> Actually, the value I've heard is 14.25. You can apply 14.25 volts to an
> automotive battery indefinitely without harming it and this is
> the voltage at
> which modern alternators regulate.
The exact "correct" value depends on battery temperature and construction.
A quick search turned up this paper with a nice curve for sealed lead-acid
... flooded (wet) lead-acid will be similar but not exactly the same.
http://www.actec.dk/SLA-charge.pdf
And it turns out that the charge voltage that is so low it will not shorten
the battery's life _at all_ is also too low to fully charge the battery,
especially if the battery has been deeply discharged in the past. So, the
absolute optimum charging cycle (in terms of maximum battery life) is to
charge to a higher voltage and then let the voltage "float" down to a lower
value. However, automotive charging systems (even in big trucks AFAIK)
don't do this.
There is actually ongoing research as to the "best" way to charge lead-acid
batteries, within constraints of rapid charge cycles and maximum battery
life. Since it appears we won't have any battery technology break-throughs,
current generation electric cars use lead-acid. Here's an interesting
presentation from the 17th annual Electric Vehicle Symposium :
http://www.nrel.gov/vehiclesandfuels/energystorage/pdfs/evs17pres.pdf
Randall
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