Consider a pacemaker delivering $2$ mA$,1$ ms

pulse every second. Pacemakers are often powered by a lithium$-$iodide battery that

can deliver a total charge of $2$ ampere hours.

$($a$)$ How long will the battery last? Ans: $114$ years

$($b$)$ Your answer to $($c$)$ is an overestimate of battery lifetime, in part because the

battery voltage begins to decline before all its charge has been delivered, and

in part because the pacemaker circuitry requires a small, constant current. For

this pacemaker, add a constant current drain of $5$ A and assume that the useful

lifetime of the battery is over when $75\%$ of the total charge has been delivered.

How long will the battery last in this case? Ans: $24$ years