Hacking the Dexcom G4 Transmitter - Battery replacement

Hacked Dexcom G4 Transmitter

After a week of hacking I have a stable Transmitter with new batteries, here is a summary of my discoveries and steps to reproduce yourself:

Kit you need:

  1. An expired G4 Transmitter – Ask around on the diabetic forums, they are getting binned every 6 months, so keep at it and you will find someone willing to donate.
  2. 2 * Replacement batteries – RENATA 391 (SR1120W)
  3. 2 * Battery holder - Adafruit 12mm Coin Cell Breakout Board
  4. Sugru – to seal the opened Transmitter once finished
  5. Dermel with Diamond Cutting Wheel – Recommended
  6. Needle File Set
  7. Jumper Wire
  8. Multimeter – Recommended
  9. Soldering iron
  10. Helping Hand


Unlike the Dexcom 7+ Transmitter, the G4 is made of two PCBs with the batteries sandwiched between. This means we need to enter the sides of the Transmitter to access the old batteries.

Original Image credits to Sean Hodgins from his great post on tudiabetes.org | Open Source Medical Devices - I want to change things

Here are the “safe to cut” areas:

Note: back cutting should be the same as the front, I cut too much off this example

Our aim is to cut approximately halfway through the existing batteries, then remove the contents of the battery itself. We wish to keep the top and bottom of the original battery’s as these will be used as the contact points for the new battery.

Here you can see the content of the battery cleaned out. The original + (hidden from view) and - of the battery still available.


Attempt 1 – Transmitter v1: fail

First attempt was to cut by hand, this was too messy, time consuming and resulted in me cutting into the PCB of the Transmitter.

Attempt 2 – Transmitters v2, v3: progress

By using the Dermel I got a much cleaner cut.

  1. Cut half way into the existing battery. Using the Dermal cutting disk move slightly up and down to enlarge the opening, do not cut too much of the battery top and bottom away, you want to keep the top and bottom of the existing battery.
  2. Using the needle file clean out the contents of the old battery and file the top and bottom for a clean finish. I found using a needle was helpful with getting material out.


At this point I found it useful to test to see if the Transmitter is working and we do not have any short-circuit due to material still inside the old batteries. This can easily be done by building a simple little device that extends the contact points for the new battery and can be inserted too test.

An image says a thousand words

Using a multimeter first check the contact points of your test device to be sure we are getting 1.5v, adding some solder to the end may help with contact once inserted. Insert one into the Transmitter, you should get a reading from the two contact points at the bottom of the Transmitter, now insert the second battery and we should get 19 mV or higher. If you are reading 0 the batteries are not making contact or you have a short-circuit.

If you do not have a multimeter you can attach both batteries and wait for a few mins to see if your Dexcom receiver displays the Antenna Symbol in the top right or if using xDrip that the yellow LED stops flashing – indicating it has detected a reading from the Transmitter.

Inserting batteries

Others have now inserted new batteries into this gap. Note that the top is positive and bottom negative, too avoid short-circuiting a slim piece of metal can be applied to the negative of the battery raising it slightly.

I found this impossible as the gap was just not big enough too insert new batteries, I could try and take more material away with a needle file but I could not see how it would be possible to insert the new battery into a space that is limited by its own height!

As said others have done this and I will be contacting them to see how it was done, I would assume with a slimmer battery?

Attempt 3: The Birth of Frankenstein Transmitter v4

I decided that the only way forward for me was to extending the contact points for the battery out of the Transmitter and mount the batteries on top. Not the most elegant, also attempting multiple ways to attach the cables to the new batteries repeatedly failed, therefore I highly recommend using coin cell battery holders for a secure fit.

  1. As with our testing device, put two wires on each side of plastic that is no longer than the gap for the new battery, put this in and test.
  2. Seal this with Sugru and I recommend leaving overnight. Test again, if this works we have a clean contact inside the Transmitter.
  3. File the top of the Transmitter flat (where it says Dexcom G4), this is where we will be attaching the coin cell battery holders.
  4. You could cut the PCB of the coin cell holders down at this point for a smaller profile, I did not do this but I believe would shave at least 4mm off.
  5. Solder the wires to the PCB of the coin cell holders, doing your best to keep this length of cable short.
  6. Test, if all is well with Sugru attach the PCB of the coin cell holders to the top of the Transmitter. Add Sugru as required around the Transmitter.
  7. Leave to set over night, if working in the morning congratulations – you have successfully hacked a Transmitter!


Final v4 Transmitter 

With the new slimmer G4 Transmitter this process maybe easier as there appears to be only one PCB, processes maybe similar to the 7+ Transmitter.

More images of hacking the Transmitter at full res can be seen here.

Thanks to Joern for his post on tudiabetes.org | Changing the flat batteries of a dead Dexcom G4 transmitter (Instruction)


**Update: 9th June 2015**

  • The extra weight of the Transmitter has caused the adhesive to give way a little, next time ill be sure to attach some extra Opsite Flexifix Transparent Film
  • I have found that others have been able to insert the thinner 365 \ SR1116W batteries, this will do away with the coin cell holders, but you will get a low battery warning which can be ignored

If you are interested in the Diabetic hacking community checkout:

Interested in helping me build an Artificial Pancreas? Get in contact!


Like this? Email me future posts!


The contents of this site is for informational purposes only and is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your Diabetes support team or other qualified health provider with any questions you may have regarding your medical condition.Sorry, have to say something like this!

Tagged under tech hacking dexcom diabetes