0
\$\begingroup\$

This is what I've understood about the process of discharging of a secondary cell. If I have a rechargeable cell, full charged, and I make it discharge at a certain constant current, let's say 1A, I will notice that its tension will go down since the EOL (end of life) tension. If I measure how much time the discharge process takes, let's say 1 hour, and if I multiply this value for the constant current, I get the capacity of the battery at that specific discharge rate (in this case the capacity is 1Ah). If I repeat the same experiment using higher currents I will obtain shorter time, but also lower capacity according to Peukert law, or some more realistic law. For example at 10 A I could get a capacity of only 600mAh. Often the constant current is written as function of the nominal capacity, so a discharge rate of 2C for a nominal capacity of 1Ah, means a discharge current of 2A. But what I really don't understand graphs Tension-Capacity, like this one:

enter image description here

Following my previous argument, given a certain discharge current the capacity is uniquely determined while tension stays inside a certain range. I wuold draw something like this:

enter image description here

Any help is very welcome.

\$\endgroup\$
7
  • \$\begingroup\$ Is that a postage stamp glued to your page? \$\endgroup\$
    – Andy aka
    Commented Feb 27, 2020 at 14:25
  • \$\begingroup\$ Well it would be easier to read if it was a bit larger. \$\endgroup\$
    – user16324
    Commented Feb 27, 2020 at 14:33
  • \$\begingroup\$ The graph seems to agree completely with your written understanding. Can you be more specific about what is confusing you? \$\endgroup\$ Commented Feb 27, 2020 at 14:35
  • \$\begingroup\$ mAh capacity at a given current (curve) is the x axis value where each curve crosses the 3.0V line. \$\endgroup\$
    – Russell McMahon
    Commented Feb 27, 2020 at 15:01
  • \$\begingroup\$ I didn't realize the graph was so small, hope this new one will be readable @Andyaka \$\endgroup\$
    – Landau
    Commented Feb 27, 2020 at 16:42

2 Answers 2

1
\$\begingroup\$

This is a graph of output voltage compared to state of charge at several discharge currents. It's plotted against SOC because obviously we know it'll discharge faster if we pull more current--this type of plot allows us to more easily see how the rate of discharge affects the power actually delivered from a single charge.

If you want to translate it into time, you need to know the rated capacity, which is probably about 3000mAh. That lets you scale 1C (3A) into a 1-hour time range. The 2C (6A) version would be compressed by 2:1 on the same timescale, the 0.5C would be expanded by 2:1, etc.

Constant current discharge is typical, but in some cases you'll see resistive or constant power (emulating a switching regulator) curves too.

\$\endgroup\$
1
\$\begingroup\$

The graph is telling you that the available capacity depends on two factors: the discharge rate and the terminal voltage. The terminal voltage is the voltage level that means "empty" for your application. If your terminal voltage is 3.0V...meaning that your system will still work when the battery is discharged down to 3.0V...then you will get more capacity out of the battery than if your system died when the battery voltage fell below 3.6V.

So, you choose the colored curve that corresponds to your discharge rate. Then draw a horizontal line corresponding to your selected terminal voltage. The intersection of these tells you the capacity of the battery for those conditions.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.