I was intrigued by the answer and comments on this question about connecting a second battery in parallel to increase capacity. The original battery has obviously lost capacity and is nearing its end of life. The OP wanted to add a second in parallel to increase capacity, which might make sense if 1) the UPS was compatible with doing so, and 2) the original battery was not faded/damaged.
Marcus indicated in the answer that adding a second battery in parallel would add "but 25% more capacity than just the new one."
I'm unsure how that figure was arrived at. If two identical batteries in parallel are twice the capacity of one alone, how does one calculate that capacity when the batteries differ in capacity or age? It seems to me that a deficient cell would obviously contribute less than 100%, but could also parasitically drag down the newer battery if they attempt to equalize. (That's my assumption.)
Is the 25% figure given based on a rule of thumb, a best-estimate based on the limited information in the original question, or something else?