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Converting from AA to Li-Ion

Discussion in 'General Electronics Discussion' started by eptheta, Dec 20, 2011.

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  1. eptheta

    eptheta

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    Dec 20, 2009
    I plan to use a 3.7V Li-Ion battery from a mobile phone as a power supply for a variety of devices that usually run on 2 1.5V AA batteries.

    This is what I plan to do. Tell me if anything will go wrong:
    1. Use a voltage regulator (adjustable LM317 or a 3.3V LM3940 with a diode to drop it down) to bring the voltage to ~3V.
    2. Connect the battery to a MCP73833/4 with all required components according to the datasheet.
    That should keep it charged (and not let it be overcharged) as long as it's connected to a power source.
    3. To prevent over-discharge, I can maybe just have a relay/transistor switch to break the circuit after voltage across the terminals of the cell drops below critical (around 3V) [When not connected to the power source]

    Please do reply. Thanks.
     
  2. davelectronic

    davelectronic

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    Dec 13, 2010
    I dont think it will be very efficient, the battery is not got the capacity, and your almost on the limit for volts above reference, the lm317 is not got a close enough tolerance, you could try it but i think its a waste of time, honest opinion. :)

    PS by the time youve added transistors / relays, even low power devices, whats left. ?
     
    Last edited: Dec 20, 2011
  3. eptheta

    eptheta

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    Dec 20, 2009
    I doubted it will work in practice too. This is all that I've thought of, so I'm pretty much out of ideas.
    Do you have any suggestions to improve upon the original concept, or would you just scrap the whole thing and approach it differently :confused:

    Have you worked with Li-ion batteries ? Or are you answering based on common (electronic) sense? I lack both, so I need all the help I can get.

    Thanks
     
    Last edited: Dec 21, 2011
  4. davelectronic

    davelectronic

    1,087
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    Dec 13, 2010
    My reply is based my modest electronics knowledge, i am not a specialist in lithium ion batterys, no i would not scrap the idea, if you want to use the same battery, then a component rethink would maybe work, or the same components and a punchier battery, more voltage, more capacity.
     
  5. duke37

    duke37

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    Jan 9, 2011
    If you want 3V and have a 3.7V battery, just put it through a silicon diode which will drop about 0.7V
     
  6. eptheta

    eptheta

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    Dec 20, 2009
    Yeah, a silicon diode would work... Thanks.

    Has anyone attempted to work with Li-Ion batteries ? Because honestly, since I've never tried it before, my main concern would be about how well/suitable the MCP73833 is for my purpose.
    I think I can tinker around most of the other problems when I encounter them.
    I'm going to start by powering just an LED, so I'll find all the application specific problems only after I get the basics set.

    In any case, I'll try this out and if/when it fails, at least I'll have a specific question..
    Thanks for the help so far.
     
  7. eptheta

    eptheta

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    Dec 20, 2009
    To prevent over discharging (beyond say 3V) my initial idea was to use a low power op amp to check battery voltage and cut off the load circuit when it hits around 3V. I then realized that I need to maintain a reference 3V for that to work, and I don't have this reference 3V !! (because my circuit is being powered exclusively by the Li-ion battery after charging)
    That eliminated uCs too, since the ADC relies on a reference voltage.

    Any ideas ?
     
    Last edited: Dec 24, 2011
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    To prevent a battery from being charged above 3V, you don't need a 3V reference.

    You need a reference and divide that to 3V or divide 3V down to your reference.

    Since you'll be (presumably) running from the ~3V rail, you really need a reference below 3V, say 1.25V. You then divide the 3V so that you gt 1.25V, and compare these voltages.

    WHen your 3V is less than 3V, the 1.25V you get will actually be less than 1.25V. When it is > 3V you will get >1.25V.

    Incidentally, you probably want to charge the lithium cell to something like 4.2V...
     
  9. eptheta

    eptheta

    188
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    Dec 20, 2009
    I have an IC that worries about overcharging, I was worried about over discharging.

    I think I figured out how to maintain a constant reference..
    What I did (on a simulator, so I don't know if it will work) is have an op amp that can work on 3V.
    (+) is connected to a resistor bridge with R1:R2=1:10
    (-) is connected to a potentiometer that I adjusted to 0.28V. I added a diode to ground which maintains approximately this same pd since it's potential drop is generally constant.

    As the voltage decreases from 3.7V to 3V, the (+) voltage goes from 0.34V to 0.27V, thus turning off the output from the op amp.

    Also, I'm using one of those mobile Li-ion batteries and those are generally 3.7V. The mobile charger stops charging at 3.7V, so I don't know if it can take 4.2V.
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    OK, perhaps better to draw out that circuit, or easier, grab an image from the simulation.
     
  11. eptheta

    eptheta

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    0
    Dec 20, 2009
    T̶h̶e̶ ̶b̶l̶u̶e̶ ̶L̶E̶D̶ ̶i̶s̶ ̶t̶h̶e̶ ̶l̶o̶a̶d̶.

