Our batteries are rated for a 300-500 charge-discharge cycles; and with each charge-discharge cycle the runtime (time between charges) is reduced by the gradual depletion and usage of the battery's chemistry inside. This is the normal function of battery technology.
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A battery will last between 300-500 charge-discharge cycles (for lithium based chemistries - NIMH can have up to 800 charge-discharge cycles and NICD chemistries can have more). A charge-discharge cycle means that a battery once at 100% draws power down to 0%. Then after recharge it will be back at 100%. This can be done 300-500 times on the same battery. Also with each charge-discharge cycle the runtime (time between charges) is reduced by the gradual depletion and usage of the battery's chemistry inside. For example you may notice in the first 3-4 months you are getting between 3-5 hours of runtime on your battery. Then in month 5-12 (after your purchase) you notice that you are slowly getting less and less runtime in between charges. This is the normal use of the chemistry inside your battery and DOES NOT mean that the battery is bad or defective, but simply has been used by you.