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A lithium battery cell is 4.2V when it is fully charged. It drops to 3.2V when it is almost dead and should be charged. A discharge voltage of 3.0V or less will destroy it. The average voltage is 3.7V.
What about a "1.5V" alkaline cell?
It is 1.6V when brand new.
It drops quickly to 1.2V when it is used.
The manufacturer rates its capacity when its voltage has dropped to only 0.8V which is half the voltage when new.
The phone is not COMPLETELY dead. The LED idea is a GREAT one! I have to say that I didn't think of it because after metering and finding a voltage between pad 1 and all of the other 3 pads I got a bit confused. I will have to think on how I could get a voltage measurement from all of those, but only get usable current to light the LED from one.
I will do that for maybe 1 minute and re-check voltage to see how it has changed. The battery label says 3.7V, 800mA however V1-4 shows 3.78V. I get the 4.2V for Li-I, but I am a bit confused about the label rating of 3.7V o the back of the battery. What are they getting at?
What is the safest way to know when to say when? I am thinking the phone measures battery life via voltage measurement (the inaccuracy of which I do understand) but, again, the 3.7V label is throwing me off.
You know it might not seem it but I am pretty familiar with those. The problem that I have is that in my undergrad the courses specifically dealing with these types of problems were not practical at all and thus were pretty ineffective. I have what I consider a `working knowledge` with hands-on circuits and do my day-to-day work as a wireless sensors guy using premade Si chips instead of discrete components.You are seeing the voltage drop across the resistor. 4.1V-3.75=350 millivolts.
The 3.75 is the drop across the internal resistance of the battery.
I think it was working fine.
If you are getting interested in electronics, this is a good lesson in Ohm's Law and Kirchoff's Voltage law.