The UB20.241 uninterruptible power supply controller (DC-UPS) is used in addition with a 24 V power supply and batteries to bridge power failures or voltage fluctuations. This configuration can prevent expensive downtimes, long restart cycles and loss of data. A unique feature of the UB20 series is the constant voltage in battery mode, which will not change as the batteries discharge. The buffer voltage in battery mode can be set to four different output values. Another feature is the utilization of two independent battery chargers for the two 12 V batteries in series. This feature makes matching batteries unnecessary and allows for precise battery charging, testing and optimized usage of the battery capacity to achieve the longest battery service life. The UB20.241 includes many battery diagnostic functions that ensure a reliable operation of the entire system. Furthermore, a temperature controlled charging extends the life of the batteries. It also includes a selectable buffer time limiter as well as ready, buffering and replace battery contacts. For safety and maintenance, an inhibit input signal is included which prevents a battery backup.