A few months ago, I bought a bundle of a few very nice vintage calculators on eBay, including a 1974 Texas Instruments TI-2500 “Datamath” – this was in good physical condition but was untested and sold-as-seen.
The Texas Instruments TI-2500 Datamath was a professional pocket calculator from 1972 with an early LED display and a rechargeable NiCd battery pack. Several different iterations exist of the TI-2500; mine is the second generation with a full-length mainboard.
The TI-2500 is pretty heavy, primarily because it has a Saft 121685 NiCd rechargeable battery pack inside, comprising four AA-size 1.2V cells (4.8V nominal) – this one was dated February 1974, so it’s no surprise that after 50 years all of the cells had leaked.
NiCd batteries leak a corrosive alkali (potassium hydroxide), even during storage, which can damage electronics. You should be careful handling and disposing of NiCad batteries and their leakage, as cadmium is a toxic heavy metal and can be absorbed through the skin – past this point I’d recommend wearing a set of nitrile gloves, and cleaning your hands, tools, and workspace regularly.
I removed all of the cells and cleaned the interior using white vinegar – some of the traces on the mainboard had minor corrosion, so I cleaned and tinned these with solder.
I wanted to replace the original battery pack with a suitable equivalent, ideally another replaceable one – I took inspiration from Usagi Electric on YouTube who made a really nice video guide on the process of fitting a rechargeable Lithium battery pack.
Lithium cells have a much higher energy density than NiCd cells, so are typically far smaller for an equivalent voltage and capacity – however, they are more picky when it comes to charging voltages and currents, so require a battery management board.
I went for the same hardware used in the video: an Adafruit PowerBoost 500 BMS board with 5Vdc input and a 3.7V 1200mAh LiPo battery pack.
I removed the original power cabling, charge connector, and protection diode, then fitted the new battery pack into the lower case using strong double-sided foam tape.
I replaced the original charging connector with a panel-mount 5.5×2.1mm female DC jack, as this allows the use of a modern 5Vdc 1A centre-positive PSU for charging.
I then soldered the replacement wiring to the BMS board as follows:
- “USB” (5Vdc charge input) goes to the centre pin of the charging connector.
- “EN” (output enable) goes to one half of the power switch.
- “GND” goes to the other half of the power switch, the outer shell of the charging connector, and the ground connection on the mainboard.
- “5V” (regulated 5Vdc output) goes to the Vdd connection on the mainboard.
I then hot-glued the BMS board to the lower case next to the new battery.
The battery simply connects to the battery header on the BMS board.
I tested the charger input by connecting the charger and checking that the charge indicator LED on the BMS lit up correctly, which it did.
I reassembled the calculator for testing on batteries, and it seemed to work perfectly, with the original power switch able to turn it on and off.
It’s difficult to describe how pretty the LED-segment bubble display on this calculator is – photos don’t quite do it justice. It’s especially impressive in the dark.
After all this work was performed, I did some finishing up: I cleaned the case inside and out using Cillit Bang general-purpose degreaser, a microfibre cloth for large areas, and a toothbrush for small areas; I also found that baking soda worked very well for removing some of the tougher marks on the case.
The calculator seemed to work OK, but thorough testing is necessary to verify correct operation, so I did as much testing as I could.
- Calculator powers on OK.
- Power switch works well
- All buttons work correctly.
- All display segments work well.
- All calculator functionality seems to work OK.
- Rechargeable battery works OK.
Another restoration complete, and a nice vintage calculator saved!
