# How 18650 cells capacity is measured (what are the amperage and cut off voltage params)?

I have lots of used Li-Ion 18650s. I want to test their capacity.

There are however 2 important parameters that I cannot find anywhere which is amperage and cut off voltage. Some people use 500mA some people use 1000mA. Some discharge to 3.0V some do that up to 2.8V.

Depending on which parameters you choose you will get different capacity. Since capacity is defined for cells it's calculation must adhere to some standards for these parameters.

To wrap it up - what is the amperage and ending voltage for capacity measuring for single 18650 cell?

• A look at the specific cell's datasheet might answer your question about cut-off voltage. Feb 8 '19 at 12:30
• There is no reason to believe that the capacity calculation "must adhere to some standards". Read the datasheet. Feb 8 '19 at 12:41
• Take care when combining different cell types and or cells with unknown soc. Compensations currents between the cells can be very high. Also the internal resistance of the cells should be the same when combining them. Feb 11 '19 at 9:54

If there's a significant difference in measured capacity between 500mA and 1A discharge currents, you should stick to lower value, i.e. 500mA (or even less). Unless you're interested in capacity under specific load, you should aim to measure the maximum value, and cells typically have higher capacity at smaller discharge current.

The cut-off voltage cannot be reasonably estimated and should be taken from the datasheet (idem for the full-charge voltage). If the additional capacity between 3.0V and 2.8V is insignificant to you, you can stop at 3.0V for all cells (or even 3.2V, which already correspond to 95-98% discharge; IMO 5% precision is plenty for the purpose of sorting used cells). What you should avoid is discharging cells with 3.0V cut-off down to 2.8V.

Since capacity is defined for cells it's calculation must adhere to some standards for these parameters.

It is totally untrue. Batteries are designed for different purposes/uses. There are high-capacity batteries with moderate discharge rates (as for laptops). They have all electrode films thinner, so the active electrochemical surface per unit of volume is bigger, which gives higher overall capacity. But thin electrodes will have higher impedance, so overall battery's ESR is higher, and it can't deliver high currents without damaging itself. So the capacity is always defined at certain discharge rate AND for specified CHARGE conditions, which vary for different cells.

There are batteries designed to deliver high current, for RC models and drones, up to 50x-100x-150x of their "nameplate" capacity. So a cell with 1000mAh capacity can deliver 150A of current for drone motors, for example. But this battery has to have thicker electrodes to have low ESR, and thus they usually have somewhat less capacity per volume of the cell. More, usually these batteries are not expected to survive more than 50-100 recharge cycles, so they are operating under highly stressful conditions.

The recommended discharge cut-off is also determined by manufacturer, by their advertised (targeted) ability to take certain number of re-charge cycles. If the voltage cut-off is higher, and charging current cut-off is higher, the battery would last longer (however at somewhat smaller capacity), 1000 - 2000 - 5000 cycles, which you would want for some equipment and cars.

So no, there couldn't be any standards for discharge current and cut offs, these are individual parameters for individual products optimized for different purposes and manufactured with different technology implementations.