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Author Topic: Low voltage cut-off circuit for li-ion cell.  (Read 1489 times)
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I'm currently in the process of finalizing a UPS-like circuit, where the off-grid energy storage is one single & large li-ion cell.
BUT I simply don't trust the cut-off circuit in protected li-ion batteries enough to blindly rely on them, so I'll need some advice on the simplest yet still efficient and somewhat beefy circuit that can simply cut-off the battery from the voltage converter when the voltage output of the cell reaches 3.15 volt and lower.
I've already understood that I'll need a mosfet for that + either a zener diode + a resistor OR a voltage divider bridge consisting of a few beefy resistors.
But after that I'm completely blank.

Thanks in advance.
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A comparator would be fairly simple.

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But after that I'm completely blank.

Then do you really have a choice other than trusting the built-in circuitry?
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"somewhat beefy circuit"
How much current are planning to pass thru?
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If you are at the beginner end of things and would prefer to buy a kit instead of doing it completely by yourself, Sparkfun (and presumably others) offers a kit that lights up a LED when the voltage is below a threshold.  Instead of the LED, you can hook the output to a digital pin that provides an interrupt (pins 2 and 3 on the Uno R3) to tell your microprocessor to shutdown.  Here is the sparkfun link: https://www.sparkfun.com/products/11087.
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"somewhat beefy circuit"
How much current are planning to pass thru?

The load of the battery is a small boost converter module that's been slightly modified (removed the on-board LED and it's current limiting resistor + the USB plug AND added electrolytic capacitors on input & output)
And if I've estimated correctly (including the typical efficiency of a cheap boost converter circuit), then we're talking about 1 Ampere worst-case.
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There are precision voltage references (basically you treat it like a zener diode) that will give you a steady 1.225v, which you feed into one side of a comparator.  On the other input, you run your supply voltage through a voltage divider until the tap between the two resistors is also 1.225v at your threshold voltage.  I would take the output of the comparator to a latch that, once turned off, doesn't turn back on again.  I designed a circuit to do this for a small ATX-like PSU to replace the one that failed in my Popcorn Hour media player.  Haven't built it yet, but the sims look good.

If you want to see my test circuit (it monitors +5v and +12v rails for +/- tolerance and issues a Power Good signal), go to http://www.falstad.com/circuit/ and let the applet run.  Go to File, Import and paste this:

Code:
$ 1 5.0E-6 5.934295036739208 50 5.0 50
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172 320 240 288 240 0 6 4.800000000000001 20.0 -20.0 0.0 0.5 Supply Voltage
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w 832 112 832 192 0
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Here's a screen shot.  Yours will be simpler, since there's less to monitor.  The latch is in the lower right.


* schem.png (31.56 KB, 741x544 - viewed 48 times.)
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