Unexpected pleasure from abused AGM batteries [RE-wrenches]

[Windy Dankoff]

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Wrenches,

An old industry buddy sent me this to share on the list.  He likes to  
lurk anonymously so I pass this to you for your late-night enjoyment  
and wonder.

-- Windy

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Hey Windy...

I just read your Wrench post on the batteries.  On target as usual.

For notes, I recently completed an extensive analysis of 24 GNB VRLA  
cells that were used by a now-defunct telecom firm.  The cells are  
four years old, and had only one year of use in float service before  
the company went under.  The power company (wonderful folks that they  
are) did their duty and turned off the power shortly after the telco  
went into receivership.  On power fail, the telco UPS system kicked  
on and kept running until the batteries were dead.  Flat dead .. as  
in 1.8 VOC.  The UPS went into low-voltage shutdown, and the system  
sat for three more years while the bankruptcy case worked its way  
through the court.  In the end, a trustee was called in to haul the  
stuff off late last year.  No one would take the battery system .. so  
I ended up with it (48 V @ 965 AH).  It's nice to have some  
sophisticated data acquisition equipment.  Turns out the pack is ok.

Before beginning my tests, I talked with an engineer at GNB who  
turned out to be a solar nerd .. and *very* knowledgeable.  He  
predicted almost to the millivolt what I'd read during each of the  
tests, and even suggested early on, and based on very preliminary  
measurements, the pack would be ok.  He was right.  He also gave me  
some interesting info.  As far as the VRLA cells go, it appears  
there's less harm done by letting these particular type of cells sit  
in a discharged state for extended periods than first thought.  The  
wear-out factor is more temperature and charge/discharge related.  He  
said GNB is now looking at cells in terms of the total W-H in/out ..  
and as an approximate rule of thumb, we could use the total C/8 W-H  
rating of the battery multiplied by 1,000 .. and THAT figure would  
give you an idea of the total "miles left" on a VRLA battery.  In  
effect, this 965 A-H pack, times 48 volts, is a 46.3 kWh battery.   
The pack will be considered "dead" .. or at least qualified for  
replacement .. when it's cycled a total of 46.3 MEGA watt hours.   
Aside from blatant abuse (such as taking a cell to near zero volts,  
or even polarity reversal, etc.), depth of discharge and other  
considerations, though important, appear to be more secondary to  
overall battery wear .. at least in the VRLA cells.  I can't speak to  
other chemistries/topologies.

Aside from slightly higher than normal leakage current (caused by a  
slight leaching of lead into the water over an extended period of  
long-term discharge), the cell capacities are all 105% to 120% of  
spec, and even the leakage current is expected to decrease slowly as  
the cells again sit in float service.

Thought I'd share this with you as long as you were on the battery  
topic on the Wrench board.

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