Best way to determine 18650 capacity, resistance and condition ?

I have done this for larger battery packs, but the same general method applies for measuring the internal resistance of the cell.  Without buying special battery test hardware, you will need an electronic load and a good quality volt meter, ideally both equipments can be controlled via a computer interface.  Testing the cell at approximately 50% charge (around 3.7V) should yield the most accurate results since this is the most linear region of the charge/discharge voltage curve.  Note ambient temperature of the cell will impact the resistance.

1. Connect the cell using a "4-wire" also called "Kelvin" connection to the programmable load.  This means two wires shorted together at each the positive and negative terminals.  One each of the positive and negative leads will go to the electronic load, the other pair will go to the volt meter.  The purpose of this is to cancel the voltage drop caused by the current through the wire resistance to the load.  Since the volt meter should have very high input resistance (>10Meg Ohms), there is negligible current flowing through the wires to cause a voltage error.

2. Set the load to something like 0.1C current rating of the cell, running in constant current mode.  If your cell is 3000 mA-hr, this would be 300mA current.

3. It is best to automate this measurement sequence, but close enough measurements can be made by manual operation.  The faster the measurements can be made, the less error will be introduced to the measurement since current flowing through the cell will cause self heating (as mentioned, cell temperature will impact the measurement)...generally the thermal mass of the cell and the general low internal resistance will be forgiving if you're not too slow.

4. Start by taking the cell voltage at the 0.1C or 10% current rating of the cell.

5. Step up the load current to something like 0.9C or 90% the current discharge rating of the cell.

6. Take another voltage measurement.

7. Disable the load.

8. Perform some simple math per ohms law to find the resistance:  Voltage = Current * Resistance.  Because this is a linear relationship (for resistive circuits), a delta measurement of current will cause a direct proportional change of the voltage (assuming resistance is constant...which is the assumption we are making).  So then (Voltage_0.1C  - Voltage_0.9C) = (Current_0.9C - Current_0.1C) * Resistance.  Simple algebra yields Resistance = (Voltage_0.1C - Voltage_0.9C) / (Curent_0.9C - Current_0.1C).

For capacity information, you can get decent battery chargers which perform this measurement for roughly $10 per cell slot.

I have done this for larger battery packs, but the same general method applies for measuring the internal resistance of the cell.  Without buying special battery test hardware, you will need an electronic load and a good quality volt meter, ideally both equipments can be controlled via a computer interface.  Testing the cell at approximately 50% charge (around 3.7V) should yield the most accurate results since this is the most linear region of the charge/discharge voltage curve.  Note ambient temperature of the cell will impact the resistance.

1. Connect the cell using a "4-wire" also called "Kelvin" connection to the programmable load.  This means two wires shorted together at each the positive and negative terminals.  One each of the positive and negative leads will go to the electronic load, the other pair will go to the volt meter.  The purpose of this is to cancel the voltage drop caused by the current through the wire resistance to the load.  Since the volt meter should have very high input resistance (>10Meg Ohms), there is negligible current flowing through the wires to cause a voltage error.

2. Set the load to something like 0.1C current rating of the cell, running in constant current mode.  If your cell is 3000 mA-hr, this would be 300mA current.

3. It is best to automate this measurement sequence, but close enough measurements can be made by manual operation.  The faster the measurements can be made, the less error will be introduced to the measurement since current flowing through the cell will cause self heating (as mentioned, cell temperature will impact the measurement)...generally the thermal mass of the cell and the general low internal resistance will be forgiving if you're not too slow.

4. Start by taking the cell voltage at the 0.1C or 10% current rating of the cell.

5. Step up the load current to something like 0.9C or 90% the current discharge rating of the cell.

6. Take another voltage measurement.

7. Disable the load.

8. Perform some simple math per ohms law to find the resistance:  Voltage = Current * Resistance.  Because this is a linear relationship (for resistive circuits), a delta measurement of current will cause a direct proportional change of the voltage (assuming resistance is constant...which is the assumption we are making).  So then (Voltage_0.1C  - Voltage_0.9C) = (Current_0.9C - Current_0.1C) * Resistance.  Simple algebra yields Resistance = (Voltage_0.1C - Voltage_0.9C) / (Curent_0.9C - Current_0.1C).

For capacity information, you can get decent battery chargers which perform this measurement for roughly $10 per cell slot.