Let me apologize for being a bit too silent these days. To be honest, this boils down to the load and stress diabetes puts on our family. On paper, last week appears to be quite nice, with a 105mg/dl average, a 34 mg/dl SD and very few hypos. That being said, we still had a few symptomatic hypos, surprisingly not in the situation that is flagged as an hypo.
Anyway, to say that we are tired is an understatement. About the only diabetes project I managed to proceed with is the replacement of the batteries of our old dexcom transmitter. In this post, I am going to dump the pictures I took and some info on the practical choices I made. Nothing ground breaking, but maybe some useful info for others.
Is this a good idea?This is something I want to get off my conscience immediately: I am not sure the battery swap is a very good idea. Just like everyone, I do feel ripped off when the time to buy a new transmitter comes. But I wouldn’t want to compromise the accuracy of the results we get in order to save 320 EUR (the price we pay our transmitters). I do, however fully realize how lucky I am not to have to worry too much about the cost of our diabetes supplies and understand that paying 160 EUR per coin battery can be very, very hard to swallow. Why isn’t it a good idea? Well, I have talked to a few battery swappers and, when you go beyond the “cool hack” enthusiasm of the initial success post, you often realize that replacing batteries is a bit of a hit or miss project: there is a high failure rate at the first stage of the operation. That basically means that, if you really rely on your Dexcom data, you are probably going to need ordering a new transmitter anyway, just in case you fail the battery replacement. Then, even when the initial replacement went well , several swappers have told me that the transmitter quickly stopped working. In some cases after a few days, in other cases after a few weeks. Other concerns include proper waterproofing (removing and replacing the transmitter for showers is not an option if you really care because it causes micro traumas and greatly increases the chance of local sub acute infection and wire encapsulation), the stability of the power delivery and the quality of the results you get. You do not want to lose your Dexcom right after a sports session because movements impacted the contact in your sensor.
Battery removalI used a Dremel with the small Dremel drill press to dig for the original batteries. I used a Dremel carving bit (probably the 26150561JA) to dig for the batteries. The main advantage of that bit is that its end is flat: unless you are extremely heavy handed, you will get to the batteries and the top tab without even scratching them.
I then lifted the tabs with a very small screwdriver. The dexcom Grey GooTM is really everywhere, be patient. There are a couple of spots where the tab seems to have been hammered/soldered? into the battery to make contact. These are the point where you want to work patiently with the screwdriver.
I then drilled the separation between the batteries in order to insert the cutter around the batteries. I would have loved to find another solution, but I did not.
We then get to the battery lifting part, where you have to make sure you take the correct approach angle. Again, the lower tab seems to be attached battery by two contact points. I worked around those points with a cutter until they let go. Make sure you hold the battery when you play with the cutter as you don’t want to apply torsion to the bottom part of the tab, the one that attaches to the board.
I started “blind”, without looking at the details of the previous tutorials (yes, I am the type of guy who hates to ask for directions) and ended up wrongly guesstimating the back battery position. So you can avoid that mistake, here is the geometry I measured, starting from the narrow end is as follows (+/- 0.1mm except for batteries)
1.4mm+11.6mm (bat1)+1.1mm(spacer)+11.6mm (bat2)+5.7mm = 31.4 mm total length
With the batteries removed, I did a few quick tests. The tabs marked in blue act as a contact that puts the batteries in series. If you break them, this is probably recoverable with a simple wire. Applying 3.1V with a power supply leads to the expected behavior (193mV in my case) on the transmitter external contacts.
I decided to use greater capacity batteries, rated 90mAh instead of the original 50/55mAh Maxell SR1120W. Time will tell if that makes a huge difference in life expectancy.
The bottom contact isn’t really a problem because it can be easily folded and act a bit as a spring. I applied a liberal quantity of mono component superglue and let it cure under pressure, for more than the recommended time.
The increase in height of the 390 batteries isn’t too bad, although it may have caused a few problems at a later stage: we’ll get to that.
Now the hard part: gluing the top tabs to the battery. Most of the people I talked with have used the scratch, glue and pray approach to attaching the top tabs. This is where I felt the source of instability lied and decided to tackle the issue with conductive epoxy glue. That thing is most often used to fix broken boards or windshield heaters and most of the brands I looked at had bad reviews except the 8331 family from MG Chemicals. There are four versions of the silver glue, one “extreme” version which is, well, extremely expensive (8330, 8330S) - around 80 EUR for the smallest pack and the 8331 8331S version, still expensive (35 to 50 EUR) but a bit more reasonable. It would be ideal to use it for several battery replacements to diminish the per unit cost.
This is when I started to run into some issues: the 8331 version is supposed to be faster, but wasn’t readily available. I settled for the 8331S which has a working time of 4 hours and a full cure time of… 96 hours at room temperature. My first attempt at gluing the tags when I released the spring loaded clamps after 4 hours and… the tabs slowly rose.
After reading the data-sheet that gave a 2 hours cure time at 65°C, I went for a 4 hours session on a pre-heat bed I happened to have around.
Here’s the result of that attempt.
It tested correctly
And I encased the thing in standard dual component epoxy and let it rest for a couple of days. Alas, after 48 hours, I could see that the epoxy had retracted a bit (I guess this is something you want when you glue things together) and that the tabs were beginning to lift (possibly in part because of the added height of the batteries). So I went back to square one…
By the way, if you are worried about a second battery change after an epoxy layer has been applied, don’t sweat it. The glue, even when it has cured, does not really stick that much to the Dexcom transmitter’s material…
For the third and final try, I went “all-in”. I first glued the underside of the tab to the battery, keeping it in place with my spring loaded clamps. Then, still under clamp, I applied a top layer on one half of the tab, let it cure, switched side and repeated the operation. And I finally added a bit of glue on top of the tab bridge. The rationale is that even if it lifts a bit, it should still have contact.
Note: if you want to test the voltage of the transmitter pads, you need to wait for the silver glue to cure. I kept them under monitoring and saw the voltage appear and increase progressively to the nominal 193 mV as the glue cured. (no picture, so many clamps and wires nothing was visible)
The final version has now been encased in standard epoxy (I will possibly put an additional layer of sanitary silicon)
and has been under test for a few days (with a slightly modified version of the excellent python script for the rapsberry pi that jamorham has kindly made available in his “Diabetes Smart Home”. (212 is the battery level that you want if you aren’t familiar with raw dexcom packets)
The next phase will be a live test, which I plan to do on myself since my son has a new transmitter anyway and I would hate to have him deal with eventually bad or intermittent data.
That’s all folks. I do hope I will be in the mood for more stuff in the near future.
Edit: you really want the new cover to be waterproof. There is a direct opening between the battery compartment and the board.