NiCad batteries have a limited number of charge cycles, and will have a life span of about 3 years average. Sometimes you can have a lucky one that will last longer. Usually after about 500 charge cycles, or about 3 years, they will start to degrade in their performance. This is actually the same for all chargeable batteries.
The exception is that the newer technology Lithium batteries do not have the memory problems of the NiCad batteries. This type of battery can be charged at any time without having to be properly discharged. Their life cycle is about the same as the older NiCad types.
The recommended charge rate from the battery manufacture should be the best to use. This is depended on the particular battery design.
If you are using a separately bought chargeable battery, and designing your own charging system, you should consult the battery manufacture for the proper range of charging specifications. This also applies to knowing the maximum safe loading, and discharge rates, when running the devices with the battery.
When you have a commercially manufactured product, they build their charger to match the battery type from the battery manufacture. These products are designed to meet speciations that are from the UL, CSA, and EC standards for safety and performance. Altering these specifications, will put the product outside of the recommended application range, and thus may pose a safety, and or performance issue.
If you have a battery that is not charging, and the charging device is in specification, this means that the battery must be changed. There is no proper way to revive an older battery that has been through its life cycle, or has been damaged.
There are publications about how to so-called, "revive" batteries, but these procedures never work for very long, nor are dependable. When these batteries go defective, it is because their internal chemical composition has gone through its life cycle, and or something in the battery has degraded, or has been damaged.