TI-86 Battery Discharger

In 2015, I got an ESP8266 with the hopes of using it to connect a graphing calculator to the internet. The plan was to swap the four 1.5v AAA batteries in a TI-86 for two 3.6v 10440 batteries and use the space freed up in the battery compartment to house the ESP8266. The TI-86 would have acted as a terminal to more powerful mathematical software like Mathematica or Matlab running on my computer. My last post mentioning this project was about Makevention 2015 and explained that there were power supply problems running the ESP8266 on a breadboard. Since then, this project has been on the back burner.

While I haven't done anything with the ESP8266 since then, I did put batteries in the TI-86 this past year so it would be ready to use if I ever wanted it. When I turned it on, the screen was solid black. This happens sometimes with new batteries when the contrast is set too high after being increased over and over as the last set of batteries decreases in voltage. This time, adjusting the contrast down didn't show any noticeable change. After a few minutes of letting the calculator run, the contrast decreased gradually until the text on the screen was visible. This was really puzzling behavior.

Searching the internet didn't turn up much for calculators that malfunction with a dark screen. While trying to troubleshoot the problem myself, I checked the voltage of the batteries with my multimeter. In all of my calculators, I have Lithium AAAs since they don't leak and damage the calculator the way alkalines can. The voltage of the AAAs in the TI-86 was around 1.8v, far above the 1.5v nominal voltage of a AAA and the 1.6v of new alkaline AAAs I've measured. After letting the calculator run a while until the text on the screen became visible, I measured the battery voltage again and it was over 1.6v. It seems the voltage drops as the calculator runs which eventually lowers the contrast, so I decided to build a circuit to discharge the batteries down to the optimal point for the contrast.

The discharger I built uses an MSP430 to switch an NPN transistor that bleeds the batteries for one second at a time through eight parallel 1/4W resistors. After each one second discharge, the MSP430 takes an ADC voltage reading from a resistor divider that lowers the battery voltage down to the range the MSP430 can safely measure. The MSP430 then displays the ADC voltage read, the calculated voltage of the batteries, and run time on a very small OLED. Part of the justification (and fun) for this project was learning how to drive the OLED since I'd like to use one for a calculator screen someday. The whole discharge process repeats until the battery voltage hits 1.55v. In my case, it took 42 seconds to hit the target voltage. 

After the batteries discharged to 1.55v, I put them in the TI-86 and the contrast was perfect! This was very satisfying. However, the next morning when I woke up and turned the calculator on, the contrast was back to solid black since the battery voltage had floated back up to over 1.7v overnight. Apparently, this is a normal part of battery chemistry. For the discharger to work, I would need to figure out how far down below 1.55v to discharge the batteries so that they wouldn't rise over 1.6v overnight. Rather than spending multiple days testing battery discharge and recovery cycles, I soldered a piece of protoboard to replace one of the four batteries and lower the average total voltage:

This lowered the contrast to the right level. Ultimately, the battery discharger was a failure though I had fun building something new with an MSP430 and learned how to drive an OLED display. After a year, the contrast still looks great.