[RE-wrenches] Batteries in parrallel Failure

[Dan Fink]

We almost had a house fire in the neighborhood a couple months ago from 
a parallel battery bank failure....an aging 2x8 L16 48v battery bank (16 
of them), and one bat in bank one got a short and thermal runaway. I 
teach in my firefighter safety classes that thermal runaway is really 
rare, but it sure happened here fast. How rare is it? Smoke was pouring 
out of the bat box. Good thing the homeowner was there. He instinctively 
hit the main DC buss bar breakers, which was logical but didn't of 
course help the fire situation.....and the result was that all Outback 
electronics from charge controllers to the 2 inverter set being fried 
from too high DC voltage coming in the wrong end.

Was something designed or wired wrong here? What is a high-voltage (up 
to 150 VDC)  off-grid MPPT PV system supposed to do when the battery 
bank effectively disappears instantly from the system?

I didn't design or install the system, but I want to know if I have 
missed something obvious here.

Wind turbines have had the same issue here  -- better hit the turbine 
shutdown switch before disconnecting the battery bank, or you'll 
backfeed high Voc and fry some expensive electronics.


All feedback appreciated!

Dan Fink
Executive Director;
Buckville Energy Consulting
Buckville Publications LLC
NABCEP / IREC / ISPQ accredited Continuing Education Providers
http://www.buckville.com/
info at buckville.com
970.672.4342 (voicemail)
970.373.1311 (fax)



Darryl Thayer wrote:
> 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? 
> 
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