Editing Uninterruptible power supply (section)

=== Series-parallel battery interactions ===
Battery strings wired in [[Series and parallel circuits|series-parallel]] can develop unusual failure modes due to interactions between the multiple parallel strings. Defective batteries in one string can adversely affect the operation and lifespan of good or new batteries in other strings. These issues also apply to other situations where series-parallel strings are used, not just in UPS systems but also in [[electric vehicle]] applications.<ref>{{cite encyclopedia|url=https://www.mpoweruk.com/balancing.htm|title=Cell Balancing|encyclopedia=Electropaedia|publisher=Woodbank Communications|access-date=2020-05-28|archive-date=2020-06-09|archive-url=https://web.archive.org/web/20200609233517/https://www.mpoweruk.com/balancing.htm|url-status=dead}}</ref>

Consider a series-parallel battery arrangement with all good cells, and one becomes shorted or dead:
* The failed cell will reduce the maximum developed voltage for the entire series string it is within.
* Other series strings wired in parallel with the degraded string will now discharge through the degraded string until their voltage matches the voltage of the degraded string, potentially [[Overcharging (battery)|overcharging]] and leading to [[electrolyte]] boiling and outgassing from the remaining good cells in the degraded string. These parallel strings can now never be fully recharged, as the increased voltage will bleed off through the string containing the failed battery.
* Charging systems may attempt to gauge battery string capacity by measuring overall voltage. Due to the overall string voltage depletion due to the dead cells, the charging system may detect this as a state of discharge, and will continuously attempt to charge the series-parallel strings, which leads to continuous overcharging and damage to all the cells in the degraded series string containing the damaged battery.
* If [[lead–acid battery|lead-acid batteries]] are used, all cells in the formerly good parallel strings will begin to sulfate due to the inability for them to be fully recharged, resulting in the storage capacity of these cells being permanently damaged, even if the damaged cell in the one degraded string is eventually discovered and replaced with a new one.

The only way to prevent these subtle series-parallel string interactions is by not using parallel strings at all and using separate charge controllers and inverters for individual series strings.