iPhone battery's capacity to hold electrical charge

Whilst there are battery meters around, there's not really a way of measuring capacity without a full charge and discharge cycle. I think they mean that, compared to how long it lasted when new, it now lasts half as long after a full charge, with the same amount of usage. For example, if you always got 2 days of average use out of it before it complained about being down to 20%, and now you only get 1 day, then you can claim on the warranty. No idea how you prove the capacity has decreased to Apple though.