The X-Rite Model 331 and Model 341 densitometers ship with four AA Nickel Cadmium rechargeable batteries with 700mAh(or greater)
Part number: SE15-19. Even though the User's Guide states that users should replace batteries with 600mAh AA NiCd batteries, Nickel Metal Hydride (NiMH) batteries are acceptable replacements. 600mAh should be the minimum capacity rating. One advantage to using NiMH batteries is that due to their increased capacity (can be over 2000mAh), much longer time between charges would be expected. Keep in mind that the higher the capacity (larger mAh rating), the longer it will take to fully charge the batteries.
Can Nickel Metal Hydride (NiMH) rechargeable batteries be used in X-Rite instruments such as the Model 331 and 341 densitometers?
Answer - Yes...
Use of AA alkaline batteries can be dangerous to use and should be avoided. If the AC adapter is plugged in with alkaline batteries installed, the batteries can be destroyed along with causing damage to the instrument.
Do not use Renewal batteries!
These are not the same as the rechargeable NiCad. They are alkaline rechargeable batteries, known as "Renewal", manufactured by RAYOVAC.