Method and apparatus for monitoring the voltage of a battery
One aspect of the invention involves: monitoring a voltage of a battery over time; evaluating whether a rate of voltage decrease of the battery is in excess of a threshold; and indicating that the battery is subject to a low voltage condition when the rate of voltage decrease exceeds the threshold. Another aspect of the invention involves: causing a circuit powered by a battery to respond to battery replacement by thereafter applying a selected load to the battery during a selected time interval. A further aspect involves: periodically sampling a voltage of a battery; and applying a load to the battery during a selected time interval before each sampling of the battery voltage.
1. A method comprising:
monitoring a voltage of a battery over time;
evaluating whether a rate of voltage decrease of said battery is in excess of a threshold; and
indicating that said battery is subject to a low voltage condition when said rate of voltage decrease exceeds said threshold;
wherein said monitoring of said voltage over time includes:
monitoring an ambient temperature; and
normalizing measured voltage values as a function of the ambient temperature.
2. A method according to claim 1 , including providing a radio frequency identification tag that includes and is powered by said battery, said tag having an antenna, and having a circuit that includes a transmitter coupled to said antenna, said circuit transmitting through said transmitter and said antenna a radio frequency signal that indicates whether a determination has been made that said battery is subject to a low battery condition.
3. A method according to claim 1 , including selecting a lithium cell to serve as said battery.
4. A method comprising:
monitoring a voltage of a battery over time;
evaluating whether a rate of voltage decrease of said battery is in excess of a threshold; and
indicating that said battery is subject to a low voltage condition when said rate of voltage decrease exceeds said threshold;
wherein said monitoring includes:
tracking the drain of charge from said battery;
determining a series of points in time for which a substantially equal amount of charge is drained from said battery between each pair of successive said points in time;
sampling said voltage of said battery at each of said points in time;
calculating the difference between said voltage samples from successive said points in time; and
comparing said calculated difference to said threshold.
5. A method according to claim 4 , including applying a load to said battery for a selected time interval before each said sampling of said voltage of said battery.
6. A method according to claim 4 , including providing a radio frequency identification tag that includes and is powered by said battery, said tag having an antenna, and having a circuit that includes a transmitter coupled to said antenna, said circuit transmitting through said transmitter and said antenna a radio frequency signal that indicates whether a determination has been made that said battery is subject to a low battery condition.
7. A method according to claim 4 , including selecting a lithium cell to serve as said battery.