The battery of the Mylaps AMB140 transponder lasted 5 years in this case. It is not meant to be replaced and is casted in an epoxy resin core. With a bit of determination, it is possible to reach and change the battery. The battery is a CR2450N that you can easily order as a replacement.
There are a few online offers to send in the transponder and have the battery changed. I found prices from 50 to 100 euros. My gokart track had a couple done, but the price is ridiculous so we tried it ourselves. I used the tools that can be seen in the photo
Rather than mechanically removing the resin until you reach the battery and potentially damaging it we tried to remove this epoxy core from the cast. The resin way the resin is cast into the case can be seen when looking carefully at the side where the metal bracket is located - the resin and the casing are both approximately the same colour.
The metal attachment bracket can be used to pull the core (that comes with the bracket from the case), but first we need to heat the transponder to soften the resin so that it releases from the case.
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Step 1: Heat in Oven
The easiest method to reach the battery is by taking the resin cast out of the casing. The epoxy has a transition temperature of about 78-80deg Celsius. Beyond this temperature the strength and hardness is significantly reduced. I heated the transponder in the oven, as i thought this would be the best controlled environment.
I found that the transition temperature for this epoxy is below 80degrees (surface temperature of the transponder, on my IR thermometer) by testing the hardness of the resin at 2deg C increments whilst heating the transponder.
At this temperature the resin will separate nicely from the casing as can be seen in the next step. Note that the transponder cools relatively quickly, so if you are not confident enough to do it quickly you can replace it a couple of times in the oven.
Note: I have not tested heating the transponder significantly higher than this temperature, I do not know if the electronics or battery can be damaged.
Step 2: Separate the Case From the Core
When the resin has been heated beyond its glass transition temperature it will become soft. It will also pop off the case easily. So, try to pry your scalpel between the case and the resin as can be seen in the photo. Make sure to push it all the way into the corner.
I put the transponder in a vice and rotated it for each cut (so that it is not clamped on the side where you want to put your scalpel) because of its temperature.
If it does not release easily and you need to actively cut the resin it has probably cured again, so put it back in the oven to soften and continue.
Once you think you released all sides it is time to release it from the bottom. For the next step, I would place it back in the oven first, just to be sure because we need to separate the two parts by force.
For this step, I opened the vice just so that the case can sit on the vice and the core would fit through the clamps. The pull hard on the bracket. You can use some hook, or I simply used a zip tie that I looped through the bracket. As a guess, I would say I need to pull at around 8kgf for it to release.
Step 3: Remove the Battery
When you flip the resin block you can already see the battery. Just use your scalpel (no power tools) to reach and cut out the battery (don't forget to heat it in the oven). The battery is around 5mm thick, so this about as deep as you need to go if you want to pry it out.
I removed the little piece of resin that can be seen in the second photo to get a small screwdriver under the battery. Be careful, if you look careully you can see the PCB that is almost directly under the battery - don't damage it. Again, if the resin is soft, the batter should pop out easily. Mine got damaged a bit because I forgot to heat the resin first.
The battery is welded to both the positive and negative terminals, so you need to release the welds somehow. I used a screwdriver and a bit of force - nothing got damaged permanently.
Now the transponder is ready for the new battery.
Step 4: Putting the New Battery in and the Core Back in the Case
I did not take any photos during the reassembly, but I figure it is best left to you how you want to have your transponder look in the end.
The steps that I took were:
Solder the new battery in by putting a bit of solder on both the battery and the terminals. Then press the two together and heat the terminal - the heat should melt the solder and the two connect.
I replaced the disc of resin that you had to cut out by a few dots of hot melt adhesive.
I then covered the sides of the case with regular multipurpose glue and pushed the core back in.
Some of the fixed transponders that I've seen were covered in tape or heatshrink.
In the end, the transponder should work fine. Good luck!