Power your fairy lights (or almost anything!) with a USB cable.
Brighten up your life and do away with pesky batteries by combining a USB cable with your battery-powered fairly lights. Cut your USB cable and expose the red and black wires. Attach these to the positive and negative wires of your fairy lights (though this trick will work with a huge range of electronics), and tighten it all up with electric tape.
Add a resistor into the circuit to protect your components, and as always be very careful when tinkering with electronics. Imperfect hacks or unsuitable gadgets can cause damage, so take care.