# Circuit to test battery

Discussion in 'The Projects Forum' started by nybound, May 25, 2011.

1. ### nybound Thread Starter New Member

May 25, 2011
2
0
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

2. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,158
1,125
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.

3. ### CDRIVE AAC Fanatic!

Jul 1, 2008
2,219
105
This idea is really unsophisticated but it's effective... Use a low current reed relay.

4. ### KJ6EAD AAC Fanatic!

Apr 30, 2011
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Last edited: May 25, 2011
5. ### #12 Expert

Nov 30, 2010
18,076
9,691
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.

6. ### nybound Thread Starter New Member

May 25, 2011
2
0
Thank you all for your responses--KJ6EAD this is exactly the type of thing I was looking for.

7. ### electronis whiz Well-Known Member

Jul 29, 2010
517
28
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

8. ### CDRIVE AAC Fanatic!

Jul 1, 2008
2,219
105
I read the text in red and thought you wanted an automatic cut off. Guess not.

9. ### CDRIVE AAC Fanatic!

Jul 1, 2008
2,219
105
I suggest you try thinking a little harder then.