Re: (ET) One cell frozen in the whole pack

["David Roden" <etpost drmm net>]

I think Darryl nailed it.  That one cell discharged itself, and that's why 
it froze.  

Sulfuric acid is a fine antifreeze (for batteries).  A fully charged cell 
won't freeze until -68 deg C. But in a flat cell, the electrolyte is 
pretty 
close to water.  At 20% SOC, the cell will freeze at -7C (19F).  Where I 
live, lately it's been that cold in the DAYTIME, let alone at night.

Here comes some battery blab. If you already know this stuff, or if you 
don't care, just hit DELETE now.

First, freezing is really, really bad for a battery.  The expanding ice 
can 
rupture the battery case.  That you can see, but even if the case doesn't 
break, there's invisible internal damage from stuff being pushed apart 
when 
it isn't designed to be. A battery that's frozen is usually pretty much 
junk.

So you want to keep an eye out of cells and batteries with high self 
discharge.  

Note that some self-discharge is normal.  At 75 deg F, a flooded golf car 
battery will lose half its charge in 4 to 6 months.  The warmer it is, the 
faster it goes.  And vice versa: a battery at 40 deg F will take close to 
a 
year to lose half its charge (which is one more reason I think that 
battery 
maintainers do more harm than good, especially in batteries stored for the 
winter).

High capacity batteries with more active material and stronger electrolyte 
self-discharge faster.  On the other hand, gel and AGM batteries tend to 
have lower self discharge, maybe in part because they usually aren't as 
aggressive about capacity.

What you want to watch out for is a cell or battery with higher than 
normal 
self discharge, where that 4-6 months is more like 4-6 weeks or 4-6 days.  

When the battery's charge starts to wind down and the on-load voltage 
falls, 
check the voltage of each 6v block, preferably with a load applied.  The 
one 
with the bum cell will probably measure about 4 volts instead of 6.

The surer way is with a hydrometer.  Give the battery a normal charge and 
measure the SG.  Come back in a day, then in a week.  The unhappy cell 
will 
be falling in SG while the others are stable.

The main danger with a high-self-discharge cell is that even if it still 
has 
a charge when you're ready to use the battery, it's proably not a full 
charge.  The longer it's been off the charger, the worse this is.  It will 
always go flat before the rest of the cells in the battery.  

This is bad news, because the flat cell will be reversed (charged 
backward) 
by the current through it when you use the battery.   And reversal, good 
gentlefolk, is usually death to a cell.

It can also be unhealthy for you, the user, at least at high currents.  
That 
reversed cell will get HOT and is apt to boil.  (My friend Bob Rice used 
to 
call these "Trojan teakettles.") This can cause that rotten egg odor you 
described.  If for some reason the boiling cell can't vent its steam, the 
battery can explode.  That would sure feel interesting, and not in a good 
way, if it happened in the battery box under your seat, no?

There are lots of reasons for high battery self-discharge, but I know of 
two 
that are fairly common.  

One is what I call mud, the somewhat-conductive gunk that builds up in the 
bottom of the battery.  This is mostly lead sulfate.  

The battery sickness called sulfation will obviously cause this, but more 
often it's just age.  

What we call sulfation is from chronic undercharging or waiting too long 
to 
charge.  It ages a battery like crack ages a person. But just like you age 
even when you take good care of yourself, your battery gradually builds up 
mud (lead sulfate precipitate) no matter how good you are to it.

Therefore batteries are made with extra space below the grids so this mud 
can precipitate out, building up in the bottom of the battery without 
causing problems.  Eventually, though, it gets to where this conductive 
mud 
touches the bottoms of the grids.  That's when self discharge increases.  

Curiously, that point arrives sooner with high capacity batteries, because 
they have bigger grids and thus less space for the precipitate.

The snake oil and blinky gadget salesmen would have you believe that every 
low-capacity battery problem is caused by sulfation, and that their potion 
or black box will fix it.  

Neither of these is true.  First, more batteries die from overcharging 
than 
from undercharging.  Second, sulfation is not reversible.  Active material 
that has flaked off the grids can't be put back on the grids, full stop.

So.  Back to high self discharge.  One cause is a buildup of lead sulfate 
in 
the bottoms of the cells, either from normal aging or from undercharging.

Well, guess what, overcharging can also lead to self discharge.  It's one 
of 
the causes of mossing, which is a deposit of spongy lead on the edges of 
the 
grids.  (Other causes of mossing are cheap or sloppy battery 
manufacturing, 
and just plain aging.)

Mossing isn't fixable either.  

It IS possible to get some more use out of a battery that has one or more 
high-self-discharge cells, but only if the discharge isn't too radical, 
and 
only if you catch the problem before it reaches a crisis (as it did this 
time).  The trick is to keep the battery on float charge when you're not 
using it.  That makes up for some excess self discharge.  

The main problem with this "fix" is that float charging overcharges the 
good 
cells.  That ages them faster, possibly even driving them into high self 
discharge too.  So, while it can buy you some time, this is sort of like 
life support for a dying battery.   (It also wastes energy.)

As all this implies, high self discharge is pretty common in older 
batteries, not so much with new or young ones.  I've had it happen only 
once 
(that I noticed), with a nearly-brand-new US Battery 8-volter. The dealer 
replaced it under warranty.

So, getting to your case (finally), if your batteries are old, I'd say 
just 
write off the stinker and replace it.  It wasn't good for much before it 
froze, and it's probably good for nothing now.  

If your batteries are still in warranty, I suggest a friendly talk with 
your 
battery dealer.


David Roden - Akron, Ohio, USA

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