1. They do not put out enough voltage (at least 8.4 volts) to properly run the instruments. NiCad and NiMh "9-volt" batteries, even when fully charged, put out a maximum of 7.2 volts.
2. Even if they did work, rechargeable batteries only hold a charge for 1-3 months. Alkaline batteries, when fresh, can go 6 months or longer. We recommend Duracell for a good fit.