[RE-wrenches] Batteries in parrallel Failure

[Hugh]

At 21:18 -0600 19/4/11, Dan Fink wrote:
>........ 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?

It's a common mistake to put a single switch/breaker that can 
disconnect the battery.  Very bad idea.  If you must have a single 
simple action then make it a double pole switch that isolates the 
loads and the charging sources separately, as two separate circuits. 
You don't want them to end up looking at each other in the absence of 
a battery or it gets expensive very fast.

A nice way to do it is to use a 3-pole fuse carrier operated by just 
one handle and put the wind turbine on one, the inverter on another 
and the diversion load controller on the third one.

But never use a single, simple breaker on a battery because it's just 
much to easy to flip it off before the wind turbine has been braked.

On the parallel battery issue I confess I used multiple strings of 
elderly batteries for most of the 1980s, on numerous sites, without 
individual over-current protection, but I am glad to report we never 
had anything approaching a fire.  Even today it's not unusual to see 
four strings in parallel here on Scoraig.  I actually prefer smaller 
cell sizes - they tend to be easier to buy if you need a replacement 
and it's easier to regroup an aging battery into something useful in 
its final years.

Another approach to parallel strings that has not been mentioned here 
is to put in cross bonds to link the strings at each battery or cell 
terminal.  That way a single cell cannot affect the whole string, it 
just pulls down the cells in parallel with it - which are a lot like 
an extension of the same cell in some cases.
-- 
Hugh Piggott

Scoraig
http://www.scoraigwind.co.uk