Re: (ET) Replacing main disconnect

["David Roden (Akron OH USA)" <roden ald net>]

Thanks for the links.  I don't mean to sound like a know-it-all, nor to in 
any 
way denigrate anyone's comments here.  However,  I think in these links we 
just have some cases of careless wording.  

More in a moment, but speaking of careless, I don't want to ever suggest 
that 
anyone should be careless with sparks and flames around batteries.  On the 
other hand, there are some people (though probably few or none are on this 
list) who I've noticed have an irrational fear of lead batteries.  Mostly, 
they tend to be auto mechanics and other folks who may be pretty 
knowledgable 
about using lead batteries, but not so well versed in how they work.

I think this attitude is partly caused by widely publicized reports of 
explosions (such as the ones pictured in the third link), and exacerbated 
by 
the dire warnings on the web, attached to jumper cable tags, and so on.  
Batteries become black boxes full of fear.  (Orange alerts? ;-)

True, lead batteries evolve hydrogen gas under some circumstances.  They 
should be treated with respect.  You should wear gloves and face 
protection 
when you work with them, keep flames and smoking materials away, and use 
insulated tools.  

But it's not as if batteries were little Hindenburgs just waiting to 
explode 
in your face.  Here are some facts.

1.  A lead acid battery evolves hydrogen under only one circumstance: when 
it 
is being charged, and the charging voltage exceeds its gassing threshold.

2.  A normally operating lead battery doesn't produce hydrogen when it's  
discharging.  

First, let's look a little more closely at that first statement. 

When electrical energy is supplied to a battery, one or more of the 
following 
things happen to that energy:

        - It drives the charging reaction, storing energy to use later.

        - It produces heat, wasting energy (but sometimes to useful 
effect).

        - It causes gassing, also wasting energy (ditto).

Gassing is caused by disassociation of the water in the electrolyte into 
hydrogen and oxygen (electrolysis).  You did that experiment in high 
school 
science class, right?  Me too.  <g>  Anyway, if the charging voltage is 
kept 
below the gassing threshold, nearly all the energy applied to a battery 
goes 
into driving the charging reaction.  A little goes into heating, and 
practically none into the electrolysis reaction.  

For most new  flooded golf car type batteries, the gassing voltage is 
about 
2.37 volts per cell.  The actual voltage will vary with the battery's 
design 
(in particular the electrolyte's maximum SG and the composition of the 
electrodes), its temperature, and its age.  But if you always hold the 
charging voltage below the gassing threshold, gassing will be negligible.

The wonderful thing about lead batteries is that when they are between 0% 
and 
roughly 80% charged, the charging voltage is, practically speaking, below 
the 
gassing threshold.  This is why you can pour hundreds of amperes into a 
nearly 
flat battery and charge it to about 70% or 80% full in just a few minutes, 
without any gassing.  You need only watch the temperature to make sure it 
doesn't overheat.  It's only in that final 20-30% of the charge that the 
voltage can easily exceed gassing threshold.  

Actually, you can charge a flooded battery all the way to 100% full 
without 
any gassing at all!  Simply adjust the charger to keep on-charge voltage 
below 
the gassing threshold, regardless of state of charge.  So why isn't this 
done 
in our tractors?  Because:

        - The current at end of charge will be extremely low, so it can 
take a long 
time to get from about 70% to 100% charged - anywhere from days to weeks. 

        - Without gassing, there is no electrolyte stirring and only 
diffusion will 
distribute the more concentrated electrolyte throughout each cell (unless 
you 
shake or stir it somehow).  Fortunately, you're not charging very fast 
(see 
above), so there's time for that diffusion to work.

        - Equalization is very difficult to achive without gassing (and 
overcharging) 
at least some cells.

Now, look at that second statement above.  A normally operating lead 
battery 
doesn't produce hydrogen on discharge.  This should be intuitive: the 
battery 
is supplying energy to an external load.  With no energy input, there is 
nothing to drive an electrolysis reaction.

However, there is one case in which I can see how gassing could happen.  
Suppose you have a battery of, say, 6 cells in series.  One cell that has 
a 
lower capacity than the rest.  This might be a result of a manufacturing 
defect, or careless use and/or maintenance.  

When the battery is discharged, this cell reaches 0% SOC first.  If you 
aren't 
aware that one cell is flat, and continue to draw current from the 
battery, 
the current from the  other other cells flows through the flat cell and 
attempts to charge it in reverse!  

The structure of a cell's positive and negative electrodes differs.  When 
you 
try to charge that cell in reverse, its charge efficiency is lower than 
normal.  Gassing voltage in reverse charging might be lower than forward 
gassing voltage.  I'm not absolutely positive that it will, but I can see 
how 
that cell ^might^ gas.  

It's just as likely, maybe more so, that it'll heat up dramatically and 
actually begin to boil its electrolyte.  All batteries, but especially VRR 
(valve regulated recombinant) batteries, have been known to actually 
explode 
from steam pressure when a cell was reversed. At least some battery 
explosions 
ascribed to hydrogen may very well be actually caused by this failure.  
(In 
fact, I've personally reversed and steamed a Hawker Genesis battery in a 
road 
EV.  When a release valve whistles Dixie, better back off on those 
discharge 
amps! <g>)

Cell reversal is obviously a failure scenario.  A battery that's well 
maintained and carefully used shouldn't reach this point, though it 
happens to 
be best of us - and of course it's still a matter of basic safety to keep 
sparks away from a battery.

Another reason for caution is that hydrogen may be present right after the 
end 
of a gassing charge.  I think I've read (but can't find the source now) 
this 
is the most common cause of hydrogen explosions around batteries.  Note 
though 
that hydrogen is light and its molecules small, so it tends to dissipate 
very 
quickly.  If you don't usually open the rear battery box cover when you 
charge 
your ET, you should.  This will prevent excessive accumulation of hydrogen 
in 
the last 20% of your charge.

So (if you've read this far ;-) you're probably thinking, "what's the 
point?"  
The point is that while batteries deserve care and reasonable caution, 
there's 
no need to be afraid of them.  They don't pump out hydrogen every minute 
they're being used.  The circumstances in which they do produce hydrogen 
are 
actually fairly predictable.  You should be careful all the time, and when 
they're apt to be producing hydrogen, you'll know - that's when you should 
be 
EXTRA careful.

Thanks,


David Roden - Akron, Ohio, USA
1991 Solectria Force 144vac
1991 Ford Escort Green/EV 128vdc
1970 GE Elec-trak E15 36vdc
1974 Avco New Idea 36vdc
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