If you are going to buy some decorative LED lights for Christmas that are battery powered, especially the Poundland type of lights, you might want to check whether they have mounted a small resistor within the circuit as part of the manufacturing process.
In my case, I bought some “Metal Lights” from Poundland which did not come with a resistor mounted inside the circuit. You can greatly extend battery life if you solder a small resistor on one of the wires yourself, e.g. a 100 Ohm one.
I only had 150 Ohm resistors at hand, so that’s what I ended up soldering in.
We’re probably talking about days of continued usage before the batteries wear off once a resistor is added, although the drawback is that the lights will be slightly dimmer.