Date:         Sun, 13 Jul 2008 17:48:30 -0400
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: Xantrex XM 1000 PRO + Batteries in Parallel
Comments: To: M'obeechi <obeechi@RUNBOX.COM>
Comments: cc: Dennis Haynes <d23haynes57@HOTMAIL.COM>
In-Reply-To:  <E1KHrds-0004Du-Tm@garm.runbox.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 10:48 PM 7/12/2008, M'obeechi wrote:
>.5C rate? What is this? How do you calculate it? Okay, googled
>discharge rate, but this is a little confusing
>http://www.batteryuniversity.com/print-partone-16.htm - I'm hoping
>that I can get three batteries together which will allow for 200 AH.

"C" is the rated capacity of the battery in amp-hours.  Lead-acid
batteries are always rated at a specific discharge rate, because the
usable capacity depends strongly on the rate of discharge.  So
(picking numbers out of my\\ the air), a battery might be rated for
100 AH at a 20-hr discharge rate, but only 75 AH at a 10-hour rate,
and maybe only 25 AH at a one-hour rate.  In other words, this
hypothetical battery would be able to supply 5 amps for 20 hours
(5x20=100) or 7.5 amps for 10 hours (7.5x10=75) or 25 amps for one
hour without falling below some defined minimum voltage like ten volts.

A common way of describing discharge rate is as a fraction of C -- so
a 20-hour rate would commonly be written as C/20 (or sometimes as
.05C).  To use Dennis's numbers, a two-hour rate would be C/2 and a
half-hour rate would be 2C.  Dennis accidentally wrote them down
backwards.  He meant to say that a 2C discharge rate, i.e. a rate
that would discharge the battery in half an hour, is too much for the
battery to supply for *any* appreciable length of time without
progressive damage -- the rate should generally be limited to no more
than C/2 to avoid this.

Does that help?

david



--
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage," '85 GL "Poor Relation"