Battery Replacement

After many years, I've finally managed to replace the batteries in a G4 transmitter.
Dying batteries is what compelled me to figure out the RF protocol years ago.  At the time my transmitter died just after 6 months, so a replacement wasn't covered by insurance.  There never was a warning from the device.

Over the last few weeks I've watched the battery level run down to where it dipped from 212 to 210.  Allegedly the point at which a Dexcom receiver will tell you to order another.  Was still functioning at 212/210, but range seemed diminished and definitely noisier.  I finally pulled it when it gave me a reading that was half of reality (something like 90 for a 175).

Finished result.  Took allot to get here.  Used Devcon 5 minute epoxy and an epoxy thickener that I had around from some past boat repair.  The Devcon 2 barrel syringe works really well, I've tried other brands that just didn't mix well, and the epoxy never cured.

I use a rasp and a really small router bit in a Dremel to get the batteries out.  The batteries are spot welded on the top and bottom.  I like to grind through the battery so I can get a screwdriver in it and lever it up a bit, then push a screwdriver in from the bottom and carefully push the tab away.  Repeat for the top.

I followed the iFixit guide by Joern.  The tricky part for me was getting sufficient contact between the top tabs and the battery.  Super glue just doesn't work.  I also tried something called "Wire Glue", carbon powder in a paint like substance, conductive.  That also didn't do the trick.  Only thing that works for me is soldering the top tabs to the battery.  Use a faceshield, too much heat and the batteries explode, I ruined a few.  I put a couple of drops of superglue around the bottom edges of the battery, then use my drill press to gently press and hold it in place for 5 minutes.  I'm surprised it holds as well as it does, you don't need to wait 24 hours for full strength.

To date, I've soldered the battery after gluing it.  Might be better to solder it in place, pop it up, and then glue it.  My solder job on this is horrible, but it makes the connection.  I can do a beautiful job soldering CR2032's, but these tiny batteries, not so much.  Sand the top of the battery, carefully sand the tab the best you can, maybe try tinning the battery top before soldering the tab on.  Lastly, glue it.
Also, hold the tab against the top of the battery while soldering, something thin and metal works, too much metal will work as a heat sink though.

You can measure the voltage on the bottom of the transmitter using the two pins.  You should see something around 0.270 mv.  This will vary depending on what kind of voltmeter you have (cheap or legit...).  Ideally, measure the voltage before you take the transmitter apart so you know what to shoot for.  The iFixit guide was 0.09 mv.  

Initially I kept getting high 100's, indicating that only one battery was really connected.  Soldering did the trick.

The next issue is batteries.  I ordered SR1120's, the battery dexcom uses.  I had a pair of Maxell and I purchased a 5-pack of no brands off of Amazon.  These were way too cheap, but I didn't realize why at the time.  I promptly blew up the Maxell's trying to solder them, then moved onto the cheapies.  These worked.  But the battery level reported by the firmware was 198 (you want 215 216).  Surprisingly this worked fine, signal was a bit week, but it was accurate.  Turns out, I bought Alkaline not Silver Oxide.  Would have been lucky to get 3 months I think.

Finally, I replaced the batteries a second time with Energizer 390/389 batteries and got the 216 level that you'd expect from a new transmitter.  These are slightly larger, hopefully I'll get 15 months before I have to repeat this exercise.

Lastly, I used duck tape around the transmitter to make a "dam", then I poured the epoxy in.

Once that set, it was easy to sand it down to something that looks very close to original.