Transponders are expensive. To me, inexplicably so, which is part of why I’ve never owned one. I pay the $50 rental fee whenever I run a LeMons race and grumble to myself about why a plastic box with some simple electronics and nothing-special NiCd rechargeable batteries would cost more than a GoPro.
A transponder with a dead battery fell into my possession. It sat in my garage for a few years before I finally took the time to crack it open. Hit the jump to find out how you can try and revive a dying TranX battery.

If you’ve ever taken the cover off a transponder, you’re familiar with the stuff what fills it. It’s some kind of hard foam resin, nearly impossible to cut or chip away at. I couldn’t find any very detailed information on how to replace the batteries in one of these except for a video or two, and many references to a company called Transponder Services in Queensland, Australia, owned and operated by Jeff Chandler.
Many people seem satisfied with his services and are willing to pay the US$90 to get their transponder fixed. I’m an experimental type and I had a free transponder. I had nothing to lose in trying this myself. Besides, the resin in mine was already cracked between the battery and circuitry. But I did consult Chandler for his opinion. He produced some important warnings and caveats for those trying this out themselves, the most important of which are:

• In most cases the old battery is leaking. You should wear rubber gloves and wear a mask. [It was too late for me when I found out, but I am not dead yet.]
• The old battery should not be disposed of in general waste. Correct disposal is at a hazardous waste depot. [Ditto above, but I’m not proud of it.]
• You can’t convert a battery-powered transponder to a hard-wired one. If it were possible, he would definitely be offering that service.

Click here to find a battery recycling center near you.
Before continuing on to my process and experience, here’s what Chandler had to tell me about his experience with do-it-yourselfers.

In the 14 years I have been providing this service I have seen many transponders wrecked beyond repair by well-meaning DIYers. I’m not scare-mongering or trying to drum up more business because I get enough to keep me occupied anyway. I am kind of semi-retired, but not old enough for the pension yet, and financially in very good shape.
The DIY repairs that I end up seeing are either damaged beyond repair or done badly. Initially the transponder worked for the DIY person but failed later. There is a small club transponder that RC model car racing rely on which are no longer made. This is the AMB20 and AMBrc.
In a club there is usually a well-meaning person with some good skills that has a crack and fixing these and either opens one or two and gives up and has possibly killed the transponders in the process, or they are successful and do the whole set of 10 or 20.
The problem is they commonly do several thing wrong which sets up a nasty chemical reaction which over time eats into the guts of the transponder and they stop working and cannot be repaired. [He wouldn’t specify.] I’ve seen many of these and it is an ugly messy sight and in some cases I can clean it up and get them working again.
I understand people wanting to have a go at repairing their transponder themselves but IMHO it is better to save yourself the hassle and have it done by someone that has the experience to do it right.
I have seen a variety of “tutorials” “I did mine like this” posts online (some in forums) for various types of transponder battery replacement and as yet I have not seen one that has the correct instructions and in many cases the methods are dangerous and done the difficult way. [He refused to give any more specific details about this.]

Armed with what little information I had, I set to work attempting to repair my own transponder so I could create a post online that illustrates the process in a potentially dangerous and difficult way. If you want to attempt this yourself, you do it at your own risk. Obviously. Sheesh.
I heated the transponder in my oven at as low a temperature as it would do, for about 30 minutes. This softened the resin. I cut off part of the plastic case and separated battery, which was already swollen and leaking before I even started. Then I started cutting away at the resin around the battery in order to read the label.

The battery was made up of 3 individual rectangular nickel-cadmium (NiCd) cells wired in series, for a total of 3.6 volts, and 1350 mAh of capacity.  I did a lot of searching, but couldn’t find anything that fit those specs and the rather small dimensions of the transponder. I couldn’t find any rectangular batteries at all. But, ignoring one packaging constraint, I found something for less than $20 shipped on batteryspace.com. Part No. CU-J605 has the same output voltage and higher capacity, and comes with solder tabs.

I had already made a note of which side held the positive terminal based on the battery I took out. I decided while shopping that I don’t know anything about batteries, so rather than pursue a nickel-metal-hydride (Ni-MH) or  lithium-ion (Li-ion) or other higher-tech replacement battery, I figured I’d stick with NiCd. That way I know there won’t be incompatibility with the charger, transponder circuitry, temperature range, or any such thing.

I dug at the resin until I could access enough of the original battery leads and soldered it all together.

Then I put it on the charger. The charger output of 200 mA and a total battery pack capacity of 6600 mAh, I figured it would take a day and a half to charge. That turned out to be about right.

The question mark I had was how long these new batteries would last. These transponders have an LED indicator that blinks, in green, how many days of life there remains in the battery. When there is less than 1 day left, it will blink red. If it blinks 3 times, you’ve got 3 days. Simple.
When I took this off the charger, though, something strange happened. It blinked red.
It blinked a single red flash for a solid 2 or 3 days. Then it started flashing in green, 6 times. A few days later, 5 flashes. Then, less than 20 hours after that, it was dead.
Like I said before, I know little about batteries, and the expert on these things isn’t going to spill much useful information. I speculate that whatever comparative circuit in the transponder calculates the amount of battery life left can’t properly understand the capacity or the discharge rate of these super-huge batteries.  If you are knowledgeable in this realm, let me know in the comments.
Regardless, I think the process worked and am looking forward to an opportunity to try it out. If you’re the adventurous, hands-on type and don’t mind having a transponder that looks a bit like Max Headroom, this might be a worthwhile experiment for you, too.