[RE-wrenches] Strings and series of batteries with reverse return linkup

Hugh hugh at scoraigwind.co.uk
Fri Dec 4 09:59:12 PST 2009 

Hi Kent,

I am sorry but I disagree with your conclusion that it only keeps the 
wire resistances the same for 2 strings.  This diagonal system, which 
I believe many people would call 'reverse return' in the states, 
actually provides for exactly equal resistance on all paths to all 
strings.  Each time you add a link in the positive path you remove 
one in the negative path.  I admit that the voltage drops will be 
very slightly different due to different currents in the various 
links but the differences now are so tiny that I would not worry 
about them.

I agree that you will get imbalances in multiple battery strings at 
times - usually during periods of prolonged low winds.  But you 
cannot persuade me that multiple strings 'don't work right' because 
they are used in most of the systems I work with since the 1970s.  I 
see just as much sulphation in large capacity cells as I do in small 
capacity cells (in multiple parallel strings).  I would say that 
going from 12-volts on up to 48-volt systems has caused me a lot more 
headaches with unequal battery states than paralleling a lot of 
batteries has.  Batteries connected in parallel will tend to take 
what they need.  Connected in series they take what they are given. 
Also, a cell failure in a 12-volt system attracts attention at once, 
whereas a cell failure in a 48-volt system can be overlooked for much 
longer.

There are plenty of points of view about batteries :-)  Part of it is 
the difference in mentality between solar users (who count out the 
amp hours and know exactly what they have to work with each day) and 
wind users (who can party like mad sometimes and then have nothing at 
other times).

Hugh

>Hugh,
>
>Feeding off diagonal corners, as you are describing, only keeps the 
>wire resistance of the parallel paths balanced when there are two 
>parallel strings of batteries and the connections between the 
>batteries are all the same length (or at least they add up to the 
>same total length).  With three or more parallel paths, the 
>resistance of the wire is going to cause imbalanced current between 
>the parallel paths. 
>
>But the internal resistance of the batteries is important too. 
>Unfortunately even with bus bars and equal length wires, imbalances 
>between the parallel paths will develop because of the batteries' 
>internal resistance.  Consider what happens when a battery gets 
>sulfated: its voltage goes up quickly while charging and it goes 
>down quickly while discharging -- in other words: its internal 
>resistance goes up.  Despite our best attempts to keep the wire 
>resistance balanced, some cell will sulfate more quickly than the 
>others, then it is a down hill spiral with the current balance 
>getting worse and worse. 
>
>With paralleled batteries keeping them equalized becomes critical 
>and the more parallel paths there are - the harder it is to get them 
>all equalized.
>
>Kent Osterberg
>Blue Mountain Solar
><http://www.bluemountainsolar.com/>www.bluemountainsolar.com
>
>
>
>Hugh wrote:
>
>>Tom,
>>
>>Sorry, I failed to write clearly. I was not talking about bus bars 
>>in this sentence.  I meant to say that I 'daisy chain' links from 
>>one string to the next.  I parallel them by connecting A to B, B to 
>>C, C to D.  Then I connect positive of A to the inverter, and 
>>negative of D to the inverter.  In this way the connections are all 
>>balanced without the use of a bus bar.  You can connect the 
>>positive of D to the wind turbine (and the negative of A) to reduce 
>>the danger of a direct connection without batteries.
>>
>>If a battery is well charged I see no problem with parallel 
>>strings. As it gets older then you do need to monitor the 
>>individual cells/units for failures and take action.  Standard 
>>smaller sized units are easier to re-group, and if desired to 
>>replace.
>>
>>Tom wrote:
>>
>>>"If you connect links  between all of the positives, and links 
>>>between all of the negatives,
>>>to parallel the battery, then the only issue is where you connect 
>>>the charge source and/or inverter."
>>>
>>>Hugh,
>>>
>>>The connection is very easy, if you look at the photos on the page 
>>>I linked to the connections to the inverter breaker (which is 
>>>where I also connected the charge source) come off the bus bars. 
>>>It's actually a very simple installation and I found it much 
>>>easier and safer to work on that a large series/parallel bank of 
>>>L-16s, which was what my first bank in that system consisted off 
>>>and it turned out to be a mess (designed and the components built 
>>>by Jade Mountain...)
>>>
>>>Tom
>>>
>>>
>>>
>>>--------------------------------------------------
>>>From: "Hugh" <mailto:hugh at scoraigwind.co.uk><hugh at scoraigwind.co.uk>
>>>Sent: Thursday, December 03, 2009 8:30 AM
>>>To: "RE-wrenches" 
>>><mailto:re-wrenches at lists.re-wrenches.org><re-wrenches at lists.re-wrenches.org>
>>>Subject: Re: [RE-wrenches] Strings and series of batteries
>>>
>>>>Just for a contrasting opinion:
>>>>
>>>>I rarely use grease or vaseline unless it proves necessary.  I have
>>>>changed out battery banks after seven years and the connectors are
>>>>still pristine whilst dry.  I agree that sometimes there will be
>>>>localised corrosion on battery terminals.  I find it rarely, and I
>>>>deal with it as and when.  I might use vaseline then.  Or spray on
>>>>some oil.  Not a big deal to be honest.  The contact resistance is
>>>>not affected by corrosion on the outside.  Probably due to some
>>>>moisture arriving on the terminal?  But vaseline/grease attracts
>>>>dust, makes a big mess and mess is not nice around batteries.
>>>>
>>>>Bus bars sound great, but what a hassle.  If you connect links
>>>>between all of the positives, and links between all of the negatives,
>>>>to parallel the battery, then the only issue is where you connect the
>>>>charge source and/or inverter.  I have learned to connect them at
>>>>opposite ends of the parallel links.  'Diagonally opposite' you could
>>>>call it.  In this way the resistance is identical to each string.
>>>>One string has a direct connection to plus.  One has a direct
>>>>connection to minus.  All have equal number of links between them and
>>>>the inverter.  If there is a wind turbine then I will often connect
>>>>this to two corners and the inverter to the opposite two corners,
>>>>making it very difficult to (both) disconnect the battery and (also)
>>>>leave the wind turbine connected to the inverter.  This probably has
>>>>saved a few inverters from instant death due to carelessness.
>>>>--
>>>>Hugh Piggott
>>>>
>>>>Scoraig Wind Electric
>>>>Scotland
>>>><http://www.scoraigwind.co.uk>http://www.scoraigwind.co.uk
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>
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-- 
Hugh Piggott

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