I had one cell in a series string of two golf cart batteries fail:
http://compusmiths.com/~w_smith/DailyBatteryVoltage.htm but the
voltage, when it dropped, took almost 11 hours to drop from 12.5 to
10.5 volts, and even the 'quick' drop at
http://compusmiths.com/~w_smith/CellShortCloseup.htm took 45 minutes
(though Excel doesn't seem to want to show the chart that way...).
Maybe having cells in parallel allows more current to dump into a
short, or maybe I had a different failure mode...
Failure mode of a 12v battery system.
I will give an idea of the events that would
accrue in the failure of a series 12V (6 cells) system,
And a series parallel 12V system consisting of
10 batteries (of 6 cells each) in parallel..
Series system.
Stage one.
One cell shorts,
The total energy capacity of that cell will be turned to heat in that cell.
Other cells will not be affected, besides moderate heating of adjacent cells
for a short period of time.
Total battery voltage under load is 8.3V to 10V
When the charger brings it up to 13V float voltage,
You will be giving the working cells a good EQ charge.
Operator may not notice, but may think the system is running low all the
time.
Automatic low state of charge cut outs may shut down the system at night.
Most systems that don't use low state of charge protection will operate
close to normal.
Constant EQ charge levels may reduce life of working cells,
and promote shorting of cells.
Stage two.
Second cell shorts.
Any automatic low state of charge cut outs would render the system unusable
at night.
The remaining cells would receive a heavy EQ charge each day the sun shines.
On systems that don't use a low SOC cutout,
only voltage variance tolerant electronic equipment will remain functioning,
Incandescent lights will be noticeably dimmer.
The charging equipment will work hard to get it up to 13V, if possible.
The battery size is considerable compared to available charging current,
so there is not enough charge current to promote thermal runaway.
Life of remaining cells will be short from the heavy charging during the
day.
Stage three.
Half of the cells are shorted.
Charging system will not be able to bring voltage to regulation levels.
Only the most tolerant electronic equipment will work at night.
Incandescent will be dim.
If there is an operator present, he should have found the problem by then.
Most smart charging systems will know there is a problem and go into an
error state.
Stage four.
Two cells left.
If the site has any contact with the outside world, they should know there
is a problem.
Remote radio sites will be obvious from the system not working.
Stage five.
One cell working.
Only small things like 12V clocks that can work down to 2V or so, will
remain working.
Stage six.
Complete short.
Panels will be dumping into a short, nothing works.
For a series parallel system.
Stage one.
One cell on a battery in the system shorts.
The problem battery will be in a quasi float charge in comparison to the
rest of the system.
During the day, it will get a good EQ charge.
During the night, it will deliver no useable capacity.
It will be a leach on the system.
Only the most picky and finicky operator will notice.
He will have to take a SG check on the shorted cell to know there is a
problem.
Most people with sealed batteries will be completely un aware of any fault
condition.
They will just think that the battery bank it getting old, and loosing
capacity.
Some may find out by pulling strings and doing voltage checks during normal
maintenance.
Some may find out by the problem battery gassing abnormally during EQ
charges on the system.
Some may find out by doing a simple "temperature by hand", check after an EQ
charge.
Some may notice by the abnormal water consumption of the problem battery.
Constant overcharging will promote shorting in the other cells of the
affected aging battery.
Stage two.
The second cell shorts in the problem battery.
The battery will become a moderate load on the system.
If it is a system of 10 or more batteries, and the dally system usage is
high, many operators will not notice.
The battery will be noticeably hotter than the rest of the units all the
time.
The battery will probably not go into thermal runaway unless the ambient
temp is high.
Gassing will be noticeable during most of the normal system cycle.
Thermal runaway may be triggered by an EQ charge.
Life of the existing cells will be short.
If it stays in that condition, and no more cells short, the battery will
safely boil dry.
Shorting of other cells is highly possible by the sediments being stirred up
in the battery by charging.
Stage three.
Half the cells are shorted.
You have just started down the one way track to Chernobyl.
Power consumption off the rest of the battery bank, will be large
Thermal runaway is guarantied.
Battery case may soften from the heat generated.
Boiling will obvious.
Remaining cells will either boil dry, or short, or boil dry and short, in
quick order.
Most interbank fuses will blow at this time.
Stage four,
Two cells left.
Any Interbank fuse or fuseable links will melt at this time.
You will have gas venting out the top of the battery like a steam pot.
Case of the battery will start melting which will cause the last two cells
to short.
The already shorted cells will boil any remaining fluid off,
and the hot plates will start melting through the plastic case.
Stage five.
One cell left, and it won't last long.
Stage six.
A complete short.
If there is any sizable charge left in the rest of the bank.
Plastic of the problem battery will probably ignite from the molten lead.
The insulation on the battery interconnects will melt off, and any remaining
insulation will ignite.
Cables will arc cut through any metal enclosure like a blow torch through
hot butter.
Any plastic conduit will burst into flame on contact with red hot
conductors.
Any wood will ignite on contact with conductors.
Flames will encompass most of the area until, all consumable materials
inside or outside a solid metal enclosure is consumed, or the enclosure
burns down (the owners house), or the fire department puts out the fire.
