@peterwe:
Over a hundred batteries have failed in 50,000 hours, which gives a 1:500 hours failure rate. Over a 10 hour flight thats 1 in 50, the chance of both batteries failing is therefore for 50x50=1:2500. All year flying with a 500 aircraft fleet means 70 dual battery failures per year. And 3500 single battery failures per year.
That is mathematical nonsense. Let me just replace your numbers with people and it's easy to see how crazy that statement is.
"Over a hundred people have died in less than 100 years, which gives a 1:100 years failure rate. In the first 10 years of life that is a 1:10 chance of dying."