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<DIV><FONT face=Arial size=2>Darryl and Wrenches,<BR><BR>I wonder about the
suggestion of using a Class T fuse. Here's why: Class T's are extremely fast on
a dead short but is that what you have during a thermal runaway event? Class T's
are also capable of many times their rated short circuit current for brief
periods of time. For instance, a 300 amp Class T can handle 750 amps for 3.3
minutes. At the extreme, it can handle nearly 2000 amps for 500ms before
opening. As current begins to flow from the other batteries to feed the runaway,
could the currents cause a fire or other harm before a Class T opens? It seems
to me that this situation would call for much faster overcurrent
protection. I'm open to be re-educated here if this thinking is
wrong.<BR><BR>I realize that the subject was concerning safety of parallel
battery connections but I can't resist mentioning the undesirable design aspect
of 8 parallel battery strings. ( I know Darryl, not your design) Since this
forum can be viewed by anyone, I would like to point out that many parallel
battery strings in a deep cycle application will lead to premature battery
failure. It will take dozens of hours of testing and corrective maintenance to
keep that bank healthy. </FONT><FONT face=Arial size=2>Current will always take
the path of least resistance leaving many cells undercharged every day. At the
same time, some strings may be overcharged which may cause thermal runaway (AGM
or GEL) or early failure. Even two parallel strings require special
attention which is why I advocate using large capacity batteries rather than
multiple strings. My design rule: no more than two strings in
parallel.<BR><BR>Larry Crutcher<BR>Starlight Solar Power
Systems<BR>www.starlightsolar.com<BR>928-342-9103<BR><BR>----- Original Message
----- <BR>From: Darryl Thayer <BR>To: RE-wrenches <BR>Sent: Tuesday, April 19,
2011 7:29 PM<BR>Subject: [RE-wrenches] Batteries in parrallel
Failure<BR><BR><BR>Batteries in parallel, How to protect from catastrophic
failure.<BR>I just got contacted to commission a solar system off grid not of my
design. Eight battery strings in parallel, AGM batteries, 24 volts @
120 AH each string (two 12 volt in series) New system, new batteries,
Silent POwer Inverter. Two kW DC array. generator to charge through
Silent power. (present installer is not sure how to do final set up and
testing) <BR><BR>I am most concerned about 8 batteries in parallel,
(especially since last week a set of AGM burned up having 5 in parallel Owner
had no way to break the paralleling of the batteries, she could only stop the
chargeing) What I am proposing for your review is requireing the
installtion of two combiner boxes Midnight solar with a breaker in each
battery string, using about 3' of #10 to connect each battery string to the
combiner box. This way if a battery failed and the paralled strings were
to "dump" into the failed battery it would trip the breaker. I was
thinking of using a 30 amp breaker in each string. The main breaker is set at
240 amps. <BR><BR>Ideas Please? </FONT></DIV></BODY></HTML>