    I̶ ̶w̶o̶u̶l̶d̶ ̶l̶o̶v̶e̶ ̶t̶o̶ ̶u̶s̶e̶ ̶a̶n̶ ̶A̶V̶R̶s̶ ̶A̶D̶C̶ ̶t̶o̶ ̶a̶c̶c̶u̶r̶a̶t̶e̶l̶y̶ ̶c̶o̶m̶p̶u̶t̶e̶ ̶t̶h̶e̶ ̶v̶o̶l̶t̶a̶g̶e̶ ̶r̶a̶t̶h̶e̶r̶ ̶t̶h̶a̶n̶ ̶r̶e̶l̶y̶ ̶o̶n̶ ̶t̶r̶i̶m̶p̶o̶t̶s̶ ̶t̶h̶a̶t̶ ̶a̶r̶e̶ ̶s̶u̶b̶j̶e̶c̶t̶ ̶t̶o̶ ̶h̶u̶m̶a̶n̶ ̶e̶r̶r̶o̶r̶,̶ ̶b̶u̶t̶ ̶I̶ ̶d̶o̶n̶'̶t̶ ̶k̶n̶o̶w̶ o̶f̶ ̶a̶n̶y̶ ̶u̶C̶s̶ ̶t̶h̶a̶t̶ ̶w̶i̶l̶l̶ ̶b̶e̶ ̶h̶a̶p̶p̶y̶ ̶t̶o̶ ̶r̶u̶n̶ ̶o̶n̶ ̶s̶u̶c̶h̶ ̶l̶o̶w̶ ̶v̶o̶l̶t̶a̶g̶e̶s̶.

    EDIT: I don't know why the picture won't go up, but never mind, I tested it and it worked fine.

    I have another question:
    My mobile battery has a nominal voltage of 3.7V. Am I supposed to charge it to 3.7V max using any constant voltage source I like [e.g 4.2V--standard for most ICs](provided I regulate current) or do I have to charge it using a 3.7V source ?
    I ask because the IC i wanted to use supports only 4.1V and 4.2V charging voltages. Is it safe to use one of the two ? or do I have to get a different one ?

    I am pretty sure that I'm not supposed to charge it to 4.1V, but using a 4.1V charging voltage won't necessarily mean that it will be charged to 4.1V provided the IC stops charging appropriately.

    In that case, is the MCP73833/4 suitable for my needs ???

    I would have looked up the battery's datasheet, but I don't think those things exist. I have an LG LGIP-430A battery and a Nokia BL-5J one, and there are no datasheets with all my questions answered. I guess there's a common one for all of them somewhere, but I can't find it anywhere.

    PS: Also, is strike-through enabled over here ? I could swear it was before..
     
    Last edited: Dec 27, 2011
  12. eptheta

    eptheta

    188
    0
    Dec 20, 2009
    Sorry for double posting, but I edited my last post significantly and wanted to bump this to the top.
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Typically, lithium cells use a current limited, constant voltage charger.

    When discharged, the charging is current limited until the voltage reacies the fully charged voltage (typically 4.1 or 4.2 volts). Then charging continues until the current drops to a small value (typically something like 5% of the current limit). At this point it is considered charged.

    Measure the battery voltage just after it finishes charging and see what it is.
     
  14. eptheta

    eptheta

    188
    0
    Dec 20, 2009
    EDIT: I take that back. Turns out that LG battery(and mobile) I was using was just kind of weird. All the other batteries (Nokia) I tested had voltages greater than 3.7V at full charge. (Hovering around 4.1V)
    I guess the chip I plan to use will be fine then.

    Thanks for making me check, I wouldn't have otherwise.
     
    Last edited: Dec 29, 2011
  15. eptheta

    eptheta

    188
    0
    Dec 20, 2009
    Hi,
    Since I went to college, I kind of gave up on this, but now that I'm on vacation, I thought I'd get back to it.

    Could you guys go over my design to prevent over-discharge of the lithium ion battery. Since I have almost no experience with the math, I have no idea if this is a sensible circuit beyond the fact that it works on a simulator.
    [​IMG]
    (link in case it disappears)
    I plan to allow discharge down to 3.6V.
    The idea behind this is that the voltage across the diode remains more or less constant at ~0.7V, so I can use this as a reference.
    The low power op amp has its (-) feed divided from this reference, and the (+) feed divided from the source.

    I guess the thing I should be looking out for is how much current is drawn when the battery is already drained to 3.6V. There's a free loop through the reference diode for it to waste current, so I put a 10k resistor and adjusted the references accordingly. The simulator says it's wasting some 0.2mA at 3.6V. Is that too much ?
    I could just put a higher value resistor since that branch is only for a voltage reference.

    I've only been designing on paper. I haven't bread-boarded and tested this yet. Will this circuit work ? Should I just use a uC's ADC (The Atmega8L for example has an internal reference of 2.56V and has an operating voltage of 2.7V-5.5V)?
     
