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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?

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    \$\begingroup\$ Most users just want max capacity and forget this also reduces number charge cycles. So what do you want Max life or max capacity? or a balance of these tradeoffs. If so I suggest charge to 4.15, and drain to 3.1V at 25'C with 20h charge/discharge rate and have more spare cells then accept reduced short term capacity to get much greater life Ah*cycle \$\endgroup\$ – Tony Stewart EE75 Dec 6 '16 at 18:21
  • \$\begingroup\$ Thanks for that Tony, but I'm not really asking how to look after my cells but rather how capacity is determined. Unless there's some kind of standard procedure then how can cells be fruitfully compared? \$\endgroup\$ – Andy Kay Dec 6 '16 at 21:42
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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.

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  • \$\begingroup\$ Thanks Neil. How could one validate that, say, a Panasonic NCR18650B has the stated capacity of 3400mAh if so many variables enter into the testing procedure? Am I being naïve to think that there must be some standard procedure for establishing the capacity of Li-Ion cells? \$\endgroup\$ – Andy Kay Dec 6 '16 at 21:49
  • \$\begingroup\$ you find out how they tested it, so charge voltage, discharge rate, and end point, then replicate those conditions. \$\endgroup\$ – Neil_UK Dec 6 '16 at 22:20
  • \$\begingroup\$ It sounds like it would be a mistake to assume that e.g. a Panasonic 3400mAh cell and a Sony 3400mAh cell have the same capacity then. This is disappointing. \$\endgroup\$ – Andy Kay Dec 6 '16 at 22:37
  • \$\begingroup\$ It's usually a mistake to assume anything. As Sony and Pana are big names, with reputations, I think you can assume that both will have more capacity than a Sunshine Industries (I hope there isn't a company out there called that) 5400mAh (there is, but they make chapati making machines, not batteries). Why don't you read their literature and find out what testing regime they use, is there some problem with research instead of guesswork? I'm thinking the first frame of XKCD 1567. \$\endgroup\$ – Neil_UK Dec 7 '16 at 7:48
  • \$\begingroup\$ The implication is that there are indeed standard testing parameters adhered to by reputable companies (a "meat thermometer"), and that this, my research "first port of call" so to speak, can't help me find out what it is. I'll delve deeper. Thanks Neil. \$\endgroup\$ – Andy Kay Dec 7 '16 at 8:28

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