Full discharge protection battery circuit

[_Vendetta_]

Hello EEVBloggers,

I have been trying to design a circuit that auto disconnect an NiCD battery pack of 3.6V when it reachs around 3V. The battery shall remain disconnected until either plugged to the charger again or the voltage reachs 3.6V again. My idea went an hysterisis solution using an LM311 comparator as show in the image attached.

I tried to water down the values of the battery using the divider. so when the battery reachs 3V (2.3V at the output of the divider), the output of the comparator is high. When the battery reachs 3.6V (2.73V volt at the output of the divider), the comparator pulls down the output.

I tried the circuit in real life it kinda does not give me the wanted result. The circuit basically pulls down at 3.2V instead of 3.6V. The rest is fine. I am aware that i maybe should try using an LMV comparator but i only have this component on hand at the moment.

I heard that i can achieve the same behavior using NCP301 with some extra hysterisis. Looking forward your remarks / advice.

Thank you.

EDIT: the Vref at the positive comparator input is 2.5V Zener.

[ggchab]

The minimum recommended supply voltage is 3.5V
The input voltage should be between VCC– + 0.5 and VCC+ – 1.5

[Gyro]

It's not obvious from your schematic but hopefully you are putting the circuit on the load side of the cutoff device (with some means of resetting it). You don't want all those resistive loads, reference current and comparator supply current drawing from the discharged battery.

If you do want it straight across the battery then you need to go CMOS, micropower reference, and take all the resistor values up by a few orders of magnitude.

By the way, you don't need to use a comparator for this application. You're not bothered about operating speed or open collector output so you can quite happily substitute a low voltage rail to rail CMOS opamp.