Battery powered window fan prototype

The window extractor fan previously built worked so well that I decided to build another one for my bathroom, battery powered this time.

My bathroom has no ventilation. In the summer I can open the window, but during winter in Montreal, getting a blast of -20C air when you come out of the shower is not great. The bathroom is in an old building and has no electrical outlet. This constraint led to the idea of building a battery powered exhaust fan. The fan assembly has to cover the window in the following pictures.

The shape of the moulding around the window and the window itself makes it hard to find a good method to hold the assembly in place. I decided to build a quick and dirty prototype in cardboard and duct tape to experiment with the best layout to use. The general design is the same as the extractor fan previously built : two 140mm fans side by side.

The base panel is made of two pieces of cardboard coming from a shipping cardboard box. Two holes are cut at the top to fit through the window’s handles. The cutting template used in the previous exhaust fan build is used again this time to cut the holes for the 140mm fans. Two templates of the 140mm fan are printed and glued on the back side of the cardboard panel.

Then, the electrical and electronics are connected together. There is a 4 AA batteries holder connected to a boost converter via a switch. The fans are directly soldered to the converter output. The boost converter is used to generate 12V from the 4 AA batteries which output only 6V (1.5V each, in series).

This initial design was functional, but the seal between the window frame and the cardboard at the bottom was not great. A duct tape lip was added to improve the seal. Duct tape was also added to all the exposed cardboard edges to protect it. The result is shown in the image below.

I am quite pleased with the result. It was very quick to build (around 1h) and it works as intended. Of course it is not very durable since it is made of cardboard and is in contact with high humidity levels, but for a proof of concept, it is more than acceptable. I have yet to the the duration of the batteries. You can see the extractor fan working in the video.

Description  Reference
Boost converter board  LM2587
140mm fan  Generic 140mm 12V fan
Power switch  EG4777-ND
Quick connect connector  A107178CT-ND
Battery holder  BC4AAW-ND