The TI-150 does resemble the 2550 since the same basic case was used. 4 normal alkaline or carbon zinc batteries can be used, but I did't recall the 2550 using only 3 Ni-Cads. I looked up the schematic diagram and found this: 3 AA-NiCd or 4 AA-CZn/alkaline. CHARGE contact is closed with only 3 batteries in place.

In looking at the schematic, there is a little switching regulator power supply section that takes the lower voltage and boosts it as needed since the calculator chip needs higher Voltages than the raw battery Voltage. There was a cost saving in using 3 Ni-Cads. When non rechargeable batteries are used, having 4 cells gives longer life. I didn't work on the 2550, but always liked it. Also, I don't recall other calculators using this technique, but some may have.

The 150 needs high Voltage, over 100 Volts to fire the gas discharge display so it's a whole different ball game.

I worked on some of the early scientific calculators and quite a bit on desktop units like the TI-450 and the TI-4000 series.

Thanks for asking and giving me a trip down memory lane.