Get yourself a few tools:

• Multimeter
• Infrared thermometer
• A battery (capacity) tester
• Battery charger

Multimeter

This is a go-to for any electronic hobbyist. Before take the cell out, measuring the voltage of cell above 3V, it is good. If the voltage below 3V and you have time, there is a chance that a cell is usable.

Multimeter is cheap and handy. I own several multimeter from $3 to $40. Get medium price around $10-15 is recommended.

Infrared thermometer

This tool is for monitoring the temperature of the cell. The cell can be uncomfortable hot when it is shorted circuit. Having a digital readings is objective, so you can monitor cell's temperature better.

Battery tester

Tester comes with dummy load for discharge test, and chargers to estimate how much energy has been charged to the cell. The first tool is a down-stream approach while the first is the uptream once. Notice that, a charger provides an approximate energy and most of the case overestimate that amount of energy. Bad cells leak electrons internally which releases in heat.

Testing a Samsung LiPo cell with ZK-FX25

If you have a working laptop battery, you may be in luck to read data from the battery like this one.

Data logger

I am fond of logging data and that for a good reason.

A top view of DIY current monitoring using TI INA219 sensor. Data is logged via MQTT server from ESP8266 and displayed on a 0.66" OLED shield

From six old cells from LG, SANYO and SAMSUNG, discharge performance is promising. The voltage dropped under load is small. The overall capacity is decent.

INA219 is compatible with an multimeter at 2A discharge current, one showed 2001 (INA219) and another was 2006 (ZOYI ZT-S4).

These tools look to fancy

Yes, sometimes it does. I don't have all these tools in place when started. A simple approach is connected a dummy load, a big resistor which a value from 3-5 ohms and power dissipation > 10W.

• Fully charge the battery using TP4056
• Measure voltage of the open circuit, that is the voltage between terminals withload load
• Discharge battery by a 3-5ohm dummy load
• Measure the voltage of close circuit

If a full-charged 18650 having the voltage dropped to 3.7V with 1A-current running, that is likely not a decent old battery. My experience tells me the capacity is less than 800-1000mAh. If the voltage dropped just 0.1-0.2V with 1A current, jay!

Keep good cells

If we have a few cells, we can keep all of them. But when we have enough cells, we need to figure out certain capacity and condition to keep and if not to dispose. My quick reference is 800-1000mAh for capacity. The cell is not leaked or heat up too much during charging. Trying to use bad cells lead to more troubles later.