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Battery Pack woes.

Discussion in 'Power Electronics' started by Kwaterland, Sep 14, 2012.

  1. Kwaterland

    Kwaterland

    2
    0
    Sep 14, 2012
    Working on a Digital Signage project for work. We needed to be able to power a 110VAC 82 Watt TV with batteries for 4+ hours. I ordered some 12v 9800aH Lithium Ion packs from China and 110Watt inverter. Seemed simple enough. Hooked up 5 packs in parallel and away we went. 2 hours in and the inverter beeped signaling a low battery charge then it went off.

    I've since hooked these packs to a 12V 40watt load and ran that for an additional 4-5 hours under the same charge. I noticing that the inverter requires a minimum of 10.5v to run and the Lithium Ion packs are only producing 9.6v after about 2 hours, however they continue to push above 8v for several hours. This must be what is causing the inverter to power off, but what do I do about this? The obvious solution to me is to find some DC/DC step ups, but I'm having a lot of difficulty finding one that doesn't ship from China, or that is rated high enough to push 82 watts.

    Anyone have any ideas?
     
  2. BobK

    BobK

    7,599
    1,641
    Jan 5, 2010
    Well, there are several problems.

    1. The capacity of cheap Chinese batteries on Ebay is often overstated.
    2. Battery capacity is stated at a particular current draw, typically the one that maximizes the capacity. So any other draw will get less that the stated capacity.
    3. Paralleling batteries is not such a good idea. The weakest one will draw the others down.
    4. The 12V rating is misleading. Lithium ion batteries measure 4.2V with no load at full charge, but they are generally rated as being 3.7V nominal. So your batteries are really more like 11.1V.

    You would probably be better off with a deep discharge RV lead-acid battery.

    Edited: Here is one that might work for you:

    http://www.amazon.com/12V-75Ah-Grou...id=1347653632&sr=1-2&keywords=12V+SLA+battery

    Bob
     
    Last edited: Sep 14, 2012
  3. shrtrnd

    shrtrnd

    3,665
    453
    Jan 15, 2010
    It ain't pretty, but BobK's idea will work.
    If you need pretty, just stay away from Chinese batteries, most everybody else's
    actually meet their specs.
     
  4. Kwaterland

    Kwaterland

    2
    0
    Sep 14, 2012
    First, thank you for your response. Any help is appreciated, as I don't know much about these things.

    The issue I have is weight. These TVs are actually going to be human mounted. The batteries do seem to be fairly cheaply made, but we have $300 invested in them so I would like to see if I could make them work.

    I am getting a lot more juice out of the batteries after the inverter shuts off, it just seems to be at a much lower voltage than the inverter can handle. Is there a better way to handle this with the current batteries? You mentioned that wiring then in parallel doesn't work very well, is there a better way to use multiple batteries at the same voltage?

    Thanks again.
     
  5. CocaCola

    CocaCola

    3,635
    4
    Apr 7, 2012
    You can't blanket statement like that... Almost all lithium cells come from China as they have the lithium mines, the stated rating are all a product of the retailer or battery pack assembler, that mostly use the same cells but rate them all differently... You will find honest and dishonest across the board, if you want more legit specifications just stick to a trusted source or big name brand that worries about their reputation, not some random Ebay guy... That doesn't mean they are not using the same Chinese cells as the next guy, it just means they are more honest in the rating, and yes there are plenty of honest Chinese battery manufactures that rate just as well as anyone else...

    Best advice is to not be price driven when shopping for batteries, and know your source...
    There are also a bunch of sites dedicated to real world testing and rating of many battery manufactures, search them out if you want a second source of ratings...
     
  6. BobK

    BobK

    7,599
    1,641
    Jan 5, 2010
    I doubt that using a boost converter would get you much more. It will not be 100% efficient, so you will be losing power by using it throughout the cycle, and it will end up drawing more current as the voltage gets lower. Maybe your best bet is to find a TV that uses less power.

    Bob
     
  7. akida

    akida

    9
    0
    Oct 4, 2012
    No one has mentioned that the inverter is only about 80% efficient and so you are losing power straight away just by using it.

    The inverter with a nominal input of 12V should handle the 14.8 volts that you would get by wiring 4*3.7v battery cells in series and that will give you a better starting voltage so losses wont affect the inverter as much. Check the tollerances on the inverter before you try this though. You can put a resistor in series with the batteries to reduce the voltage down to a safer level. Don't forget the cars charge at 14.4volts.

    So all in all,
    1) bigger/more cells to increase mah value
    2) Put more cells in series to increase voltage. (the batteries you have should be able to be broken down to 3 individual cells.

    Hope I'm not too late and this information comes in handy =)
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,690
    Jan 21, 2010
    True, perhaps it is a bit higher than 80%, but probably not a lot.

    True. A fully charges lead acid battery can have a voltage this high anyway.

    Also, the inverter probably cuts out at around 10.5V. 4 cells in series allows you to get down to 2.6V per cell, rather than 3.5V per cell. So you'll get a lot more capacity out of the battery. HOWEVER, discharging below 3V per cell is really to be avoided.

    If you could adjust the point at which it detects low voltage you may be able to get away with it.

    No you can't.
     
  9. akida

    akida

    9
    0
    Oct 4, 2012

    Perhaps you misunderstood my meaning. I simply meant that less voltage would be avaliable to the inverter, not that the batteries would put out any less.

    I know I'm a bit rusty on my electrical theory so correct me if I'm wrong but from my understanding two loads in series share the avaliable voltage rather than parallel which would share the avaliable current.

    I double checked that against this outline of the theory: http://www.ehow.com/how_4900110_reduce-voltage-resistors.html
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,690
    Jan 21, 2010
    A resistor will drop voltage proportional to current and dissipate power proportional to current squared.

    When the load is low, the voltage would remain (too) high. When the load is high, a great deal of power may be dissipated in the resistor.

    A series diode may be a better option, but it wouldn't be perfect. And in this application, the last thing you want to do is to waste power.
     
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