still woefully inefficient inside the atmosphere, you don't have any power to spare on those extras.

    A DIY project of the same capacity will cost more than the $300 120W-hr block w/2 batteries, and the DIY path will b much less efficient than the single regulator/charger in the link.
     
  10. wayneh

    Expert

    Sep 9, 2010
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    I'm not sure I follow your question, but those are basically the same thing. An amp-hour is a measure of charge, current times time. Like a volume of water. When it's at 5V (like height or pressure, in the water analogy) it has energy = power times time = watt-hours that is 5VAh or 5Wh. (Watts = power = volts x amps).

    It does however, take more energy to charge a battery than will be stored in it, due to inefficiencies.

    Oh, and a battery's mAh rating doesn't necessarily mean the current that can be sustained for an hour, it might be determined over several hours. For example, 5 hours at 200mA = 1000mAh. There are rules and defined conditions for rating batteries. You'll generally get more mAh from a lower discharge rate, so the discharge rate is part of those defined rules.
     
  11. MajorTom525

    Thread Starter New Member

    Nov 28, 2012
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    Okay. Thanks for your help guys, I really appreciate it!
     
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