We do recommend that you change the rechargeable batteries approx. every 6 months. The cells within rechargeable batteries depreciate over time as they are constantly charged/discharged and can give a false reading.
The Parent Unit can switch back on albeit for a short amount of time if at all when removed from power.
We also recommend that you purchase AAA NiMh rechargeable batteries, these are available from all good retailers or on line.
It is vital, for optimum performance from this type of battery, to charge 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.