Guru
Joined: 05/10/2019
Location: United KingdomPosts: 8894
| Posted: 03:12pm 10 Mar 2025 |
A "primary cell" in this context is where the energy is provided by a non-reversible chemical reaction - a non-rechargeable source. An 18650 is a "secondary cell", in other words the chemical reaction can be reversed.
Using a fixed resistor, the current through it falls as the battery terminal voltage falls (ohms law). A constant current sink provides a fixed, known current throughout the discharge curve. Unless the battery is completely flat (and probably damaged) we can be pretty sure of it's off-load terminal voltage and we know the current so the internal resistance is the only variable. That's what ultimately decides the capacity.
You can also estimate the capacity by measuring the terminal voltage while you connect and disconnect a known load. This gives a guide to the internal resistance and hence the state of the battery. This method is often used for automatic "battery health" indicators in switchgear. It isn't a good guide to the capacity, but does give a pretty good indication of whether the battery could accept a short term emergency lighting or breaker tripping load.
It's all a guide anyway as the amount of charge entering the cell and the discharge capacity both vary quite a lot with temperature.
If you want to measure the C/20 capacity then it's not accurate to load it with a fixed resistor.
e.g.
C=1500Ah It will discharge to end point voltage in 1 hour at 1.5A
C/20 It will provide 75mA for 20h
If start voltage is 4V3 and the load resistor is 56R then current is 76.8mA
At end point (3V) the current has fallen to 53.6mA
At a voltage of 3V7 the current would have have fallen to 66mA so the run time of a 1500mAh cell would be about 22.5 hrs. A constant current of 75mA would have given the correct value of 1500/.075 = 20 hours.
As the voltage has fallen then so has the load so the time to end point has slowly increased.
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Edited 2025-03-11 01:30 by Mixtel90