Battery strings [RE-wrenches]

[Hugh Piggott]

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I come from a background of working with systems that normally have 
multiple parallel battery strings. In many cases they are wired by 
the owners, and there is no attempt made to equalise the resistance 
using bus bars or diagonal corner connection of take-offs.  I agree 
that equalisation of connection resistance is good practice, but I do 
feel that the advantages are over-stated.

It is true that eventually/sometimes cells will fail, and that it is 
useful to locate these failed cells when it becomes apparent that the 
battery as a whole is not performing well.  In multi-string batteries 
the failure (short circuit or rapid loss of charge) will often lead 
to overcharging of the other cells in the string (and rob the other 
strings of charge).  Either that or the cell goes open circuit.  But 
I see no particular reason why the failure rate should be much higher 
with paralleled systems than with a single string of big cells.  My 
gut feeling is that with more smaller cells you will get more smaller 
failures.  Generally the battery thrives if it is well charged, and 
the failures become more likely if the battery is chronically low. 
This rule applies regardless of wiring.  There can also be big 
variations in manufacturing quality.

A failure in a single string of big cells is a very big problem.  The 
correct size of cell is probably hard to source, and very expensive 
to replace.  Without it the system is crippled.  Failures in a 
multi-string battery are relatively easy to fix.  Isolate the failed 
string.  Either buy a replacement cell or battery (which will be a 
widely available type) or just break the string up, and use the good 
parts to fix future failed strings.

I tend to think that there is too much prejudice against multi-string 
batteries.  If they are properly charged then I see no reason why 
they should fail sooner than a single string of big cells.  Just to 
cement my position as a complete battery nincompoop I have been 
paralleling completely different battery types on my own system over 
the last few years - tubular plate tractions alongside pasted plate 
standby cells.  Both are expensive types of battery that I have 
acquired on the cheap.  It works very well.  I do have occasional 
failures of cells after long periods of calm weather, but no more 
than in the past.  In the case of the traction cells (which were a 
bad batch from the manufacturer) I have learned to take the offending 
cell, shake it briskly for a while to stir up the electrolyte, and 
put it in series with the wind turbine (outside the rest of the 
battery) so that it gets a stiff fifty amps for a while (hot and 
frothy), shake it again, leave it to think for a while and then it is 
ready to use again.

I wonder if part of the reason why people think that single strings 
are so great is because only the best quality batteries are available 
(at a price) in this format.  So they therefore have less failures 
presumably.  My feeling is that multiple strings of batteries, even 
of different ages, and even of different types, can work well in a 
regime where there is a good overcharge from time to time, as is 
normally the case with a well sited windpower system (so long as the 
bulk voltage setting is high enough).  Of course my approach is 
coloured by the fact that this is a much cheaper approach and much 
easier to fix when things go wrong.  But I do think it is a shame 
that most punters will be frightened to do anything like that, 
because all of the experts (I am not a battery expert - I just use 
them and curse them) agree that you must never parallel more than 
about two strings of totally identical batteries or the sky will fall 
in.

Parallel strings work for me.

PS screw in anchors are too expensive, and tricky to install without 
expensive equipment.  It's cheaper and more reliable to get a local 
to dig a big hole, and put some heavy scrap metal down there 
connected by a ludicrously heavy chain.
-- 
Hugh

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


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