New Batteries for a SARLink PLB

Recently I purchased a used, SARLink Personal Locator Beacon for use as a safety measure when camping of hiking. These are very cool devices that can transmit your GPS location to a satellite and summon emergency help in life threatening circumstances. The SARLink is self contained and only requires the user to unfurl the antenna and press and hold a button to send an emergency broadcast. However, the unit is designed with internal batteries that have a shelf life of about 5 years and the date code on the used unit I bought said that it's batteries had expired the very month I received it. Unfortunately, the manufacturer wants to charge $150 for this service, which is more than I paid for the device. So, being a cheap bastard, I found myself thinking; "how hard could it be?" to replace the batteries myself.

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The manufacturer of the SARLink, ACR Electronics, does not sell replacement batteries and goes to great lengths to discourage anyone from attempting to replace them on their own by stating that this is a "life saving device and you need to have the tools, hardware and software to perform a battery replacement." So, I'm not recommending, or even encouraging anyone else to try this, as I would not like to be responsible for you failing to be rescued from dire circumstances. However, I'm just the cheap, curious sort that can't resist a challenge like this. I was a bit concerned about the manufacturer suggesting that special "software" was required to perform a battery swap and wondered if there was some kind of anti-tamper mechanism that would disable the unit if I disconnected the batteries, but I decided to crack open the unit anyway and take a look. Upon removing the 6 screws that hold it together and removing the back part of the case, I found this inside:

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As you can see, there are two, 6 volt, shrink-wrapped battery packs, each with identical markings, that attach to the main circuit board via JST-type connectors. The batteries were nestled in a specially molded plastic cradle to hold them firmly in position and the markings on the shrink-wrapped covers indicated that the packs are LiMnO2 (Lithium Manganese Dioxide) chemistry, which implies that each pack consists of two, 3 volts cells. This hunch was confirmed when I peeled back the shrink wrap to reveal each pack consisted of two, CR123A Lithium cells:

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To test the theory that removing the packs and, thereby, removing power from the circuit might trip an anti-tamper mechanism, I removed the batteries for about an hour, reconnected them and then tried the self test button. This seemed to produce the correct response from the unit, seemingly indicating that battery replacement was feasible. (Note: the manual indicates that one of the self tests, the full GPS test, can only be performed a limited number of times (12), so perhaps this is where special software is needed to reset this count.) Then, removing the tape that held the two cells together revealed that the cells were connected by welded metal tabs and fiber spaces, which you can see in this photo.

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CR123A cells are rather common, as they're often used to power digital cameras, but it was clear that I'd need to purchase replacement CR123A cells with welded tabs already attached in order to create a workable set of replacement packs, as it's not recommended to risk damaging a Lithium cell by trying to solder wires directly onto the positive and negative terminals. After a bit of web searching, I found suitable cells at Interstate Batteries for about $5, each. I also ordered some 3/4 inch, clear shrink tubing from a seller on eBay to I could repackage the cells into packs as was done with the originals. Then, once I received all the components I'd ordered, I used the salvaged wires with connectors and the fiber spacers from the original packs to fashion a new set that looks like this:

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Note: the above image shows both battery packs covered in shrink wrap. Unfortunately, I had to remove the shrink wrap because it was too thick and made it impossible to reattach and seal the rear cover.

SARLink Tear Down

For those who are curious about the internal circuitry of a PLB, here are some photos I took while I had the unit disassembled. Unfortunately, the component side of the circuit board is covered in some type of resin, so it difficult to read most of the part numbers.

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