So i want to make a project using lithium ion batteries, this project involves me leaving the circuit/batteries unattended for over a month. So i want some kind of undervoltage protection which shuts off the output completly when the voltage goes down to around 3.5v. I have a few relays and voltage regulators/op amps at home. But i don't really know where to start. So i need some tips on how to start building this protection. Thanks!

 

Do your batteries not include a built-in discharge limiter?

 

You're better off getting an 18650 pcb monitor, ready-made,

http://m.ebay.co.uk/sch/i.html?isRefine=true&_pgn=1&_nkw=Lithium+battery+pcb&_sop=15

anything you make will have to work from a 3.5v supply like an op amp comparator.

 

Do your batteries not include a built-in discharge limiter?

No they do not.

You're better off getting an 18650 pcb monitor, ready-made,

http://m.ebay.co.uk/sch/i.html?isRefine=true&_pgn=1&_nkw=Lithium+battery+pcb&_sop=15

anything you make will have to work from a 3.5v supply like an op amp comparator.

I see thank you, well mostly i wanted to make this for learning. I could just buy one from ebay but the shipping time is pretty long to sweden.. Could you explain the "3.5v supply like an op amp comparator." a little more? Thanks!

 

If your battery is going to cut off at 3.5v, then the only supply is your camera battery, so your circuit will have to be powered by it. A comparator is an op amp circuit.

 

A comparator is a lot like an opamp but it has no inherent provisions for a graceful rolloff of frequency response. Use comparators as comparators and opamps as opamps and your life will be a lot easier.

 

No they do not.



I see thank you, well mostly i wanted to make this for learning. I could just buy one from ebay but the shipping time is pretty long to sweden.. Could you explain the "3.5v supply like an op amp comparator." a little more? Thanks!

The datasheet I read said don't discharge below 2.75V.

 

Thanks for all the answers. I'm trying to make some simulation for learning. but i still don't know where to start..

This is the only thing I've come up with so far..

Any example circuits are appriciated.

Edit: Would this work?

Edit again: And no it won't lol.. I get confused with op amps/comparators haha..

 

No it wont work, pins 1,2 are linked together,

This is the sort of thing you need,

https://www.google.co.uk/search?q=v...tage+comparator+circuit&imgrc=PEEvsIOjF-mGuM:

You need a fixed reference on the(-) inverting pin, like 3v, and alter the resistors on the non-inverting pin (+) to set the switch over point

 

No it wont work, pins 1,2 are linked together,

This is the sort of thing you need,

https://www.google.co.uk/search?q=voltage+comparator+circuit&client=ms-android-sonymobile&prmd=isvn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiJzuns0O7PAhVhKMAKHcq7ACsQ_AUIBygB&biw=360&bih=512#tbm=isch&q=battery+voltage+comparator+circuit&imgrc=PEEvsIOjF-mGuM:

You need a fixed reference on the(-) inverting pin, like 3v, and alter the resistors on the non-inverting pin (+) to set the switch over point

i see thank you very much! I also found this: http://danyk.cz/p_ochr_en.html

Works really well when i simulate it. What do you think?

 

Yes, simply better.

 

Yes, simply better.

Great thanks, i only have a small issue. I want to replace the zener with a TL431 which has a 2.5v reference, but i don't get the results that i want. atm i have it like this for testing:

If i understand this correctly it should give me 2.5v, i.e the output should terminate if i go under 2.5v, and allow current to flow if i go higher, but even at 3.8v current still won't flow until i go up to 4v.. any idea why?

 

Great thanks, i only have a small issue. I want to replace the zener with a TL431 which has a 2.5v reference, but i don't get the results that i want. atm i have it like this for testing:

If i understand this correctly it should give me 2.5v, i.e the output should terminate if i go under 2.5v, and allow current to flow if i go higher, but even at 3.8v current still won't flow until i go up to 4v.. any idea why?

The TL431 "programmable Zener" is actually a comparator with a built in 2.5V Vref - and all in a TO92 package (and various SMD styles).

The input resistance is about 100k or so, that means fairly low standing sense current. Using it outside a nfb loop means the comparator will switch - but in the on state it can't go below about 2V, because Vcc and output are both the same pin.

You might get away with discharging a lithium cell as low as 2.5V, the datasheet I read says 2.75V - you can put a voltage divider on the comparator input, but it'll draw slightly more sense current.