We recommend these are changed approx. every 6 months. The cells within all rechargeable batteries depreciate over time as they are constantly charged/discharged and can give a false reading.
As a result the battery life of the Parent Unit, when not connected to power reduces. To the point where the unit powers OFF almost immediately despite showing a full battery seconds before.
We recommend that you purchase AAA NiMh rechargeable batteries, these are available from all good retailers or on line.
For the best performance from this type of battery, charge them for 15hrs before their first use.
To ensure the longevity of your battery life you need to fully charge and fully discharge periodically, this keeps the cell life to a maximum.