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

Thread Starter

Seb1929

Joined Feb 20, 2015
26
Hello!

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!
 

MikeML

Joined Oct 2, 2009
5,444
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.
 

paulktreg

Joined Jun 2, 2008
786
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.
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?
 

GopherT

Joined Nov 23, 2012
8,012
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)

image.jpg
 
Last edited:

paulktreg

Joined Jun 2, 2008
786
Seb

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?
 

dougalere

Joined Mar 4, 2015
128
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?
regards
dougal
 
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