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
