Conventional way to measure Li-Ion cell capacity?

Question

I have an 18650 Li-Ion cell taken from an old powerbank, and was interested to find its capacity so I built a capacity tester. This consists of a meter (displaying volts, amps, and mAh), two 6V 5W lamps in parallel for a dummy load, and a TP4056 charger board with a DW01 on it to cut off the discharge current when the cell voltage falls to around 2.4V. I fully charged the cell then allowed it to discharge at just below 1A through the TP4056 board and lamps until (after about 15 mins) the lamps turned off. The meter showed a value of 315mAh. The cell then recovered back above the 2.9V over-discharge release threshold (and the lamps turned on again) only to fall below the over-discharge cutoff threshold (and the lamps turned off again). Allowing the setup to bounce around like this for 24hrs, the meter showed a value of 1,610mAh (with the cell still bouncing!)

The fact that the cell so rapidly discharges to cutoff at 315mAh out of at least 1,610mAh led me to conclude that the cutoff capacity of the cell will depend on the current being drawn from it. I tested this by removing one of the lamps and repeating the procedure. The current draw was just below 0.5A and (after something approaching 3 hours) the lamp turned off. The meter showed a value of 1,310mAh. (I assume that if it were left running again it would bounce around for at least another 300mAh.)

This leaves me unsure about how to measure the capacity of a Li-Ion cell. If it is the amount of charge released up to the point where there is insufficient charge in the cell to recover back above the 2.9V over-discharge release threshold then I will never achieve that capacity in practise because the cell protection board will terminate the discharge prematurely. But if it is the amount of charge released up until the cell protection board terminates the discharge then the cell capacity would be dependent upon load resistance.

Can anybody tell me the conventional way to measure Li-Ion cell capacity please?

Answer 1

The cell capacity is dependent on the charge voltage, the discharge rate, and the end point voltage. The charge voltage and end point voltage also affect the cell lifetime.

This means there is no one 'right' way to measure the capacity of a cell, as both lifetime and capacity are desirable, but are in conflict.

Generally, what the people responsible for the headlines describing the cell will do is state the capacity using a charge/discharge regime that will result in short lifetime, and then state a lifetime 'up to' some much larger number of cycles that can only be achieved when delivering half that capacity. But such is specmanship!

You have to decide whether you want long lifetime, or best capacity, or a compromise, set your charge voltage and endpoints, decide on a discharge rate, and measure to those.

Let's look at this 18650A data sheet from Panasonic. They are a big name reputable manufacturer, and to their credit they tell you what conditions they have used, and quote their cycle lifetime under the same conditions that they quote the capacity.

They charge the cells at 0.5C. That's slower than most commercial chargers, and results in an improvement in capacity. The endpoint voltage is the normal 4.20V. Their charge endpoint current is C/50, which is a longer charge than usual, most chargers will cut off at C/20 or C/30. The discharge is to an endpoint of 2.5v. I don't know of any commercial battery control systems that would take it this low, 2.7v to 3v is the norm.

The three extreme conditions means that you won't see the capacity they quote when using normal commercial charge/discharge controls with faster charge and more conservative endpoints. But you should if you replicate their conditions.