AC> On (18 Aug 96) Roy J. Tellason wrote to Alec Cameron...
 AC> And here is my answer to first message, before I read the 
 AC> second:
Ditto.
 AC> On what basis do you decide to retire an aged battery? If you 
 AC> discharge test it before condemning it, what test preparation 
 AC> do you first undertake?
 RJ> I would not get rid of one unless there were a shorted cell,  or a bad
 RJ> connection inside the battery which prevented any power drain (a
 RJ> condition I've encountered more and more frequently over the 
 RJ> years).
 AC> A problem that I come across, is self drainage ie standby loss. 
 AC> The text books used to put this at 1 or 2 % daily. Some cells 
 AC> seem to fade very much faster and I suppose this might be the 
 AC> "mud" at cell bottom bridging the plates. And not all of it 
 AC> easy to see even with transparent cases.
This is true.  I have seen it on one or two occasions,  mostly a rather 
muddy-looking sludge,  but often it's not very visible,  perhaps being caught 
in the separators...
 AC> Highly-regulated *should* mean, that the output is exactly 
 AC> matched to the needs of the battery ie fast recovery when flat.
 RJ> One would hope so.  I wonder how you'd get a charger to determine 
hether
 RJ> a battery was either severely discharged or had a shorted cell,  though?
 AC> I suspect that some advanced chargers in generating stations 
 AC> and submarines, have individual cell monitoring and/or measure 
 AC> or infer the cell internal resistance by assessing dI/dV ie the 
 AC> cell conductance. At a generating station I worked in we had a 
 AC> few single cell chargers, so that in-service diagnosis and 
 AC> correction was possible on individual cells weighing several 
 AC> hundredweight.
Yeah,  but that's not going to help much with what I've got here.  Within 
this small office at the moment are two group 27 deep cycle batteries,  two 
6v/33ah gels,  and four 6v/12ah gels.  No way to test individual cells on 
those except a hydrometer (one of those is handy too),  but I was thinking 
more along the lines of some way for a "smart" charging circuit to be able to 
tell when it was a lost cause...
 RJ> ..................................  I can't quite see the sense of 
eally
 RJ> pushing a battery quite hard.  It would appear that a long,  slow charge
 RJ> would tend to let things happen in there a lot more evenly,  too.
 AC> A hefty charge on a cool battery has two advantages I can think 
 AC> of- rapid return of the battery to its desired state of 
 AC> readiness, and compensation for internal [series] voltage 
 AC> drops. The resistance of internal busses is significant. Some 
 AC> are of copper clad with lead! Not a well known fact. 
That could get interesting if the lead coating were breached,  and may well 
explain why so many of the failures I saw were "bad connections" where you 
could see a full charge on all of the cells with a hydrometer but couldn't 
pull any significant amount of power out of it.  Something inside had gotten 
to a very high level of internal resistance.
 AC> Resistive internals, results in full charge being acquired near 
 AC> the cell terminals, before it "penetrates" in to the larger 
 AC> amounts of more distant plates. Important I think with foil 
 AC> constructed cells eg nicads.
I wonder how much this applies to the stuff I've got here,  though?
 RJ> One of the things that makes it hard to figure some of this stuff is 
hat
 RJ> a lot of the batteries sold for RV/Trolling use aren't rated in
 RJ> amp-hours,  but rather in "reserve minutes",  which is how long 
 RJ> you'd get to run it at a specified temperature (usually 80 F) 
 RJ> and with a specified draw (most often 25 amps).
 AC> The battery specs in OZ are also puzzling, we no longer rate in 
 AC> amp hours or plates per cell. The rated data here is now CCA- 
 AC> Cold Cranking Amps. For your 2 litre car you might use a 250 
 AC> CCA and for your V8 you go for a 500 CCA, and so on. Me, I just 
 AC> go to K-MArt and buy the largest battery that can be fitted in 
 AC> the engine bay.

So do I,  but mostly because you can get the same CCA rating in many 
different sized packages,  but a bigger box is going to have more reserve 
power.  What's a litle surprising when you look at those charts long enough 
is that there are some 4-cylinder motors which require _more_ CCA than the 6 
cylinder or V8 engines in the same car in the same year.  GM was a great one 
for this in the mid- to late eighties,  with many of the four-cylinder cars 
wanting a 630 CCA group 75.
