Battery strings [RE-wrenches]

[John Raynes]

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I second Kurt's comments about isolating individual cells when they are 
cross wired.  I just came across my first replacement job (one which I 
didn't initially install) where the bank was cross-wired.  I could tell by 
looking at currents between cells as to the general location of the cell 
that was starting to short, but I wasn't smart enough to say for sure 
without shutting down the system and disconnecting the cabling, and letting 
the cells find their resting potentials.  On a bank wired with standard 
series strings, I make a few quick measurements and point to the problem 
right away.

This particular system had three strings of L16s.  I couldn't get to the 
battery replacement for about 3 weeks, so we removed one string with the 
shorted cells, and kept him running for the time being at 2/3 regular 
storage capacity.  Everyone was happy (at least as happy as you can be when 
your battery bank fails!) and stress levels minimized everywhere.  This is 
harder to do with a single string system unless its wired with individual 
2-volt cell assemblies.  Limping along temporarily after losing one or two 
2-volt cells in a single string doesn't give me heartburn, but losing 6V 
worth of potential on a 24 or 48 volt system is a pretty large 
percentage.  So we stick with multi-string systems unless we're supplying 2 
volt individual cells.  I'd rather stay at 3 strings or under, but we'll do 
4 if we have to on a 24V system.

Another thing to consider about cross wiring: when the cells are 
paralleled, if one cell shorts, the connected cells stay chronically 
undercharged until the problem is detected and fixed.  If the cells are in 
series strings, the connected cells get chronically overcharged.  I'd 
rather see overcharge in most cases, unless customer maintenance of 
electrolyte levels is not happening as it should.

I've been monitoring and recording string currents in all of the systems 
that we install or maintain, whenever I'm on a site.  I do a test under 
constant load conditions (and under constant charge or eq if conditions 
allow), and see how close the string currents are.  What I'm finding is 
that, with basic good cabling practices, usually the maximum current 
imbalance is about 4-5%.  If there is more imbalance, cables are not 
usually the cause.  I'm watching one bank with about a 15-20% imbalance, 
that's been there from the start, and doesn't get better or worse.  One of 
the cells in one string reads higher than all the rest, and it always has, 
and equalization has no effect.  Something in the battery construction must 
have come from a different lot of material, I guess.  In that case, I'm 
recommending rotating batteries next season, to better distribute the 
overall workload over the lives of the batteries.

Anybody other wrenches monitor and track cell potentials and string 
currents on systems that you maintain?  I'd be curious to trade notes here 
or off line.

John Raynes
RE Solar
Torrey, UT


>One thing to consider with paralleling the individual cells/batteries 
>inside the string is the increased difficulty of isolating a string for 
>maintenance or testing.  We connect individual strings (typically of large 
>AGM telecom cells) through pullout fuses to a central busbar system on 
>which all the load fuses also terminate.  *Every* connection to the 
>central busbar is through a fuse or breaker, allowing for isolation when 
>needed.
>


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