Low Voltage Cut-off Discharge Protection

Hi,

I would like to create a discharge protection circuit for my project. I have 4 li-ion batteries in series that I need to sample each of them in order to check if any of them are below 2.7V. How would I go about it?
It should take very minimal electricity thus I chose to use a FET transistor as a switch. And also I saw this product on eBay - Low Voltage Buzzer Alarm
It buzzes upon low voltage detection, can I hack it to meet my needs? If I add a capacitor and somehow take the signal and convert it to open the transistor when signal is "0" and close when "1" it should do. However I've tried to do it but from the positive and negative terminals I wasn't able to actually find the common ground of it. The negative isn't ground...
What to do? Anyone have an idea?
I just need a circuit to detect the voltage difference between the chip's output plus and minus terminals and make it that if it detects voltage to output "0" and if it doesn't detect(which means the alarm doesn't work so the batteries' voltage is safe) to output "1"
So how to do it?

I’ll just throw you some thoughts & information…

The Arduino’s analog-to-digital converter measures relative to ground, so directly-measuring the voltage of a battery in the middle of a series is “tricky”.

However, there are multiple analog inputs. So you can measure the voltage of one battery, two batteries, three batteries, and four batteries all at once. Then, you can simply subtract (in software) to find the voltage of any one cell.

The Arduino can measure between 0V and +5V (Vcc) and it can be damaged by negative voltages or anything higher. So, you’ll need [u]voltage dividers[/u] (2 resistors) to knock-down the voltage. Then of course, you compensate for the the voltage divider in software to get the true voltage. Current will flow through the resistors, but if you use megohm-range resistors the current will be small and you can probably live with it. Virtually no current flows into an Arduino input (as long as you stay between 0V and Vcc).

Once you’ve got the voltages, turning-on an LED or buzzer with an if-statement is pretty easy.

How would I go about it?

Use a battery protection PCB. They are designed for the task, cheap and also protect from overcurrent and overcharging.

I would prefer if I could accomplish this without Arduino. As for a battery protection PCB, I've searched for a one with these parameters but couldn't find the right one: 2.7V discharge cut off, 4.2V charge cut-off

Why is your choice of cutoff parameters so critical?

The batteries I use should be cut off at 2.5V but I prefer to cut off a bit earlier.
Anyways, I've a thought of a solution. However I don't know if it's feasible.
I have these components: LM393, 100K ohm resistor, 10K ohm resistor, 1N4728 3.3V zener diode, 10K ohm trimpot, step down regulator, and a FET trasistor.

Could I somehow make use of the LM393?