Date:         Fri, 16 Jun 2000 01:02:14 -0400
Reply-To:     David Beierl <dbeierl@ibm.net>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ibm.net>
Subject:      Re: dual battery? hobigesuss!/Lets try this again!
Comments: To: THX0980@aol.com
In-Reply-To:  <77.57ad2c7.267b0695@aol.com>
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At 00:27 6/16/2000, Beckett Cantrell wrote:
>rated at, lets just say, 150amp/hrs, then you can expect to get just that,
>ie, you could run an accessory that draws 1 amp for 150hrs, or an accessory
>that draws 10amps for 15hrs, or any given combination up to the max capacity
>of said batt. The Hella kit has nothing to do with the out-put of the batt,

Just to be picky about a detail here -- the battery capacity will be
(usually) rated at C/20, i.e. the amount of draw that will discharge the
battery in 20 hours.  As you increase the rate of draw, the total available
capacity becomes less -- I forget the numbers right now, but if for example
you discharge at a one-hour rate your total capacity will be much less than
half of the rating.   Nothing to do with the present discussion, but you
need to keep it in mind when actually using the beast.  An Optima battery,
for example, rated at (guessing) 62 AH at C/20, and 54 AH at C/10, would
not run a 7-amp refrigerator for 7 1/2 hours -- probably more like five or
six (or if you keep to conservative battery-management practice, using only
35% of rated capacity*, more like two hours).

* This figure is the difference between 50%, which is as low as you should
discharge the battery if you want it to last a long time, and 85%, which is
as high as you can charge it quickly.  The last 15% of charge will take
several hours no matter what, whereas by using "smart" regulators you could
charge up to that point in very much less time.  Unfortunately the standard
automotive regulator doesn't understand how to do this, so even getting to
85% charge takes far longer than it needs to.  Basically you want to keep a
constant charge rate of 15-20% (or more) of rated capacity until the
battery voltage rises to 14.1 volts (*adjusted for battery type and
temperature*), then maintain constant voltage of 14.1 (with constantly
declining charge rate) until you get tired.  Automotive regulators don't
even know what the battery temp is, let alone what to do about it.  See
http://www.amplepower.com/ps_reports/v1-2/index.html (and related links) if
you want a thorough discussion of this.  Then decide whether you want to
spend $300 for a regulator <g>.

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