Why do I sometimes rate a cell lower than other testers do?

Published by Mooch in the blog Mooch's blog. Views: 1781

The performance of a cell over months of use in our devices doesn't just depend on that how that cell does for one or two discharges when new. It depends on how fast the cell ages over time.

What affects this aging? Assuming you're not overcharging or overdischarging, temperature has the biggest effect on your battery's health. The hotter you run them, the quicker they will start to lose capacity and run at a lower voltage.

To better determine how a cell will perform over time I run multiple discharges at the cell's specified or accepted continuous discharge rating (CDR). If the cell gets too hot, much over 75°C, then I know that the CDR rating is too high for that cell. I then run another lower-current discharge to measure the capacity loss and damage that the high temperatures might have caused. If there was a capacity loss then I have confirmation that running at the cell's specified CDR is harmful. This means that the CDR needs to be lowered.

When picking a CDR I'll select the discharge current level that results in the cell temperature rising no higher than approximately 75°C. While this is an extraordinarily high temperature to operate a cell at, it does allow for powering the small devices we use now while still maintaining decent battery life and safety margins.

If you see test results elsewhere that rate a cell a lot higher, or lower...ask questions! What temperatures did the cell reach during testing? Did the tester do multiple discharges to see if the cell started to show signs of damage at its CDR? What method did the tester use to measure temperature? Some popular methods are very poorly suited for measuring cell temperatures.

All these questions may seem overly geeky but the answers will make you a better, safer vaper and help you to pick the tester(s) you trust to review the cells you're putting up to your face.

Another question might be, why do I say "cell" instead of "battery"?
To prevent confusion with the portion of a vaping device that powers the coil, often (confusingly) called a "battery", I use the term "cell". It's also the term used by cell manufacturers and my clients so I'm just used to it. :)
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