Me and my friends are making a UPS model. So far we have established a battery charger circuit from mains and over charging protection for backup battery. We are working on charging indication using arduino, but we can not figure out a few things - firstly, when mains supply is on the battery supply should be off, second, after power is restored it should go back to power supply, third, if power is low battery supply should start.
We are basically not able design something for this logic. Please guide us!

 

It doesn't have to be complex.

The approach "may" depend on how much Power, AC-or-DC-Output, etc.

The Battery-Charger "IS" the normal Regulated-Power-Supply.

The Battery acts partially like a huge Electrolytic-Capacitor,
reducing the required size of the Output-Smoothing-Capacitor of the Regulated-Power-Supply.

When AC-Input-Power is lost,
nothing needs to be "switched",
because the Battery was never disconnected from the Load.

If your Load is DC, the above is the only really practical way to set things up.

If You need an AC-Output that is the same Voltage/Frequency as the Input,
quit messing around and just purchase a commercial UPS.

Tons of excellent, working, UPSs are thrown in the trash every day,
when all they really needed was a new Battery.

Check with your local Computer-Repair-Shop, and also check Thrift-Stores like Goodwill.
This is what I use, only I replace the Battery with
a huge Optima-brand "Advanced-Glass-Matt" Automotive Battery
for literally hours of seamless back-up Power,
and they last around ~6-to~7-years before starting to get weak, and they are completely sealed.
.
.
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Thanks