    Last edited: Jun 2, 2012
  16. khankll

    khankll

    110
    0
    Feb 6, 2011
    can u be a boit more specific in ur demands?
    u say u wana use liion instead of 2 aa cells .. yes ..?
    first the primary advantage oif liions is higher density then its recharfgeability upto 500 times or so...
    though Nimh cells are notr so energy dense but they are rechargeable u could instead swap from normall aa to rechargeble aa and use decent charger..

    if u wana stick with liion..let me say they r dangrous ..and need proper care and feeding..

    my suggestion to u would be to first match the voltages... for e.g ur device is if rated for 3 volts .. then charge ur liion cell in the specific mobile phone.. and then take it out use it for the expected time .. when u think it is near draoined then just remove it frm ckt and recharge it in mobilephone..

    or if u dont want to use mobile ohone to charge the cell use any liion charger... that could be a dedicated ;

    and how will u be fitting the prismatic battery of mobilephone in the equipement ? how will u be connecting it and making it stay secure ?


    if u want to use liion then y not the liion rechargeable cell such as 18650 etc..? they are cyinderical and they have a lot of dedicated chargers ,.,,

    or do u want to use ur two batteries bl5j etc which are just lying spare ? and u want to mod them for replacing 3 aa batteries..?

    what are some of the devices that will be running this cell ?

    and most liion batteies have protectio circuits hich will prevent overcharge,overdischarge and shortcircuit.. but short circuit protection may keep ur house safe from burning but it will also destroy the protection ckt and hence the cell..

    ...............
    currently i m also using few nokia bl5c .. for my led lamp.. in our country we are so lucky that electricityt comes 8 times daily to us.. whareas in most advance countries they mayu not be so lucky...

    i have removed the protection ckt and in the cell has two contacts just soldered wire to it and charge it using universal charger ,, then put it in m led lamp and at .5amps of current draw i get 2.5-3 hours of excellant lightning before the voltage drops to below 3 vols at that instant i recharge it.. originally this lamp had a leadacid 4 volts battery ,,wgich stoped working along woith the cheapo ckt that it had..
     
    Last edited: May 31, 2012
  17. BobK

    BobK

    7,673
    1,684
    Jan 5, 2010
    Li Ion battery packs normally contain current limiting electronics so that they are not dangerous. If you were to buy raw cells, you would have to add protection.

    Bob
     
  18. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    You're not showing any current limit on your LED and that's bad.

    Also the battery voltage will go up and down with load, so the LED will not just go off when the voltage drops. What will happen is that the battery voltage will form part of the feedback loop and the transistor will be turned off slowly to maintain the battery voltage at the set point until it finally drops belore it (which may take a loooong time)

    The practical problem is that the dissipation in the transistor may rise to a relatively high value about mid-way through this process.

    Your options include adding hysteresis (+ve feedback) -- well that's really the best one.
     
  19. eptheta

    eptheta

    188
    0
    Dec 20, 2009
    I guess I should make my intentions clear. For all my projects so far, I've been using 9V batteries which are bulky and have to be replaced often. I effectively want to switch to using lithium cells that can be easily recharged from a convenient source like a USB port. While removing the battery to recharge it is a perfectly good solution, I'd prefer to keep it easy, and charge the battery on site. (It must get annoying to remove and recharge the cells from your lamp every few hours instead of just plugging the device in like you would a laptop)

    At first I intended to replace the AA batteries of generic electronic devices like keyboards etc as a quick fix, but I now want to incorporate Li-ion supplies in any further projects I have, so knowing how to do it right will be invaluable.
    I am content with using any sort of commercially available lithium cells, but I happen to have a few spare mobile batteries, so I thought I'd use them for now. No compulsion to stick with them though.
    I have the charging part done (with a dedicated IC- the MCP73833), it's only the discharging that has me concerned.

    Sorry about that, I meant to include a 1K resistor in series with the LED.
    I never thought about that. I'm not too familiar with op amp feedback (no practical experience, just what I studied as part of my coursework), so I have to read up a bit more.
    Also, would it be more sensible to use a microcontroller and its ADC for this?

    EDIT: I read up a little on positive feedback and this is what I gathered:
    At 3.6V (my desired cutoff) the voltage across the battery fluctuates because the load goes off. It seems I need to allow a little bit of fluctuation in voltage levels for which the Schmitt trigger seems appropriate. When the output goes low the first time at 3.6V, the output is latched to LOW and voltage variations are tolerated within a band. The output only turns high when it crosses a threshold value.
    I still have to figure out the appropriate feedback values, but am I on the right track ? Also, I'm a little hesitant because all the reference designs I saw on the net rely on a nice constant voltage supply for their voltage references, but I have one that goes down gradually from 4.2V to 3.6V.
     
    Last edited: May 31, 2012
  20. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Yes, you have the idea for positive feedback.

    If the batteries are in series to get the required 9V (ish) then you can't easily charge them from a 5V USB charger because the ground connection is likely to be tied directly to the USB ground. This means that you will short the batteries out.

    What you could do is get a couple of isolated DC-DC converters and use these to charge individual cells. This is real overkill though.

    Google for "LiPo cell balancing" for a more typical approach.
     
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