> The lithium battery I have chosen shows constant voltage over
> the life of the battery and drops suddenly (http://www.tadiran.com)

Taking this 1Ah battery as an example

http://www.tadiranbat.com/pdf/tl-2150.pdf

10uA continuous discharge gives you somewhere around 22
months (~16,000 hours) life. What's interesting is that the
calculated Ah rating is 1Ah at 1mA. Note that for 10uA the yield
is 0.72Ah. It looks as though this yield curve needs to be figured
in. Say you reckon on 3 x 16,000 hours at 3uA continuous =
48,000 hours = ~ 5 years. The yield, however, is going down
and the lifetime is not based on 1Ah but perhaps 0.62Ah. Could
you then reasonably expect the battery to reach 2V terminal at
some point less than 5 years ? Although as you say, and as is
shown in the discharge graph, voltage does tend to plummet
suddenly so the product will probably go from live to dead in
quite a short time

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