A good charger will recondition and somewhat restore a battery, helping it to recover from neglect or severe depletion of charge, and will get the maximum possible life out of it, more recharge cycles and a slower reduction in capacity per recharge.
A good charger can also give you real-time reporting on the capacity of the battery, and help you to understand which NiCd rechargeable batteries are nearing the end of their lives and which are still in good shape. This is not just interesting but essential, because it means you can match up batteries of similar remaining capacity for your devices.
It is not a good idea to have, eg, a 2500 mAh battery paired with a 1500 mAh battery that means you’re only using 1500 of the 2500 mAh capacity of the better battery, or alternatively, you’re harming the lower capacity battery by over-discharging it. Better to have two NiMH batteries of each about 1500 mAh.
To manufacturers, the number of recharge cycles a battery will accept depends on many things such as the type of use it is getting, the temperature it is being used at, and the type of recharging it is being given, too.
A battery doesn’t just go stone cold dead after a certain number of recharges. All that happens is there is a steady degradation in the amount of storage capacity the battery has. At what point has this become unacceptably short? With rechargeable batteries freely available and inexpensive, many people consider that when the batteries have lost a third to a half their capacity, it is time to replace them. But if you are willing to accept less storage per charge, there’s no reason not to keep reusing and reusing them well beyond their rated life.
You probably already know there’s a complex science associated with charging and maintaining lead acid type batteries golf cart and car batteries, and that you can harm those batteries by incorrect charging, or overcharging, or by allowing them to discharge down to zero.
You can harm a 18650 battery by charging it too quickly, and you can harm a battery by overcharging it. Cheap chargers and fast chargers threaten to do either or possibly even both of these things to your valuable batteries. Do not economize on your chargers.