My two cents.. You can monitor the current by using Arduino. You need a shunt resistance in series with the power supply and measure the voltage across the shunt resistor (This is how the ammeters are build). Of course you need to calibrate that to get the correct values. You can refer the link Make a Shunt Resistor | REUK.co.uk if you want to build your own shunt. Once your arduino starts reading the current flow you can turn on or off the relay at the desired values.