I imagine this will be pretty easy for some people, I don't have much experience with circuit design though.

I want to create a very simple circuit to test if a lithium-ion battery pack is charged enough to be providing a certain amount of voltage. I have determined that I need 12V min; with a full charge the battery pack supplies 16.8V. I've already made a cable to provide contact to the V+ and ground leads, what kind of circuit would I need to light up an LED if >=12V?

I am hoping for something simple, maybe just a load and limit resistor with an LED. A transistor and battery would be fine but I prefer not to use any op-amps. I have the following specs, do I need anything else to calculate the resistor, etc, values?

Max. Continuous Charge Current: 2.2A (limited by cell)
Rec. Charge cut off: 44mA or by time 3h
Max. Continuous Discharge Current: 2.5A (limited by connector)
Rec. Discharge cut off: 12V
Internal Impedance: approx. 400mOhm

 

You WILL have to use opamps, or something similar

OR how about just getting a simple VOLTMETER. You can make it into a permenant battery tester by adding the proper mating connector and placing it in a project box. You could get fancy and use a small leaf switch, so the meter only powers on when you place the battery you want to test in the tester.

The display of the meter will show you how much voltage you have.

I've seen small analog meters smaller than the size of pack of cigarettes.

It really is the 'simplest' way to do it.

 

This idea is really unsophisticated but it's effective... Use a low current reed relay.

 

You can use a 1 zener diode, 1 resistor and 1 LED. Here's a circuit that will work: http://www.reuk.co.uk/Make-a-Simple-Battery-Status-Monitor.htm. You will have to calculate slightly different component values of course.

 

Don't laugh. It was sophistocated enough for me to put a $4 Harbor Freight meter in my Variac cabinet. A tiny transformer provides the 9 volts to power it.

 

Thank you all for your responses--KJ6EAD this is exactly the type of thing I was looking for.

 

all i would think you would need is a resistor and an led. there are many online calulators that can tell you the value i would ges at least 500ohms would be needed. a transistor would work also bet then you need a pair of resistors 1 to limit led power and one for the value conected to base.
if you want a few led E.G. red= low yellow = ok green = good you just pit the leds in paralell withe a series of resistors on one side then conect the other side with wire

 

Max. Continuous Charge Current: 2.2A (limited by cell)
Rec. Charge cut off: 44mA or by time 3h
Max. Continuous Discharge Current: 2.5A (limited by connector)
Rec. Discharge cut off: 12V
Internal Impedance: approx. 400mOhm

I read the text in red and thought you wanted an automatic cut off. Guess not.

 

all i would think you would need is a resistor and an led.

I suggest you try thinking a little harder then.