BLE has it almost correct.. well, half correct. CYCLE means to go around in a cycle or a circle. Discharge then charge. If it only discharges, then it's a discharger. If it only charges, well, I guess you would call it a charger, right? A cycler will discharge THEN recharge appropriately.

There are several commercial cyclers on the market that will help you with this little project. The batteries in a pack are connected as a pack by the manufacturer and are usually 'sealed' in a form of shrink wrap. Then they are treated as a pack not as an individual battery.

Cyclers 'look' at the condition of the pack by discharging them to a preset level then re-charging them. They usually (often) have some sort of digital display that shows how much amperage was removed when discharging. Then they repeat the process after the battery pack has fully charged.. which will cycle down to a set point, display this amperage, then recharge to the set charge, depending on the number of cells attached and the type of batteries in use. Repeated cycling usually, often, will restore a questionable battery pack to normal. Battery packs with bad batteries are just thrown away, they you buy a new battery pack. They are cheap enough that you can do that. It is not worth it to keep a questionable pack just for the sake of a few dollars. It can cost you a lot more in destroyed aircraft and any damage they may do when crashing.