12V Low Battery Indicator

[audioguru]

Why do you want a warning that the voltage on a Lithium cell is low? Because it is bad for it.

A protection circuit in a Lithium battery cell disconnects the load when the voltage gets low. Your circuit does the opposite, it connects an LED to discharge the cell more which will destroy the cell if you do not disconnect it yourself.

On one of these forums an ad from China showed photos of a protection circuit that look identical to real protection circuits. These photos are being sold to battery manufacturers to cheat consumers.

Here is a video of what is inside many 18650 Lithium cells sold on e-bay:

 

[MikeMl]

Here is the circuit modified for 3.3V. I plot the total supply current I(R3) which shows that it is less than ~400uA when the battery voltage is >3.3V, and about 2.7mA when the LED turns on as shown by I(D2). If you use a super-bright LED for the indicator, you will be amazed at how bright it looks even with only 2.7mA. You will not want to leave this circuit hooked up 24/7 as it will kill the battery eventually. Maybe a press-to-test switch?

 

[bortre]

Many thanks MikeMI, you really help me out here :gbigthumbsup:

And yes, I know that this circuit will drain my battery, once the voltage in my battery passes these 3.3V. However, the device this is going into is a battery-mod for an atomizer in an electronic cigarette running at 1-1.5 Ohm, so the battery will get hit hard anways and the reason why I can't use a protected 18650 as these don't deliver enough amps. That's why I use unprotected batteries and the reason I need a low-voltage-iindicator, so that I don't drain the battery too much.
400uA leakage isn't that bad either, as I need to charge the battery once per day anyways. When I fire the atomizer there's some 3 amps flowing resulting in 9-10 Watt drainage from the battery for some 5 seconds per draw and I do draw alot on this thing over the day

So yes, i understand your concern audioguru, but I guess you see the reason why I do it this way

 

[audioguru]

Doesn't an E-cigarette have an LED that glows when it is used? Then when the voltage gets low the LED gets dimmer and dimmer then does not light?
That is the low battery indication.

 

[bortre]

Doesn't an E-cigarette have an LED that glows when it is used? Then when the voltage gets low the LED gets dimmer and dimmer then does not light?
That is the low battery indication.

Not all eCigarettes have that. The ones that you can buy for $30 do have a PCB integrated with Voltage-regulation, LEDs and all that, but if you use a mechanical battery-mod with a mechanical switch, then all this is not available. Basically I'm going to use a 23mm stainless-steel tube in which I fit the battery with a bottom-cap that has a phat copper-spring to press the battery to the positive post in the topcap, where there's a mechanical switch installed into. The low-voltage-indicator PCB is fitted into this topcap aswell to have atleast some sort of battery-indicator.
No other electronics will be present in the whole device.

The atomizer, which vaporizes the fluid is nothing more than a 5cm piece of 0.32mm Kanthal wire formed into a coil and screwed in between a positive and a negative post. Through this coil you fiddle some cotton or silica-wire that wicks the fluid from a tank. And when you hit the switch on the battery-mod the coil will heat up and vaporize the fluid. There's no voltage-regulation whatsoever going on. It's directly applying the voltage from the battery to the coil, which has between 1 and 1.5 Ohm.

Look at this atomizer -> **broken link removed**
and this battery-mod -> **broken link removed**
to understand where I'm going with this whole project