I have read a few posts lately about people buying extra / back up / replacement batteries for their cameras.

Since 2005 till the present we have 3 models of DSLR that take the same battery. Apart from when a battery grip was purchased secondhand that included a battery we have never bought another battery and all the batteries that we have still give somewhere near the same number of shots per charge as they did when they were new.

Relate your bad or good experiences with genuine or non genuine battery purchases here. Most importantly, why did you have to buy another battery in the first place.