 AC> BUT if you are ripping 5 amps into a 10 amp hour battery then 
 AC> this might strip active material from the plates and also 
 AC> concentrate the electrolyte, both are harmful results.
 RJ> Removing active material from the plates?  Isn't that a normal part of
 RJ> the process,  that some material is going to flake off the 
 RJ> plates over the life of a battery?
 AC> Severe vibration, 
I notice that they sell different batteries for "stationary" applications.
 AC> excessive charging especially of a "full" battery are 
 AC> aggressive in removing active material.
Sounds like overcharging isn't necessarily a good thing,  then.
 RJ> And I don't understand your comment here about concentrating 
 RJ> the electrolyte,  as that's one of the usual things that 
 RJ> happens when you charge a lead-acid battery!
 AC> It's not the concentrated fluid itself that is harmful, but the 
 AC> reason it gets that way ie the electrolysing of the acid into 
 AC> gases. With a diminished volume of [concentrated] fluid, areas 
 AC> of plate are exposed and dried and these are then lost as 
 AC> storage areas.
Oh yeah,  that's definitely a bad thing.  I had some older emergency lights 
given to me while I worked there,  and they had regular "wet" batteries in 
them with a continuous charger in each unit.  The one that I did hook a cord 
up to and try to check out swung the ammeter way over as soon as I plugged it 
in,  but the battery was fairly new and fairly well charged.  One of these 
days I'm going to tear into the electronics in that unit and see what I can 
find out.  (BTW,  these are why I ended up with those 6v/33ah gels.)  The 
original batteries which I took out of those boxes were completely bone dry, 
and the plates were pretty severely warped.
 AC> Deep discharge [standby power] is very wearing and such 
 AC> batteries are not expected to have long lifetimes.
 RJ> Actually,  it's not the discharge that does it but the leaving of a
 RJ> battery in a discharged condition -- when you do so,  you end up with
 RJ> lead sulfate crystals on the positive plates,  and those don't 
 RJ> usually dissolve with a charge put back in.  You've therefore 
 RJ> lost significant plate area.
 AC> I think you have missed something. Yes, a prolonged neglect of 
 AC> a discharge battery will damage it irrespective of whether it 
 AC> is designed for "floating" fully charged, or for regular full 
 AC> discharge as with a wheelchair or golf cart.
 AC> But if you try and use an auto starting battery in golf cart 
 AC> service with daily full charge/ recharge, you will very soon 
 AC> retire it as it has lost storage ability due to the shedding of 
 AC> active material, and some corrosion of internal structure. 
I know that the internal structures of the two types are *very* different,  
with the one being optimized for cranking and the other optimized for longer 
discharge periods at a much lower rate.
 AC> I don't know how significant the corrosion is, but I have sure 
 AC> seen some very sick post mortems! With daily charge/ full 
 AC> discharge there is no chance for sulphation to occur.
True.  I don't think that sulfation is the culprit in those instances.
 RJ> This is why letting a battery sit around for a real long time and then
 RJ> charging it up doesn't usually work,  you end up with a charge but no
 RJ> real power behind it for starting applications.
 AC> A text book I read, not recent, said that for a profoundly 
 AC> sulphated battery you should drain and save the electrolyte, 
 AC> refill with water, and "charge" at about the 40 hr rate for 
 AC> several weeks then replace the original fluid. 
Several weeks?!  This is probably where the stories I've heard about "fresh 
acid" come from...      
 AC> It is a long time since I did that [30yrs] it seemed valid BUT 
 AC> I suspect that the main improvement was due to the draining of 
 AC> the sludge which had previously been "shorting" the plates at 
 AC> cell bottom. 
That too,  assuming that there is enough room to get it out,  or that you 
rinse it extensively.  They sure don't seem to be designed to flush out.
 AC> You will have noticed that there is a helluva gap below the 
 AC> plates, before cell bottom.
Yep.  And that's why it's there...
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Same here.
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