Ti-89 Titanium Rechargeable Batteries? NiMH?

I use alkaline batteries, but they cost a lot over time. I was wondering how do NiMh batteries do in a Ti-89 Calculator. Provided, that I will probably do Cpu intensive calculations, could an overall cell potential of 4.8 volts do the job of the 6.0 volts provided by the alkalines? Which ones should I use?