The nonstop power supply compared with water stream from tank
 
The basic concept of the nonstop power supply can be represented by the following water tank model.  The AC and DC inputs of the nonstop power supply are compared with two water tanks, which are named AC water tank and DC (battery) water tank. 
The AC/DC input voltage of the power supply corresponds to the water level of each tank. With the dual water tank configuration, the water always flows into the secondary tank from the primary tank being at a higher water level. When the water level of AC water tank is high (normal), water flows from the AC water tank, and the water is partly used to fill up the DC (Battery) water tank. This is compared with battery charging operation. When the AC water tank is empty (no AC input), water flows out from the DC (battery) water tank into the secondary tank without discontinuity of secondary water flow. 
When the AC water tank is at a low level, the DC (battery) water tank feeds water to compensate for the water flow shortage from the AC water tank. Therefore, the water flowing into the secondary tank is always regulated.