Unexpected pleasure from abused AGM batteries [RE-wrenches]

[Eric Stromberger]

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Hi Darryl,  We have discovered this lately about the Trimetric.  It 
seem to become more inaccurate as the batteries become more sulphated - 
that is allowing them to drop below the 50% state of charge.  Ralph at 
Bogard offered little help in explaining the reason for this. We are 
now not trusting the Trimetric, which we use to consider THE guide.  
We're asking customers to check the Trimetric accuracy with specific 
gravity tests.

Eric Stromberger
Mendocino Solar Service


On Apr 15, 2006, at 5:59 AM, Darryl Thayer wrote:

> Great story Windy, but what do wrenches do to assure
> complete recharge after cycling?
>
> My situation is the battery's are required to supply
> power at night, next day they get recharged on a three
> stage charge cycle.  (Absorb to absorb volts, limit
> current at absorb volts for time, drop volts and hold
> at float the remainder of day) After 6 months I
> checked the battery set, it was at 49.8 resting volts.
>  So I increased the Absorb time, checked once a month
> until the resting volts were at the maximum of about
> 2.2 volts per cell.  Which means I am slightly cooking
> the batteries.  So i set the the absorb time back.
>
> What do I do to get an automated battery charge
> reading, and get an automated correction to keep the
> batteries at full and not overfull.  with flooded
> batteries I can add water and check with the
> hydrometer.  By the way I have a trimetric, and I am
> unable to tell the net battery status with enough
> accuracy.
>
> --- Windy Dankoff <windydankoff at mac.com> wrote:
>
>> 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
>>
>> -------------- quote --------------
>> 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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