Simple low power consumption Battery Protection Circuit / Cut Off

Thread Starter

Lumenosity

Joined Mar 1, 2017
461
Before I posted this, I did a quick search here on several variations of the topic above but didn't find quite the answers I'm looking for.

However, a while back this forum was very helpful with a similar project and I was able to build a protection circuit that is still in use today and has been perfect. With a few differences, that circuit might work here as well but in this new project, there are 3, 1.2v batteries instead of two and I do not need an LED indicator in this new project. Also, the previous circuit was not a cut-off, just a warning. Here is a link to that project.

https://forum.allaboutcircuits.com/threads/simple-circuit-using-led-to-alert-low-voltage-but-i-need-lower-threshold.141757/page-3#post-1216269

So I need a similar circuit that does not utilize an LED, and that stops the flow of current below approximately 3.6v DC

The circuit in this new project consists of 3, 1.2v rechargeable NiMh batteries. The device is a motion activated, outdoor LED light we use above our garbage cans to automatically illuminate the area when I take garbage out at night. The problem is that the device KILLS batteries because it will continue depleting them until they are at zero voltage.

It uses 3, D Cells in Series so the total voltage is of course approximately (1.4v x 3 cells) = 4.2v fully charged.

What I want is to add a battery protection circuit that will stop the device from drawing current from the batteries when the voltage drops below the per cell voltage minimum of 1.2v or (1.2v x 3) = 3.6v

It would be nice to start with a POT to be able to adjust the cut off voltage over a range of maybe 3.9v to 3.0v, then after finding the best compromise, replace the POT with a resistor.

If there is already a thread with such a thing (pretty sure there is), I didn't find it so a link would be great if it already exists.

If not, this should be a very easy thing to do using a TL431 ?
 

crutschow

Joined Mar 14, 2008
23,787
Here's a circuit with LTspice simulation that should do what you want.
Its cutoff is adjustable between ≈3V and ≈3.9V with pot U2 (shown for pot positions of 0%, 50%, and 100%).
It uses a low-power TLV431 instead of the more common TL431 to minimize current drain.
The P-MOSFET must be a logic-level type (Vth maximum of <2V).
R4 provides positive-feedback hysteresis so the circuit doesn't oscillate around the trip point.
For minimum battery drain, connect this circuit between the activation circuit output and the LED.

upload_2018-12-24_19-49-15.png
 
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