Li-ion under-voltage protection circuit

[sweben]

sweben submitted a new Showcase Item:

Li-ion under-voltage protection circuit

Hey all,

In my long quest to automate as much of my home as possible I want to have battery powered modules, mostly for sensors.

To achieve this, I made a comparator circuit connected to a transistor, which allows me to cut the power on my circuit when the batteries reaches 3v.

The circuit:

Note the voltage divider, it divide the battery voltage by a factor of 10, in order to make sure the battery voltage is always below 2.3v (Because the comparator is powered by 3.3v, and cannot take a higher voltage on it's pin than on it's power supply)​

Module prototype:

the transistor base resistor is 1.2k on this picture (seen above the comparator socket), I had to change it to 220ohm in order to have enough current flowing through my circuit. (thanks Arouse1973 for helping me find out the issue)​

setting up the threshold voltage

The modules ready for the batteries
​

I didn't work much with comparators before, so I learned a lot this weekend, it was very interesting! I hope this could help some of you

Cheers,
Benoit

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[HellasTechn]

I would recommend to set the cut out voltage to something about 3.6 to 3.7 volts.

3.7 volts is about the voltage indication that a Li-ion battery is almost depleted.

 

[Harald Kapp]

The resistance from Vbat to GND by the resistive divider is a bit on the low side. It will drain the battery even with the olad turned off. You should increase this divider's resistance considerably.

 

[sweben]

I'll raise the cutoff on the modules, but 3.7V is still quite high isn't it?

@Harald Kapp: What do you think, changing by a factor of 10? 18K/2K?

 

[sweben]

@Harald Kapp: So, using 18M/2M would work the same? the current will truely be minimal there.

Also, for the next generation, I'm considering getting proper comparators instead of amp ops. I'm looking at some LM2903D, can I just use it the same way as the lm358, or do I need additional components?

 

[Harald Kapp]

The answer will have to wait until I can see your diagrams, they seem to be blocked by the firewall here.

 

[Harald Kapp]

So, using 18M/2M would work the same? the current will truely be minimal there.

These values my be too high a you'l have to account for the input current of teh comparator.
I suggest not more than 1.8M/200k, probably 900k/100k.

Also note that ypur circuit is prone to oscillations when the battery voltage comes in the treshold range:
When the power is turned off due to low-batt indication, current flow from the battery stops and the battery will recover a few mV, thus turning power on again. Add some hysteresis to the comparator to form a Schmitt Trigger circuit and prevent oscillations.

 

[Old Steve]

Is that a 2K pot? That will also drain the batteries even when the load is switched off. You need to consider a much higher resistance pot, as suggested by Harald for the voltage divider.