These are the older rechargable’s that were popular for a long time. They are not used as much these days because NiMH batteries have much higher power density. However they are cheaper, and are still used in many cases where performance is not as important as price. Another useful quality is they discharge slower than NiMHs. (That is, left alone, they will retain their charge longer.) Battery cells are 1.2V, often bundled in "packs" of 5 to make 6V.
Pros: Inexpensive, rugged, come in "standard" sizes, easy to recharge.
Cons: Lower power density, If not fully discharged they remember the shortened cycle and are thus reduced in capacity (length of use per charge).
and so require "full discharge/recharge" cycles every once in a while to reduce this memory effect (caused by the growth of crystals on the battery plates).
These are more popular rechargables, These are a good replacement for standard alkaline batteries in many cases. The battery cell voltage is 1.25V per cell, that's less than the 1.5V of alkaline but more than the 1.2V of NiCads. The most annoying thing about them is their high self-
Pros: Good alternative to Alkalines in most situations, high power density, "standard" size, better capability than alkalines, pretty easy to recharge but not as rugged.
Cons: More expensive than Ni-
The typical expectancy life expectancy is 2 to 5 years.
In general, the nickel-
Under charging can cause low service where overcharging can cause loss of Cycle life