On Sat, 20 Aug 2005 16:12:46 -0400, Larry wrote:

(Mic) wrote in :

The only caveat to large banks of batteries is proper internal fusing.
If a cell shorts out in a battery, the battery voltage will drop
approximately 2 Volts. All batteries in the bank will start
discharging into the shorted battery, unless fuses take the bad
battery out of the circuit. Thus, battery banks need to be fused
internally as well as externally."


If a cell shorts out in a battery, the cell explodes boiling its
electrolyte into steam, damn near instantly, unless it's already dead. The
other cells in SERIES with the dead cell have no current through it. The
other batteries may explode if not properly fused, even if the other good
cells in the blown battery oppose them, overcharging like mad in the
process. All batteries MUST BE FUSED! I like about 150% of the starter
current and the wiring must be able to handle that current level to blow
the fuse....not the crap house wiring I see on house batteries all the
time.

There's no way I know of to keep the shorted cell from exploding,
distributing its acid over everthing in every drawer in the boat....what a
mess.

I had a cell in one of two parallelled 4D batteries develop a short,
without exploding, or any other serious consequences.

I noticed while cruising that I was unable to get my batteries above
13 volts or so, despite using a 40 amp charger for 24 hours - but I
was still able to start the engine and run all accessories.

After the cruise, I was checking battery water, and found that one
battery required lots of water in all but one cell, while the other
didn't need any. Switching to the thirsty battery only, I was unable
to start the engine, and the voltage on that battery was only 10 volts
or so, while the other was over 12.

I expect that the "shorted" cell was really a low resistance, rather
than a zero-ohm short.

I've since replaced the 4Ds with 4 Golf Carts as a house bank, and a
Group 27 for starting. I don't have fuses on the two series pairs of
GCs, but do have a 1-both-2 switch so I can easily test each series
pair independently.

A problem I see with fusing sections of a battery bank is selecting a
fuse rating high enough that it won't blow when starting, or running
the bow thruster, but low enough that it will blow on a fault.



--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
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Peter Bennett wrote in
news.com:

I expect that the "shorted" cell was really a low resistance, rather
than a zero-ohm short.


It wasn't a shorted cell. It was a "dead cell". That cell had converted
lead and acid into lead sulphate crystals, so had a low gravity situation,
causing the ongoing charging problem because its voltage wouldn't come up.
These cells have quite low resistance, that's true. Lead sulphate is a
good conductor dissolved in water. What's missing is the acid. The other
cells simply forced current through the bad cell, attempting to charge it
backwards in the process, as you used it. It won't explode.

A normal battery cell that develops a short, where the plates actually
touch, is different from a dead cell. A thousand amps boils the
electrolyte as the acid-lead chemical reaction goes crazy. Acid steam
results and blows the cell, and the case, apart. Other cells may become
shorted from the impending distortion of the shorted cell pressing them
from the middle of the battery case. It isn't pretty....but it's quick!

--
Larry