I’d originally posted this on another duo thread, but figured it would be better off here
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Duo PRAM battery fabrication:
I ordered 2x of these from Amazon. They’re 3V lithium rechargeables, with solder tabs to make the job a little easier, and hopefully keep too much heat from the soldering iron from reaching the cells.
https://www.amazon.com/dp/B07W4NWHJ8/
The first image shows the battery I took out of the machine. The red wire is connected to the positive (+) terminal of the top battery, the white wire is connected to the negative (-) terminal of the top battery and the (+) terminal of the bottom battery, and the black wire is connected to the (-) terminal of the bottom battery. I desoldered the wiring harness to reuse for the new battery.
I bent the solder tabs on the new batteries to put the stack together. The (+) terminal of the top battery got bent all the way over, and the (-) terminal got bent flat so that it (just barely) protruded out the side. The (+) terminal on the bottom battery got bent flat to make a long tongue, and the (-) terminal on the bottom battery got bent all the way over. Then, I soldered the (-) terminal on the top battery to the (+) terminal on the bottom battery, and finally resoldered all of the wires, with the white wire connected to the middle “tongue” and the red and black wires connected to their respective terminals. Some heat shrink tubing and Kapton tape finished it off.
Try not to hold the iron on the tabs too long as you’re soldering to them - you want as little heat as possible to get into the cells. Mine is working, but I suspect I was a little slow and may have shortened their life expectancy - but it’s hard to tell.