why three batteries in the pen of surface pro 3?

Greetings,

      The N-Trig pens usually take 1 AAAA battery to power the sensor board and erase & right click buttons in the pen. The pens do not need to be paired on Bluetooth to operate for pointing and drawing and using the lower buttons. Microsoft has taken this pen and added a simple Bluetooth beacon transmitter in the cap to act like a wake on LAN message that is custom tweaked to launch Onenote or the login screen. The purple button on the top is the Bluetooth device portion of the pen.

     I bought a Surface Pro 3 today and the store indicated, as far as they knew, only the single AAAA battery was replaceable. That did not seem logical to me so I took the pen back to the workshop for some closer examination. All 3 batteries are in fact safely replaceable, there are two #319 watch batteries inside the cap.

    The AAAA will need to be replaced more often than the watch batteries, it should last a long time. It is changed like you would expect to change the ink in a pen. The top screws off where the metal meets the plastic. Positive side towards the writing tip side.

    The two #319 watch batteries power the Bluetooth beacon and should not need changing for a couple years unless you are holding the button down all the time. The batteries will go bad from natural decay before you consume their charge in some cases. Should you need to replace the batteries here is how: You will only need a skinny Phillips screwdriver. Unscrew the pen top like you are replacing the AAAA. Inside the cap is a spring that holds the AAAA down, pull it out and you will see what looks like a Phillips screw head. Its not really a screw but is a threaded metal insert inside the cap. It has very  finely machined threads so be careful when you put it back together not to cross thread it or over-tighten it. It's well made so you don't need a lot of force to tighten it securely. The watch batteries are stacked inside the insert. #319 batteries are tiny; imagine 3 grains of rice together. The Phillips-head insert is kind of like a cup and its easier to put the batteries in the cup flat-side up instead of the pen-cap/top when re-assembling. Test the purple Onenote button before putting the rest of the pen back together, it does not need the AAAA for the one note trick. Putting the spring back in the pen cap a little tedious because its so small but it just shoves in there, its oversized so it stays in tight once fully seated straight.

I hope this helps clear up some confusion I have seen around about the digital pens batteries.

PS This device rocks so far.

Hi TKsurface,

Thank you for reaching out and taking the time to clear out this doubt with the Microsoft Community! We appreciate your interest! We too are very excited about the upcoming Surface Pro 3!

In this case, all current available information about the Surface Pro 3 and its accessories can be found in our Surface Blog, our News Center and the Surface website.

We hope you find what you are looking for!

If you have any other questions don’t hesitate to ask! We are always here to help!

Best Wishes,

why three batteries in the pen of surface pro 3?

Is one for Bluetooth? Which one? AAAA or watch batteries?

The pen is made by N-Trig

You can find a lot of info about it here (it's Spanish so you may need some help of Google translator)

http://wintablet.info/2014/05/n-trig-duosense2-como-funciona/    (Spanish)

It's not clear which battery will be used for BT. The available information talks about just one battery and the second battery was added for the SP3 I believe.

http://www.n-trig.com/Content.aspx?Page=DuoSenseActivePen  (English)