Lithium halide primary cell having end of life indicator means

DETAILED DESCRIPTION Generally, the present invention is adaptable to lithium halide batteries having at least one lithium anode, one of the major surfaces of which is contact with a depolarizer such as the depolarizer disclosed in U.
S.
Pat.
No.
4,148,975.
The invention comprises a stepped portion on the major surface not in contact with the depolarizer.
The stepped portion comprises about 15 to 5% of the total thickness of the anode and has a surface area of from about 40 to 60% of the surface area of the major surface in contact with the depolarizer.
In a preferred embodiment of the present invention, the thickness of the step portion of the anode comprises approximately 10% of the total thickness of the anode and has a surface area of about 50% of the surface area of the major surface in contact with the depolarizer.
It has been found that the surface area of the electrolyte formed at the inner face between the anode and depolarizer is proportional to the internal impedance of the cell at any time during discharge.
The stepped portion having the preferred dimensions results in a doubling of the resistance at that point in time when only 10% of the unused lithium anode remains.
This increase in the internal impedance of the cell occurs suddenly, and is detectable by a physician observing a significant drop in the heart stimulation rate afforded by the pacer device.
By providing such indication when 5 to 15% of the lithium anode remains unused there is sufficient time to replace the battery before its end of life.
Other advantages of the present invention will become apparent from a perusual of the following detailed description of a presently preferred embodiment taken in connection with the accompanying drawings.