Hi. I bought a trio set of the 4600 cordless phones, plugged them in to charge for a couple of days. Unplugged them, including the base, before setting up and within a day all three were dead. Standby time is supposed to be 170 hours, around a week. Contacted BT who gave me a return number. In the meantime I plugged them in again and this time left the base on and all three lasted over a week. Inadvertantly switched the whole lot off a day ago and again they were flat before a day had passed. Simple question, with no base station do the phones "wind up" their transmit power, looking for a base, hence flattening the batteries more quickly?
Its most likely that the phones would very their transmit power level depending on how far they are from the base unit. This would keep power consumption down, and avoid overloading the base unit if the phone is too close.
The same applies to normal mobile phones as well.
In your case, switching off the base unit would be the most likely cause of your poor battery performance.
You will find that the same applies to wifi connected devices. The incoming signal level and quality is used to determine the transmitted power level. Without this, a wifi device which was close to the router, would overload the receive section of the radio module in the router.