[RE-wrenches] Help me do the math: amortized cost per amp-hour

[Bell, Steve]

HI Mick,

 

The Concorde Battery web site has a great technical manual worth
checking out;
www.sunxtender.com/pdfs/Sun_Xtender_Battery_Technical_Manual.pdf

 

It would sound like the this battery bank mostly operating at an average
daily DOD (Depth of Discharge) of about 10% and that daily load will be
increasing to about 20% DOD. 

 

 According to the Concorde manual (pg 37) the expected battery life
cycle at 10% DOD is about 5000 cycles. The expect life cycle at 20% DOD
is about 2800 cycles (about 7.6 years). BTW, these values are based on
an "End of Life" when the battery has permanently lost 20% of its
original rated amp-hours. In real life, most batteries stay in service
beyond that value.

 

Obviously, the occasional deeper DOD during cabin visits will somewhat
effect these numbers.

 

I also agree with Ray's comments that batteries need to be cycled
somewhat (15% to 20%) to harvest their full life cycle. We all learned
that when we continuously floated battery banks with the old SW
inverters. Banks that are always floated only seem to deliver about 70%
of the expected life. Moderate cycling is good for a battery.

 

I think increasing the daily DOD from 10% to 20% should not
significantly reduce battery life. This is assuming the batteries get
fully recharged on a regular basis.

 

Steve

 

Steve Bell

Technical Support Specialist 

 

SunWize Technologies, Inc.

#1 Sun Street

Stelle, IL  60919 USA

(815) 256-2222 ext. 23

sbell at sunwize.com

www.sunwize.com

________________________________

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Mick
Abraham
Sent: Thursday, June 10, 2010 11:02 AM
To: RE-wrenches
Subject: [RE-wrenches] Help me do the math: amortized cost per amp-hour

 

Greetings, fellow Energy Techs~

A client has a nice new set of 2v Concorde AGM sealed batteries: 915 amp
hours in one 24 volt string. His cabin is often unoccupied with only the
refrigerator running. During those "idle" days the battery is called
upon to deliver an average of 90 amp hours every night...about 10%
discharge.

The client wishes to rev up some more loads to run during these idle
times. Coincidentally, those loads would also require an additional 10%
discharge from the battery each night. The added loads surely have a
cost in terms of decreased battery lifetime. I wish to do the math to
amortize the battery cost and arrive at a "dollar value per month" for
the additional loads.

The pack plus installation cost about $4,700. I don't know the number of
"lifetime amp-hours" which could be expected from these batts, but I
hope one of you can help me get that number. Could we then divide $4,700
by the lifetime amp-hours to get the cost per amp hour? If so...I could
do the rest of the math from there.

*************************************************************

Side note: I realize that various factors such as shallow cycling,
repeated overdischarge, repeated overcharge, etc. can affect the number
of amp hours which any battery can deliver. In this case, let's assume
the following:

* No repeat overcharge because I'm strictly following Concorde
recommendations and I've even calibrated the Outback charge devices

* No repeat overdischarge because I >finally< have the generator
automation dialed in and reliable (fingers crossed)

* Prompt recharge after discharge...because there's an OK sized solar
array, many sunny days plus the genset backup

* Electronic sulfate dissolver is installed on the battery string

* Average cycle depth of 50% discharge. I'm using this value to factor
in the times when the cabin is occupied and therefore cycling harder on
the battery

* Limited heat stress on the battery. The cells are in a crawl space in
alpine terrain so they will range from cold to cool, never warm


**************************************************************
 
Thanks in advance for your assistance. The Wrench List is the bomb!

Mick Abraham, Proprietor
www.abrahamsolar.com

Voice: 970-731-4675

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