Low Battery Indication circuit

[kinabaloo]

Here is an easy to make battery level indication circuit.

With a freshly charged battery, both LEDs will shine brightly. Gradually the green LED will dim and be only faint around 3.9v. The red LED will be faint at around 3.4v.

Most e-cigs stop the battery at about 3v. The absolute minimum for a Li-ion / Li-po is about 2.5v (otherwise it will not recharge).


Thanks to RjG for the initial idea. Deciding that a reproducible circuit was needed, one that could be put on paper and easily be constructed, led me to investigate ...

I found that standard red LEDs will have a typical forward voltage of around 1.7v whereas for a green one it is about 2.2v (with a current around 20mA). That is the key idea behind how this circuit works, using known values

The resistor values set the current at about 20mA (fairly bright) for a fully charged battery.

Although 4 LEDs are used, just display the top or bottom two; the others will be hidden, with the resistors.

For even greater simplicity, just use the two red LEDs on the left (for a good idea of the 3.4v mark - need to charge soonish); display only one of them.

~~~

Selected Links:

Technical aspects of LEDs: http://www.maxim-ic.com/appnotes.cfm/an_pk/3070

Beginners guide to LEDs: http://www.kpsec.freeuk.com/components/led.htm

[CoderGuy]

What a great idea! I have been thinking it would be great if there were an indicator.

A great design idea would be one that screws between the battery and atty and lays flat onto any battery (so you could move it easily from battery to battery).

I would like to pre-order now if someone wants to build one

CoderGuy

[kinabaloo]

What a great idea! I have been thinking it would be great if there were an indicator.

A great design idea would be one that screws between the battery and atty and lays flat onto any battery (so you could move it easily from battery to battery).

I would like to pre-order now if someone wants to build one

CoderGuy

Thanks

Anyone could build this! Not much more than the usual LED + resistor used to indicate atty on.

If put on a board with the LEDs to stick through the case; solder two with longer legs and perhaps push over and flat the other two.

[mnealtx]

I was thinking of something similar after reading the other thread about LED indicators - what about using a single, dual color led? Would it be possible to, for instance, have the green LED fade out around 3.6v and have the red illuminate at 3v?

I'll have to go hit your links...

[framitz]

I was thinking of something similar after reading the other thread about LED indicators - what about using a single, dual color led? Would it be possible to, for instance, have the green LED fade out around 3.6v and have the red illuminate at 3v?

I'll have to go hit your links...

I'm still looking at the RGB LED from Radio Shack, the characteristics of the device should be useful in making a color shifting light to show status with minimum circuitry. My wife is going out and I'm going to ask her to pick up a pair so I can play. Based on what I see about the item It should shift from blue/green to red as voltage is reduced, we'll see . . .

[kinabaloo]

Thanks guys.

Anyone who's more adventurous can take a look at this circuit :

Battery level indication : a circuit

Because it uses op-amp comparators and a zener (with resistor divides feeding high impedance op-amp inputs) the circuit is fully customisable (doesn't depend on LED forward voltages at all).

[mnealtx]

Thanks guys.

Anyone who's more adventurous can take a look at this circuit :

Battery level indication : a circuit

Because it uses op-amp comparators and a zener (with resistor divides feeding high impedance op-amp inputs) the circuit is fully customisable (doesn't depend on LED forward voltages at all).

That's the thread I was thinking of in my prior post - take the op-amp and drive a red/green LED for a 'fuel guage' - make the input through the other leg of the NO pushbutton switch and you can see your battery level every time you take a drag:

Voltage above 3.9V - drive green only
Voltage below 3.9v and above 3.4v - drive red / green to make yellow
Voltage below 3.4v - drive red only

Maybe some sort of bistable multivibrator...*shrug*

[kinabaloo]

That's the thread I was thinking of in my prior post - take the op-amp and drive a red/green LED for a 'fuel guage' - make the input through the other leg of the NO pushbutton switch and you can see your battery level every time you take a drag:

Voltage above 3.9V - drive green only
Voltage below 3.9v and above 3.4v - drive red / green to make yellow
Voltage below 3.4v - drive red only

Maybe some sort of bistable multivibrator...*shrug*

1 x 2-color (3 leg) LED - neat.

Maybe then can use an 8-pin DIL chip dual op-amp

"bistable multivibrator" *scratches head*

[mnealtx]

1 x 2-color (3 leg) LED - neat.

Seemed like an elegant solution with a little lower power use, IF it can be done.

"bistable multivibrator" *scratches head*

flip-flop circuit - i figured it could be used to possibly trigger a leg of the op-amp, if need be.

[kinabaloo]

flip-flop circuit - i figured it could be used to possibly trigger a leg of the op-amp, if need be.

No transients so no need of any memory; it's just a real-time A to D model