I'm trying to build a circuit that will let me know when one of a string of series lithium-ion cells drops below a certain voltage.
I pretty much want to build exactly this (http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=4176), except with an adjustable setpoint and more extensive indication.

I'm looking at comparators which I understand fairly well, but the problem I'm running in to is getting a reliable reference voltage for each cell. I'm thinking either voltage following opamps or optocouplers or :confused:?

I'd buy three of those, except shopping from Hong Kong is $30, they don't cut off at the right voltage for the cells I'm using, and, well, this is DIY.

Thanks.

Batteries are LiFePO4 lithium-ion, 3.3v nominal, 3.65v charge, 2.1v cutoff.

What exactly are you trying to do? Are you trying to masure the voltage of each individual battery in series or measure the voltage across a parallel combination of serial coupled batteries? I'm really not sure what the Lipo monitor is for either.

I'm trying to measure the voltage of each cell in a 12-cell battery pack wired in series. This is what the lipo monitor does except it doesn't do exactly what I need, so I want to build my own.

I see. I think you could do it with comparators. One voltage is the voltage across the positive side of the battery and the other the negative side plus whatever voltage you want to be the trip voltage set with a voltage dividing bridge. The bridge would need to be set with a dedicated DC power supply, so it's not tied directly to any one of the battery voltages which will decay over time and would never trip the comparator.

Here's (http://i34.tinypic.com/a1sme1.png) what I've come up with.
Datasheets:
Voltage monitor (12): TC54 (www.solarbotics.net/library/datasheets/TC54.pdf)
Quad optoisolator (3): ACPL-847 (www.avagotech.com/docs/AV02-1429EN)

I'm guessing the opto resistors should be about 100 ohm. Pull-down resistor on the right should be 1k or so, the funky thing is a piezo buzzer, and the transistor can be any general purpose NPN (2N3904?). Vreg will come from a LM2576HV (www.rose-hulman.edu/~herniter/Data_Sheets/LM2576.pdf) switchmode regulator, 5V version.

How does it look?

What is the voltage drop of the LED's? The complete voltage drop between the LED and the base-collector junction cannot be more than 5V, otherwise the transistor will never turn on. It looks like a decent setup.

The 2.1V is the drop-off the voltage detector is set to? If so, it looks like the detector will have enough voltage to forward bias the optoisolator. You may need to figure out the current resistor for the optoisolator input and lowball it enough so that you will not exceed the current specifications at full power. When you discharge the cell, its internal resistance increases and the voltage drop may be too low across the LED because the resistor is too large to forward bias it, so you would never get an indication.