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

Mick Abraham mick at abrahamsolar.com
Thu Jun 10 09:02:17 PDT 2010


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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