Can I connect any capacity/any number of batteries to an Inverter/UPS to increase its runtime?
The capacity of the battery that should be connected to an Inverter depends on the charging current the Inverter can provide as this determines whether the battery gets charged fully in a reasonable amount of time without getting damaged. Usually the battery capacity should be no more than 12 times the charging current that can be provided by the Inverter (e.g. a 5A charger can accommodate only 60AH (5*12=60)). In case the battery discharge is less frequent (say once in 10 days), then in such extreme cases the battery capacity can be increased to 20 times the maximum charging current of the inverter. In addition, the charging current provided by the inverter should be below the limiting current of the battery to ensure the battery does not get damaged during charging.
When connecting batteries multiple batteries in parallel, please note that the inverter’s battery charger should be able to provide the charging current required by each individual battery (e.g. if the limit current of one battery is “A” amps, for “N” batteries in parallel, the limit current for charging of inverter should be “A*N” amps). This is not a factor when connecting batteries in series as all batteries in series use the same charging current from the inverter.