mzst's blog: Dexcom G4 (non slim) Transmitter battery replacement

In this post I will show you how I changed the batteries of a Dexcom G4 (non slim) transmitter. This is not an easy task because it involves cutting the batteries horizontally in half and you can easily destroy the transmitter if you cut in the wrong place. The batteries are sandwiched between the two circuit boards.
On the Dexcom G4 Slim this seems to be much easier because there is only one circuit board, so you don't have to cut the batteries in half. It has already been done, but the Forum thread got deleted.

Preparation

First I took some x-rays of the transmitter to precisely locate the batteries. Along the process I took x-rays to control and correct any mistakes. For now only consider the top left and right image.

I then measured the pictures taken and drew a plan of their position (units are in mm). I marked the position with a pen on the transmitter and started cutting it open with a Dremel and a disk.

Front Battery

You have to start cutting in the center of the battery and then expand the gap it by moving the Dremel disk up and down slowly.

Cut until right before the pins that connect the upper with the lower circuit board. Notice the black dot on the right, that is the encapsulation of the pins. Do not cut any further than this.

I then started prying away the old battery, taking care not to destroy the metal tabs that are tack-welded to it. Sadly I did not take any pictures of this step, but you can see it below on the back side battery.

Back Battery

Left and right dots are the connectors, in the center you can see the front battery.

Here you can see the upper metal battery tab…

... and here the lower. Be careful to not break it off because this one is only connected to the PCB via a tiny metal band in the center of the larger tab (see arrow).

Side views

Battery replacement

The Dexcom transmitter uses two Maxwell SR1120W Silver Oxide Batteries which are connected in series, and have a voltage of 1.55V and a capacity of 55mah each. The SR1120W measures about 11.6 X 2.1mm. The transmitter battery status is send to the Dexcom receiver via a value that ranges between about 207-217. When this value reaches 207, it triggers the alarm to replace the transmitter.
The value normally stays constant at around 216 for a couple of months and then quickly drops in a matter of weeks/days. This makes sense because if you consult the datasheet of silver oxide batteries you can see that the voltage remains constant for a long time and then quickly drops in the last 1/3 of the graph.
With an xDrip setup you can read out this value in the System Status tab.

This is what remained of the batteries.

I experimented with different Brands and two form factors (SR1120W aka 391 & the slimmer SR1116W aka 365)

I came to the conclusion that the slimmer Renata 365 (47mah) had not enough juice to power the transmitter for more than a couple of months (even if only one was used combined with a 391). I also tried the Energizer 391 (60mah) but this one did not fit in the back side. In the end I used a Maxwell SR1120W in the back side and a Energizer 391 in the front side.

Renata 365 in the back and Energizer 391 in the front, showing a mere 210 in xDrip system status, which then dropped to 208 after a couple of minutes.

I had the feeling that the Energizer 391 is slightly thicker than the Maxwell SR1120W because it was very difficult to insert. Because of the tight fit I had to insulate the half of the lower tab with insulation tape so that the battery wouldn't short out. However I think this is not a problem with the Maxwell one (at the back side it worked just fine).

Energizer 391 (Front) covered in resin + Maxwell SR1120W (Back), showed a changing value of 217/216 in xDrip, which then stabilized at 217.

Finishing

I filled the empty space with light curing resin so it can be easily removed in a future battery change and covered it with a layer of epoxy glue for waterproofing.