Date: Sat, 21 Aug 1999 01:14:48 -0400
Reply-To: David Beierl <dbeierl@IBM.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@IBM.NET>
Subject: Re: Optima d750-S vs. d750-U
In-Reply-To: <37BE2C3F.16DCCAE4@mindspring.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed
At 21:34 8/20/99 -0700, Todd Last wrote:
>The web site cautioned against using CCA and Amp hour ratings and to
>instead look
>at the Reserve and Cycle life to judge the quality of the battery.
Hi Todd --
CCA and Reserve time are *starting* battery ratings. I don't believe that
deep-cycle batteries are ever rated in those terms, as they are not built
to deliver high cranking amps at any time. CCA (Cold Cranking Amps) says
how many amps the battery can deliver for thirty seconds at zero
fahrenheit. There's a much more generous number you find on marine
batteries that is specified at 32 f. instead of zero. Reserve Capacity
tells how many minutes the battery will carry a 25 amp load when fully
charged (and, I believe, have a little left to start with) at some benign
temperature like 60 fahrenheit. This is a useful number to give an idea
how far you might get driving when your alternator stops alternating.
Amp-hour ratings say how much total power is available from the battery at
a specified discharge rate. Ordinarily the rate is 20 hours to discharged,
b/c that rate gives high numbers. I believe Optima also specifies a
10-hour rate (I think they are 65 AH at c/20, 52 AH at c/10). A one-hour
rating will be a pitiful fraction of the 20-hour. For a deep-cycle battery
these are the ratings that make sense, although you have to bear in mind
that you will seldom in practice achieve the theoretical capacity, and if
you want to have the battery to last a long time you should assume a
working capacity of 35% of theoretical. That is to say, it takes three
hours or so to charge from 85% to near 100%, which is not economical if
you're running the engine to charge; and the cycle life degrades
considerably if you discharge below 50% (like 1000 cycles at 50% down to
200 cycles at 10%). Also, of course, the battery loses capacity as it
ages. I believe the industry agrees that a battery is used up when its
capacity reaches half of original, and that's what the cycle-life ratings
are based on. Depressing, isn't it?
Cycle life will be specified as so many charge-discharge cycles down to X
per cent of charge. Assuming that X is the same and the mfr is telling the
truth, more cycles implies a better battery, but this says nothing about
the capacity of the battery.
All these ratings are subject to "specsmanship," exaggeration, sample
variations, lack of initial freshness etc. etc. Consumer Reports doesn't
do deep-cycle batteries, but they periodically take a look at
(consumer-grade) starting batteries and they always get a good proportion
of batteries that don't meet their claimed specs.
My two cents...
david
David Beierl - Providence, RI
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"
| 
