Glenn,
At one time (either very long ago or recently depending on your age) I
did a great deal of battery work.

Batteries are now rated by:
Reserve capacity (minutes at 25a to 10.5v(12vnom)
20 hr (current * 20hrs to discharge to 10.5v)
50 hr
Cold Cranking Amperes (30sec current to 7.2v at 32F)
0CA (0F Cranking current (same but 0F)

None of these numbers can be used for anything except comparison. If
you run most any battery to its limit by these numbers, it will be
damaged.

If you are going to use the rating to size the bank, go looking for the
for a set of discharge tables (I'll keep looking for the set that I have
- too). You might be able to get a set from a real battery supplier,
but autoparts people will have no idea what you are talking about. This
set of tables will tell you what capacity you can expect (expressed as
percent of 20 or 50hr rate) that you might get at another rate and
temperature to a give terminal voltage.

My actual advice that seems to work best - put in all the battery that
you can afford (weight-space then cost).

How soon to launch?

Fair wind and Smooth Sea
Matt Colie



Glenn Ashmore wrote:
Interesting question came up this evening perusing the Surrette L16H battery
specs. The specs say they have a 20 hour rating of 400AH and a 50 hour
rating of 476AH. Almost 20% more. That is to be expected as total amp
hours increase with slower draw rates. The 20 hour rate is used to compare
one battery against another but if in normal good practice a bank is only
discharged from90% to 40% over 24 hours, wouldn't it be more appropriate to
use the 50 hour rather than the 20 hour rating when estimating the actual
usable amp hours?.