short lived UPS system [RE-wrenches]

[Richard Perez, Home Power magazine]

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Hello Wrenches

OK, I'll put my two cents in on this lead-acid battery topic. This 
info was gathered from my personal experience, feedback I've had from 
HP readers and RE dealers, and reading the published technical 
literature on L-A batteries.

Lead-acid batteries do not benefit from cycling. The only battery 
technology which exhibits "memory effect" is the small sintered plate 
Ni-Cds (flashlight cells). All other technologies last the longest 
(calendar lifetime and cycle lifetime) when shallowly discharged.

I think that the key here is equalizing charges. In a series/parallel 
array of L-A cells it is inevitable that the SOC of some of the cells 
becomes unequal. The equalizing charge is the only way to bring all 
cells to full charge.

If equalizing charges are not performed, then sulfation and dendrites 
(whiskers to plate material which form inter-plate short circuits) 
set in. As some cells get sulfated (consider them lazier), the other 
cells work harder. This situation is easy to diagnose--sulfated cells 
have radically lower voltage under discharge, radically higher 
voltage under charge, and these cells consume more water than other, 
healthy cells. The equalizing charge is the answer.

Batteries with a high number of parallel elements are more 
susceptible to this problem. I think that a L-A battery should have 
no more than three parallel strings of cells, two is better, and one 
is ideal. The higher the number of parallel elements, the more 
critical it is that equalizing charges be performed on schedule.

Even batteries in float service need periodic equalizing charges. It 
is very difficult to find that voltage "sweet point" for cells which 
are floated. This voltage set point varies with temperature, battery 
manufacturer/model, service, and even the cabling connecting the 
array of cells.

Some makers of sealed AGM cells now recommend "equalizing" their 
product. This is not a true equalizing charge since the battery 
cannot deal with the gasses produced, but merely raising the 
regulation voltage for a period of time. This is, however, 
recognition that even sealed L-A cells can exhibit unequal states of 
charge in a large battery array. Once again, a controlled overcharge 
is the only way to bring all the cells back to an equal SOC.

Roy is correct in saying that once a neglected battery pack has 
reached the state where the unmindful customer complains, it is best 
to replace the whole battery pack and not just the worst elements. 
Symmetry and sameness is as important in a battery as it is in a PV 
array.

Richard

At 17:02 -0400 09/25/2003, EH Roy wrote:
>Todd,
>
>I'm sure Richard Perez can explain the physics behind this but lack of
>cycling and equalizing of batteries kept in a continuous float condition
>causes something called "mossing". Essentially fine pieces of lead flake off
>and collect at the bottom of the cell. When they collect to the point where
>they reach the bottom of the plates it shorts the plates and charging takes
>more and more energy because of the short. The short results in increased
>battery temp during charging - you'll notice it. Increased water usage is
>another symptom. If you try to do a vigorous charge of batteries that have
>undergone mossing you will notice that the electrolyte is cloudy because of
>the fine particles of lead in suspension.
>
>As usual, don't try to replace only the shorted battery with a new one. The
>others in the bank have almost assuredly undergone mossing as well and it
>will only be a matter of a short time before more plate shorting occurs.
>Replace the whole bank.
>
>I learned this from Trojan when one of our Y2K customers ended up with a
>failed battery bank due to mossing because all he had done for maintenance
>was check electrolyte levels.
>
>Richard, I encourage you to add, clarify, and correct as appropriate.
>
>Thanks,
>E. H. Roy
>Solar Works Inc.
>64 Main Street
>Montpelier, VT 05602
>802 223-7804 x306
>ehroy at solar-works.com
>
>
>
>-----Original Message-----
>From: Todd Cory, Mt. Shasta Energy Services
>[mailto:toddcory at finestplanet.com]
>Sent: Thursday, September 25, 2003 11:45 AM
>To: RE-wrenches at topica.com
>Subject: Re: short lived UPS system [RE-wrenches]
>
>
>I still don't understand the need for cycling flooded LA batteries. I can
>see
>the need for equalization charges here and there, but cycling? Why? These
>are
>not ni-cads so IMO should only suffer from needless cycles not benefit from
>that.
>
>Todd

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