Lithium cells / batteries. More efficient to step-up or step-down

Discussion in 'The Projects Forum' started by panici, Dec 19, 2013.

  1. panici

    Thread Starter New Member

    Dec 19, 2013
    2
    0
    Hey guys,

    I'm building a battery powered LED project.
    I need a maximum of 1A @ 5v. Current draw will typically be less.
    I want to use "14500" lithium cells.
    I plan to use a separate regulator/board (to meet my current demands) and bypass the onboard regulator of the arduino i'll be using.
    Size/weight are issues. Through-hole components preferred for ease of prototyping.

    1. Is it more efficient to step up a single 3.7v cell or step down two cells in series (7.4v)?
    2. Will I run into issues using 14500 cells in parallel to increase my capacity?
    3. Will I run into issues using 14500 cells in series? (They have onboard protection circuits)

    Thanks!



    Possible equipment options:
    Batteries: (upto 2C discharge rate) http://dx.com/p/trustfire-protected-14500-3-7v-900mah-rechargeable-lithium-batteries-2-pack-26124
    Voltage Regulator:
    Both: http://dx.com/p/mini-dc-dc-voltage-stabilizer-regulator-module-red-126106
    Step up (TDFN package): http://www.maximintegrated.com/datasheet/index.mvp/id/5770
    Step up (TDFN package): http://www.maximintegrated.com/datasheet/index.mvp/id/5127
    Step up (SOT23 package): http://www.maximintegrated.com/datasheet/index.mvp/id/2451
     
    Last edited: Dec 19, 2013
  2. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
    1,758
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    Use a Simple Switcher e.g. LM2475 and the two cells in series. Properly charging them will be something you need to consider.
     
  3. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,281
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    Stepping down will use a smaller inductor and be a bit more efficient as a result. It will also use a NFET transistor instead of PFET (probably) which will also help.
    I would buy one from Ebay. Hard to build one for the same price. Check out Buck Regulator.
    The series configuration will need to be balanced.
     
    Last edited: Dec 19, 2013
  4. BobTPH

    Active Member

    Jun 5, 2013
    782
    114
    Using a larger cell, like and 18500 or 18650 is better than parallelling two smaller cells. I would use two in series and a buck converter.

    Bob
     
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    Definitely buck down from a higher voltage.

    The problem is the cell voltage: an Li cell is 4.2V peak, but drops to about 3V at end of cycle so that only gives about 6V from two cells. That's not enough to run a buck down to 5V.

    Adding a third cell would make the design a lot easier.
     
  6. panici

    Thread Starter New Member

    Dec 19, 2013
    2
    0
    Alright, thanks for your help guys! :)
     
  7. BobTPH

    Active Member

    Jun 5, 2013
    782
    114
    Buck converters can be made to operate with an input all the way down to the output voltage. In this case, the switch just stays on permanently.

    Bob
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    But that's a terrible design: look at the VIN range with only two cells. The efficiency is terrible on a buck when the VIN approaches VOUT. Adding a third cell in series is the way to make it better. If only two cells can be used, he would probably be better off just using a linear reg. The power saved in a 1A design doesn't justify the added cost, size and noise of a switcher. For a linear, I would use a LP2975 controller and an external FET and it would be very cheap.
     
  9. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Not so bad.
     
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  10. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
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    Because of the great current capacity, I'd lean towards the step up. Your voltage sag will stop the buck, before your current runs out in a boost.
     
  11. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,281
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    Lipos are pretty "stiff' until they go dead. With such large ones I don't think he needs to worry much about the voltage sag. His voltage should only sag a couple of millivolts at 1 amp.


    [​IMG]
     
  12. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    I was talking about the typical cheap stuff like simple switchers LM2575, LM2576 type. Unless you have at least 3V of headroom, not worth the trouble because the eff gain is microscopic.
     
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