Message text written by Randall Young
>David Massey wrote:
>
> P.P.S. CCA is not a measure of capacity. That is measured in Amp-Hours.
> Few manufacturers list the A-H rating of the battery in a prominent way.
Last time I looked, most battery manufacturers were posting "Reserve
Capacity" or RC, which is an indication of battery capacity. RC is
given in minutes, and is the number of minutes a battery can deliver a
fixed current before reaching a specified terminal voltage. (Sorry, I
don't recall the current or voltage off-hand.) It's still really an
amp-hour rating, just presented in a different fashion.
Although IMO the switch to RC was simply to confuse the battery buying
public, there is actually a rationale for it : RC is supposed to more
accurately reflect what users are actually interested in : how long can
you leave your headlights on and still start the car ?
The problem with amp-hour ratings of lead-acid car batteries is that the
number varies greatly with how fast the battery is discharged. The
previous standard was the "20-hour" rating, where the discharge current
was fixed at the value that the battery could deliver for 20 hours. The
new 'RC' standard instead uses the same current for all batteries. The
name and units were changed mostly to prevent confusion with the old
standard (which typically resulted in higher AH ratings than the new
one).
Randall
<
Additionally, Starting batteries are optimized for current capacity and do
not tolerate well deep cycling. Other types of batteries are optimized for
cycling and can not deliver the starting current as a same sized (and
weight). Concequently, Amp-Hour ratings for starting batteries is not a
primary concern. Starting batteries have thin plates with maximum surface
area (to get the current) but as a concequence when deeply discharged and
recharged the plates do not reform to the original integrity and
configuration. Deep cycle battery plates are much heavier and can tolerate
deeper discharges.
Furthermore, battery current capacity diminishes greatly at lower
temperatures and a starting battery is sized for worst case conditions
(ie: starting at -40 degrees). For those of us who only drive our cars in
temperate weather conditions we find our selves with much more battery than
we need and can squeek by for a few more years with an old battery that is
not performing to spec.
So buying an oversized battery can lead to less frequent replacements but
not always since heat can cause batteries to fail suddenly. A friend in
the pre-owned auto business once told me that more batteries expire in
summer than in winter. Most batteries replaced in winter are just old
batteries that are becoming marginal. Summer time failures are sudden. I
once test drove a two year old car that started fine on the dealer's lot
but when the engine died it wouldn't restart. The battery failed as we
drove.
Dave
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