Normally, I don’t repair consumer electronics, but in this case, a good friend asked me to have a look at these expensive Sennheiser headphones. Apparently, some water had entered the case, and since then, these were not operational. When charging, it blinks the red led 3 times, pause, 3 times, and so on. No other sign of activity and no success to reset it by pushing the button even for a long time.

To have a look inside, you have to first remove the ear protectors, then remove 4 screws, and gently pull-out the speaker. Exercise great care! There is a flat cable to connect the microphone to the intermediary board (the same board that has the battery connector). The flat cable and connector can be easily damaged, better use small tools and a microscope.

The battery connector had signs of visible corrosion. Probably cause by the combination of water and electric power. The battery has about 4 Volts when charged, so it can easily cause electrolysis of water and generate corrosive products if the water contains traces of salt, etc.

Unplugging and re-connecting it, using some contact cleaner, I was able to establish a stable (electrical) connection again.

The battery is contained in a small case, broke it a little when opening, all these internal parts are pretty fragile. But still functional. Inside, the cell is just attached by some double-sided sticky tape.

Here, a close-up of the microphone connector. It needs to be opened with a needle or tiny screwdriver by lifting the black part carefully. Easy to break!

For the current headphones, there battery was still good, but there are spare cells available. The quality of these may be variable so you may better check them before using as a replacement (e.g., by giving them 10 or 20 cycles by an external lithium ion charger). The price is quite OK, but you wouldn’t want to open up the speakers every few months for a battery replacement. The Sennheiser factory battery may last for about 3 years of use.