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Using an LED for battery indication

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Destek

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I am testing a small hand-held transmitter for my company. It transmits a signal on 433.92 Mhz and uses a button battery.
WHen you push a button an LED lights up and this LED is supposed to also let you know the battery is good. If it doesn't light up then the battery is dead and you should replace it.
So far so good.
The battery is a CR2450 (3V .620 AH) and it looks like it will easily last 2 months - which is just OK - not great though. When idle the unit draws about 20 microamps and when transmitting it will draw just under 20 milliamps.
I ran some tests today using a power supply, not a battery. I found that the unit would actually stop transmitting when the voltage was lowered to 2.15 volts.
The LED, however will continue to light up until the voltage is dropped all the way down to 1.92 volts.
Now I am using a power supply. I set the PS current limiter to the lowest I could to keep the transmitter operating at the full 3 volts.
My question is: is my test valid and do I really have a problem? Based on my test it would seem that a battery nearing the end of its life would continue to indicate as operational when, in fact, it isn't. Will an actual battery behave differently from my power supply - invalidating my test?
Thanks for any advice you can give me!
 
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This is a very good example for the testing process and real operation. And I do this kind of mistake offenly.
Keep in mind that the battery acts like a limitted reservoir. So when it discharged to the critical point, it's voltage and current capacity will drop very rapidly.
So after testing from the workbench power supply, you shold test the operation from the actual battery and monitor the voltage and current too.
 
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    Destek

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adding to "pico"s points:

you can cutoff the battery to transmitter if voltage is <=2.15v.
(how to do it , depends on your circuit)
 

The battery detection system obviously isnt accurate and this is common.
However your power supply method isnt the best test, when batteries go dead they tend to go high resistance, that is the voltage drops more and more with load, so that the battery volts will be high enough to extinguish the led untill the transmitter powers up and drags the voltage down.
Try connecting a 100r potmeter in series with the transmitter from the bench power supply, and rather than vary the bench supply's volts, vary the potmeter and see if the led goes out when the transmitter stops working, I bet it does.
 

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