if there is any reason why I cannot/shouldn't use a slightly lower amp hour battery.
shortened battery life ..
higher workload for the alternator as well i'd guess
and as it's winter .. colder temps will add to the workload
cranking a cold engine .. heating up glowplugs if it's a diesel ..
more usage of headlights .. and the ptc heater if fitted ..
car batteries don't like deep discharge .. it shortens their life ..
( a.f.a.i.k. )
i don't know how much difference 3Ah will make
actually .. just bumping your post ..
as someone with electrical knowledge might give you a better answer
when my last battery was on it's last legs ..
the starter motor began chirping ..
( when the starter gear doesn't disengage quick enough )
the door locks squealed on locking/unlocking ..
then cranking got slower .. few days later it refused to crank ..
i think it were the original battery as well .. 11 years ..
now have fitted an 80 ah battery ..
stater motor and locks all happy now ..
even my portable 12v hi-fi defaults to a higher volume at switch on
which surprised me
anyhow .. post Bumped

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