"Caroline" <caroline10027remove@earthlink.net> wrote in
news:MIKtc.13684$Tn6.4370@newsread1.news.pas.earth link.net:

> I'd say replace before failure. My anecdotal reading indicates that a
> failing battery puts a strain on the alternator and may contribute to
> premature alternator failure. E.g.
>
> "A weak battery that is not storing enough power will cause the
> alternator to work harder and possibly cause premature alternator
> failure. Dirty or corroded battery terminals can severely reduce the
> lifespan of the battery and alternator."
> http://www.trustmymechanic.com/35.html

Except that when CRANKING the engine,the alternator is NOT in use.
(the real battery test;cranking,the greatest strain on the battery,100s of
amps drawn in a short time)

Lead-acid batteries fail from loss of electrolyte(evaporation of H2O) or
from the paste falling out of the plate grids and piling up at the bottom
of the cell,shorting it,or from sulfation,causing the internal resistance
to increase,causing a greater voltage drop under loads.

Alternators fail when a bearing fails,when a rectifier diode fails,or the
slip rings brushes fail from wear.(bad contact)

--
Jim Yanik
jyanik-at-kua.net