Li - Ion battery level indicator (3.7-4.2V)

Discussion in 'The Projects Forum' started by Seb1929, Apr 5, 2015.

  1. Seb1929

    Thread Starter New Member

    Feb 20, 2015

    I want to make a battery level indicator with 3-5 led's that will support 3.7-4.2V battery. For many hours i have tried searching for one but with no luck. Can i get some help finding a schematic or someone explaining.

    Thank you!
  2. bertus


    Apr 5, 2008

    Have a look at the following schematic:


    Lower R3 for lowering the range.

    RodneyB likes this.
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
    You are proposing basically using a voltmeter to estimate the capacity of LiOn battery. Since the discharge curve of a LiOn is basically flat, this is a very poor indicator of state of charge...

    Gadgets that use LiOn have a microcontroller on board that actually integrates ∫i(dt) to report remaining charge. Save your time, or do it right.
  4. paulktreg

    Distinguished Member

    Jun 2, 2008
    If you look at a typical Li-Ion discharge curve then 4.2v down to 3.7V is pretty much the useful life of the battery (90%). Looks OK to me so why complicate things?
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
    Look at the voltage vs load current, voltage vs temperature curves in this data sheet, and you will see that it is difficult to divine much about remaining capacity just by reading voltage.
  6. Seb1929

    Thread Starter New Member

    Feb 20, 2015
    So if you want to do it right, what would you do?
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
    For a simple solution, you can just use a comparitor that turns on a red light when you reach the manufacturers recommended minimum operating voltage (3.2 to 3.7 are common recommendations per cell in series).

    In other words, additional LEDs do not help unless you want to see that you are getting close. It will not be a fuel level, just a warning light. You should include a potentiometer or switch to change the comparitor level as you change your battery.

    I can send a schematic if you need one - just a variable voltage reference (eg 317 regulator) with a small pot, a voltage divider for your battery voltage and a comparitor - I just made on that plugs into a two-cell battery's balancing plug.

    To get the two LEDs, it is best to use an op amp as a comparitor instead of a comparitor (ask me why if you need the answer)

    Last edited: Apr 5, 2015
  8. paulktreg

    Distinguished Member

    Jun 2, 2008

    Do you need to be "right"?

    What is your application?

    Do you really need to be integrating sh*t?

    Is GopherT battery low do something about it not good enough?
  9. dougalere


    Mar 4, 2015
    thanks seb
    i was just about to post a similer question, i go through a lot of batteries,
    and there are now a number of different types of battery each with different characteristics.
    a couple of possibilities, please dont try any of these until the senior members
    have advised- rechargables must be treated with caution,
    one is to drain the batteries with a suitable bulb then calculate the charge time.
    eg. a one amp/hour battery will take 5 hours to charge at 200 milliamps,you may need to use an ammeter to see what your charger is suppliing
    what i also do, i have a charger that takes 10 batteries side by side, sometime i'll use one as a test battery,after the allotted time i'll take it out and see how long it holds up under a load ,
    it would be nice to have an led or meter indication though.
    just a thought, would the charge current change as the battery charges?