The 99.99% number was provided as example. The 99.99% ballpark number was created, as example, to keep the concept simple.
The term 'average life expectancy' as it was introduced by another referred to how batteries were sold in those days. Yes, the number of months stamped on a battery meant every battery should last beyond that date. But in the early 1970s, if a warranty was for two years, then the product was often failing in less than three. Therefore many assumed that 24 months was the average life expectancy. Reliability in the
1970s was so bad that many cars needed a whole valve job or new cam by 20,000 or 40,000 miles - typically just after the warranty had ended.There should be little relationship between warranty and life expectancy. Warranty is some number less than the life expectancy. But in the 1970s, life expectancy for some products was often little beyond the warranty period for too many products. Therefore for a 24 month battery, many simply assumed that 24 months was, also, its life expectancy.
This was a period of Firest> w_tom wrote: