How acidic is bat urine? You may think this is a strange question because the answer is obvious, but the fact is it isn’t as obvious as one may think. For instance, one research article on bats in Oklahoma (Shackelford, Jenni, and William Caire. “Variation in pH, volume, osmolality, and sodium and calcium levels of the urine of hibernating myotis velifer from western oklahoma.” The Southwestern Naturalist 38.2 (1993): 159-163.) found that bat urine was only slightly acidic.
Research in the UK agrees finding that bat urine is only mildly acidic with a pH of 5.5 (see http://www.ai-journal.com/articles/10.5334/ai.1703/print/). Now you may read this and say well, we told you bat urine was acidic. But the authors of the UK research said that testing bottled water also revealed that pH levels ranged from 4.2-8.7.
Bottom line, bat urine is acidic but it doesn’t damage items with the speed that one may think. Of course, long-term exposure to bat urine is problematic, particularly for calciferous stone like limestone. So be sure not to over-state the material damaging properties of bat urine.