Originally Posted by comfun1
Lou, you seem to know a lot about batteries, so tell me, is there a way to tell in advance if a battery is about to fail and leave you unable to start your vehicle? Or maybe I should have said is there a way to tell when you need to replace your battery before it fails?
Carnac the Magician (Johnny Carson) holds an envelope to his forehead...
As batteries are designed with a specific number of charge/discharge cycles for that style and manufacturer of batteries.
That "by design" age is an upper limit. What you do with that battery determines how long it will last.
A "design" charge cycle is a discharge to 80% capacity and a full recharge every time. Seriously EVERY TIME. 80% You have to be kidding me; who does that?
Well, anyone who wants his batteries to last the 1000 or so cycles the manufacturer determines is the age limit on their battery.
During normal use of a car battery for instance, as long as the car starts in one or two cranks you will never discharge it below 90% or so and then drive it long enough to replace the charge you used. Typically, about 6 years of use. If you drive long distances you will get longer life from your battery because you will fully charge it before you need it again.
Ask someone who drives a couple of miles to and from work in the dark how long HIS battery lasts. The battery never gets a full recharge and winds up deeply discharged at some point. Then one cold winter morning 2 or 3 years later... nada.
So, how do you know if age is unreliable? If your battery does not "hold a charge" you could be in need of a new battery or it could just be deeply sulphated from being in a low charge state for a long time. A battery load test will let you know if it is healthy enough for continued use.
Read up on charge charge cycles and battery health.
Several good files here in the FAQ and elsewhere.