Battery strings [RE-wrenches]

[Ray Walters]

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Hi John;

I look at the different current flows in each string sometimes, 
especially on weirder configurations. I found just what we all 
predicted: newer batteries mixed with older batteries having up to 50% 
differences in current flow, more than 4 strings having large 
differences, systems that pull off one end overworking the closest 
batteries and hardly using the string on the end...
BTW what brand of batteries were those with the cell that just wouldn't 
match the others?

Ray Walters

John Raynes wrote:

>
> 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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