So here I am with a simple yet very efficient little relay assembly module which will do all the above functions without letting you know about the implementations, everything is done automatically, silently and with great fluency.
Converting an Inverter into UPS
Looking at the diagram we can see that the unit requires two relays, however one of them is a DPDT relay while the other one is an ordinary SPDT relay.
The shown position of the relays are in the N/C directions, meaning the relays are not powered, which will obviously be in the absence of the mains AC input.
At this position if we look at the DPDT relay, we find it to be connecting the inverter AC output to the appliances through its N/C contacts.
The lower SPDT relay is also in a deactivated position and is shown to be connecting the battery with the inverter so that the inverter remains operative.
Now let's assume that AC mains is restored, this will instantly power the battery charger which now becomes operative and supplies power to the relay coil.
The relays instantly become active and switch from N/C to N/O, which initiates the following actions:
The battery charger gets connected with the battery and the battery starts charging.
The battery gets cut OFF from the inverter and therefore the inverter becomes inactive and stops functioning.
The connected appliances are instantly diverted from the inverter AC to the mains AC within a split second such that the appliances doesn't even blink, giving an impression that nothing had happened and the are kept operative continuously without any interruptions.
A comprehensive version of the above can be witnessed below: