Well, the process was simple in theory. I found the point where the two power sources are connected with diodes, and soldered in a third diode, then soldered wires to the diode and a nearby ground point. I used a power supply plugged into a UPS to provide power - I didn't have any batteries with terminals I could solder to, and I did have a UPS available. I slowly turned up the voltage to make sure nothing went wrong, and made sure the voltage at the diodes rose, so I knew it was powered by the supply. Kept an eye on the power supply's ammeter (with the current limit set to 10mA) and a voltmeter connected across that rail the whole time. Desoldered the battery from the PCB, then soldered in the Tadiran. Realized it didn't fit, so I swore profusely, then bent the leads around to make it fit. Then I disconnected the power supply, put the can back on, and realized that even though the battery looked like it fit, it was still blocking the damn can. Swore profusely again, this time in two languages
, because I had to very carefully bend the leads and tape over them to keep them from shorting to anything, and finally got it to fit. I put a spacer of hot glue under it and taped it down to the PCB. (Making sure all the glue and tape were only over the digital section - I don't know if the glue has any electrical effects that might interfere with measurements at high precision. Who knows what the dielectric constant of El Cheapo hot glue sticks is..) Then I put it all back together and started it up to verify I didn't ruin it.
No pictures because I'm not particularly proud of how it looks...
It's in there pretty securely, but looks pretty redneck. Just imagine a battery about 50% too long with its leads bent all over the place, taped down to a PCB...