At least with a breaker arrangement you could charge different strings separately which would help an otherwise bad situation. Also, I think the CB breakers would be adequate in this case. They have a 5000 AIC (too low for a HUP) but my specs on an AGM battery are showing 2300A short circuit.
R. Walters r...@solarray.com Solar Engineer On Apr 20, 2011, at 5:12 PM, Starlight Solar, Larry Crutcher wrote: > Darryl and Wrenches, > > 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. > > 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. 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. > > Larry Crutcher > Starlight Solar Power Systems > www.starlightsolar.com > 928-342-9103 > > ----- Original Message ----- > From: Darryl Thayer > To: RE-wrenches > Sent: Tuesday, April 19, 2011 7:29 PM > Subject: [RE-wrenches] Batteries in parrallel Failure > > > Batteries in parallel, How to protect from catastrophic failure. > 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) > > 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. > > Ideas Please? > _______________________________________________ > List sponsored by Home Power magazine > > List Address: RE-wrenches@lists.re-wrenches.org > > Options & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > List-Archive: > http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out participant bios: > www.members.re-wrenches.org >
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