Especially as it gets colder, I often make my wife a hot water bottle. As a friend of alternative solutions I came across an electric one at some point and bought it straight away. My wife was pleased – but we were both a little disappointed by how much warmth it gave.

It turned out that this bottle contains only two small heating elements of maybe ten watts, and they heat the bottle itself just as much as anything else. So I got to work.

The basis is a plush penguin – my wife loves penguins. Into it went 40 watts of heating. Behind the heating elements sit heat-reflective blankets so the warmth radiates outwards and the power bank inside stays cool. Should it get too hot after all, a temperature-sensitive switch cuts the power automatically above about 75 degrees.

A second version is currently being built: bigger, softer and with even more heating power.