[Milsurplus] Lead Acid Batteries

[Peter Gottlieb]

I hate to beat a dead horse but I just wanted to point out a few things.

First of all, there are very few "gel cell" batteries out there.  Gelled 
electrolytes just never became popular, perhaps due to ionic mobility issues. 
What IS common though, are batteries where the liquid electrolyte is absorbed in 
  porous glass or other mats between the plates.

Lead acid batteries must remain charged or (mostly) irreversible sulphation of 
the plates will occur.  Mild sulphation can be reversed but lead sulphate is 
larger than the material it replaces and eventually the plates will swell and 
can short through the insulating mats and even crack the battery case.  Some 
UPSes used to have tight fitting battery compartments and the swelled batteries 
were impossible to remove after the steel chassis were deformed.  Obviously the 
batteries do not need to be on float continuously but they cannot be allowed to 
completely discharge.

My understanding from Yuasa, Panasonic and CSB (China Storage Battery) is that 
the absolute best charging method is to constant voltage charge to 14.2 or so 
until the current tapers to a low value then drop to a maintenance float of 13.7 
or so.  Very close second best is to constant voltage charge to 13.7 and this is 
what is mostly done.  How much better the higher voltage is depends on the 
engineer you talk to with some saying it is insignificant and others saying it 
promotes better cell charge balance and faster charge but at the risk of 
overcharge and possible gas generation and venting resulting in moisture loss 
and internal terminal corrosion.

Battery construction is a compromise.  Float and charge/discharge cycles take 
their toll on the plates and to get longer life you need heavier plates but this 
reduces surface area and thus capacity and conductance.  A heavily used battery 
might only last 3 years while one in float standby service can get 5 years of 
life but that is about it.  Longer life batteries are available but they are 
larger for the same capacity.  I have seen batteries guaranteed for 25 year 
service life and have no doubt they can achieve that but those are larger than 
you want to carry with you in a manpack.

Peter


gl4d21a at juno.com wrote:
> I was interested to read the comment about keeping lead acid batteries charged.  I was recently in communication with a manufacturer of commercial equipment about the treatment of the gelled electrolyte batteries in his equipment, and he recommended leaving them on charge no longer than a couple of days at a time.  So, a blanket statement about keeping them charged needs a lot of modifiers, and humming and hawing.  
> 
> A number of years back I designed some equipment for a customer which used gelled electrolyte batteries, and I found a charger manufacturer who could supply a dual rate charger which initially charged at 14.something volts until the current dropped to a preset value, then dropped to a maintenance charge of 13.something volts.  Both voltages need to be temperature compensated, and set to the particular electrolyte chemistry.  So, the comment about a bad idea which went downhill from there about lead acid batteries in electronic equipment is well justified.  Way too many variables to be practical.


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