As reference regarding NiMH rechargeable batteries:
NiMH batteries only put out 1.2vdc vs 1.5vdc for alkaline, so with the 3 in these units you get 3.6v vs 4.5v. A device that takes 4 normal cells to make 6v needs 5 rechargeables to get the same supply voltage.
NiMh batteries lose 25% of their charge per month sitting on the shelf, alkaline batteries have a typical 100% charge shelf life of 5 years. In a device where we would like the batteries to last 6 months 25%/month is significant.
From topic http://forum.mios.com/index.php?topic=629.0: