Editing Uninterruptible power supply (section)

=== Testing of strings of batteries/cells ===
Multi-[[kilowatt]] commercial UPS systems with large and easily accessible battery banks are capable of isolating and testing individual cells within a ''battery string'', which consists of either combined-cell battery units (such as 12&nbsp;V lead acid batteries) or individual chemical cells wired in series. Isolating a single cell and installing a jumper in place of it allows the one battery to be discharge-tested, while the rest of the battery string remains charged and available to provide protection.<ref>{{cite web|work=The Data Center Journal|title=Maintaining and Testing Your UPS System to Ensure Continuous Power, Section: Maintaining a Battery Bank|url=http://www.datacenterjournal.com/facilities-news/306-electricalElectrical/2639-maintaining-and-testing-your-ups-system-to-ensure-continuous-power}} {{dead link|date=July 2016|fix-attempted=yes}} {{cbignore|bot=medic}}</ref>

It is also possible to measure the electrical characteristics of individual cells in a battery string, using intermediate sensor wires that are installed at every cell-to-cell junction, and monitored both individually and collectively. Battery strings may also be wired as series-parallel, for example, two sets of 20 cells. In such a situation it is also necessary to monitor current flow between parallel strings, as current may circulate between the strings to balance out the effects of weak cells, dead cells with high resistance, or shorted cells. For example, stronger strings can discharge through weaker strings until voltage imbalances are equalized, and this must be factored into the individual inter-cell measurements within each string.<ref>[https://web.archive.org/web/20120401023942/http://www.btechinc.com/btech-focus-battery%20failure.shtml BTECH Inc, BTECH's Focus – Predicting Battery Failure] and [https://web.archive.org/web/20140622185615/http://www.btechinc.com/docs/S5/S5%20Installation%20Manual.pdf Installation Manual], page 18, showing sensor wires for each cell/battery on a battery string, and also note that the current transducer sensors to detect cross-string series-parallel current recirculation.</ref>