Editing Uninterruptible power supply (section)

=== Common battery characteristics and load testing ===
When a lead–acid battery is charged or discharged, this initially affects only the reacting chemicals, which are at the interface between the electrodes and the electrolyte. With time, the charge stored in the chemicals at the interface, often called "interface charge", spreads by [[diffusion]] of these chemicals throughout the volume of the active material.

If a battery has been completely discharged (e.g. the car lights were left on overnight) and next is given a fast charge for only a few minutes, then during the short charging time it develops only a charge near the interface. The battery voltage may rise to be close to the charger voltage so that the charging current decreases significantly. After a few hours, this interface charge will not spread to the volume of the electrode and electrolyte, leading to an interface charge so low that it may be insufficient to start a car.<ref>{{cite book|last=Saslow|first=Wayne M.|title=Electricity, Magnetism, and Light|year=2002|publisher=Thomson Learning|location=Toronto|isbn=0-12-619455-6 |pages=302–4}}</ref>

Due to the interface charge, brief UPS ''self-test'' functions lasting only a few seconds may not accurately reflect the true runtime capacity of a UPS, and instead an extended ''recalibration'' or ''rundown'' test that deeply discharges the battery is needed.<ref>{{cite book|title=Maintaining Mission Critical Systems in a 24/7 Environment|author= Peter M. Curtis|pages=261–262 |url=https://books.google.com/books?id=GVczgy4BkU8C&q=ups+battery+rundown&pg=PA261|year=2011|isbn=9781118041628|publisher=Wiley}}</ref>

The deep discharge testing is itself damaging to batteries due to the chemicals in the discharged battery starting to [[crystallize]] into highly stable molecular shapes that will not re-dissolve when the battery is recharged, permanently reducing charge capacity. In lead-acid batteries, this is known as [[Lead-acid battery#Sulfation and desulfation|sulfation]], but deep-discharge damage also affects other types such as [[nickel-cadmium batteries]] and [[lithium batteries]].<ref>{{cite book|title=Emergency and backup power sources: preparing for blackouts and brownouts|author= Michael F. Hordeski|url=https://books.google.com/books?id=jXIwvQTzX6UC&q=deep+discharge+crystallize&pg=PA70|isbn=9780881734850|year=2005|publisher=The Fairmont Press, Inc.}}</ref> Therefore, it is commonly recommended that rundown tests be performed infrequently, such as every six months to a year.<ref>{{cite web|url=http://d5.leonardo-energy.org/webfm_send/4011 |access-date=August 1, 2012 |title=Maintenance Manager's Guide, Section 2.1|author=Leonardo Energy }}{{dead link|date=June 2016|bot=medic}}{{cbignore|bot=medic}}</ref><ref>{{cite web|author=APC Inc|title=Knowledgebase article: What is the expected life of my APC UPS battery?, Answer ID 8301|url=http://nam-en.apc.com/app/answers/detail/a_id/8301/kw/runtime+calibration/session/L3RpbWUvMTMyMTMxMzA4My9zaWQvM0pOZkE3Sms%3D}}{{dead link|date=May 2014}}</ref>