Detect mains current to power supply with backup battery

[DaveO]

I have just replaced my main power supply.

The new unit has an AC to AC transformer  ( 220VAC to 16VAC ) connected to a PCB, and a 9 amp hour battery also connected. Output from the board is 12VDC.

This new box does not have any outputs that show when the mains has failed and the battery power is being used.

I would prefer not the have to modify the board in the box in any way.

Is there an easy and reliable way to detect if the mains power has failed ?

I want to add a relay ( open to suggestions ) that I can connect COM and N/O to my Gnd / opto-inputs that allows me to detect when the mains has failed.

[DaveO]

Additional note :

My reason for wanting to sense the current is 2 part :

1. if the mains fails.

2. if the PCB in the power supply fails, and supplies only battery power  ( like the last supply that died yesterday ).

I assume that in this case, the current flow on the AC line to the transformer would reduce considerably, making it detectable.

I have seen posts relating to non-invasive sensors, but the output here appears to be ( although small ) still AC.

What about using an ACS712 hall effect sensor break out board on one of the AC lines ?  Wouldn't this be the safest way to get an isolated maximum 5VDC reading back to the Arduino ?

Alternatively, would there be a better way to monitor the 9aH battery to detect when current is being draw from it, instead of it being charged by the PCB ?