Battery Bussing [RE-wrenches]

[Windy Dankoff, Dankoff Solar]

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>Consider this. A battery bank has 8 each T-105 batteries from the same
>production batch with the same specific gravity .....      .........
>......   under what conditions will interconnect current flow differ?


This is an aside, related to the discussion.

Many of us have pondered over the years of observing premature cell 
failures in multi-string battery banks. No matter how well you 
attempt to establish symmetry between two or more parallal strings, 
you should expect a greater chance of premature cell failure than a 
single-string battery bank.

One of my old customers has batteries in an insulated box outdoors. 
They were a 5-parallel string at 24V. Three times in 14 years, the 
batteries located on the outside of the pack failed first. I had to 
conclude that it was temperature differences that were the culprit. 
The outer units were less temp-stable than the inner ones. I've heard 
of other cases where inner cells fail first because they run hotter. 
(One lesson is to leave space between batteries for air circulation.)

A more common culprit is terminal corrosion. A slight added 
resistance in one string, from one or more slightly corroded 
contacts, will cause less current to flow through that entire string 
and thus more to flow in the other(s). Yet, the charge controller 
responds to only one voltage reading, being the net of the parallel 
bank. This can result in uneven states of charge and failure of one 
string to finish-charge, especially with an RE charging source.

Big cells/single strings are the only battery banks that I believe 
are likely to live to their full potential, but we have to do what we 
can with customers and their budgets.  Use your favorite goop 
(vaseline or whatever) on terminal parts BEFORE assembly. An annual 
maintenance check should include comparing current between battery 
strings under a high-current charge or discharge condition, using a 
clamp-on ammeter.

Windy

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