[RE-wrenches] 24 volt Battery Bank comparison, Why a single string?

[boB Gudgel]

On 6/1/2011 8:41 AM, Larry Crutcher, Starlight Solar Power Systems wrote:
> Hi boB,
>
> We know that current will flow through the lowest resistive path. In a 
> perfect world, the resistance from end to end of a battery string will 
> be identical. Real world conditions show that one or more cells have a 
> slightly higher internal resistance. Add to that the imperfect 
> terminal crimp, improper torque at battery terminal, and perhaps 
> terminal corrosion and we now have paths of differing resistance. The 
> parallel strings have an unequal distribution of current and ergo 
> undercharged cells. These cells retain a small amount of amorphous 
> lead sulphate that, without correction, will begin to crystalize. We 
> all know what will happen over time.
>

The issue of maintenance is a not brainer of course...  i.e. the need to 
add water...

Imperfect crimps would be a problem with a single string of batteries 
too.  If the resistance
is higher in that cell in the one string, then yes, that one string will 
not be working nearly as
hard as the other strings.  Bad workmanship is not an excuse for not 
using multiple strings
as far as I know.


If one string is discharged more than another parallel string, then its 
finishing (ending) Amps
will will certainly happen later than a string that wasn't discharged as 
deep, but if using a
timed Absorb cycle, why does this matter ??   They're all receiving the 
same voltage and
should be treated the same in that case.  Right ??    At least for timed 
charging cycles.

As Darryl pointed out, AGMs like to get hot when unbalanced and this is 
definitely what I
have seen.   But that again can happen for a single string rather than 
multiple strings.

One thing that hasn't been mentioned, unless I missed it, is the 
possibility that a shorted
cell in one string can drag down the other strings it is in parallel 
with.   The more strings
in parallel, statistically, there would be more of a chance for that to 
happen.

That would be more of a self discharge issue I would think which isn't good.
boB




> Granted, this can be prevented by the proper knowledge and attention 
> but I see far too many failures from neglect and/or lack of 
> understanding. My experience comes from 10 years of working with 
> hundreds of small 12 volt systems with 2 to 6 parallel strings of 
> batteries. In our shop we often see 2 to 3 year old battery banks that 
> are damaged and I believe this is a contributing factor. It is often 
> too late by the time the problem is discovered. So, my opinion is to 
> reduce this possibility by decreasing the parallel paths whenever 
> possible.
>
> More strings of a given size battery means less stress per string. 
> But, if you decrease strings you are normally increasing cell size to 
> maintain the capacity so this should not be an issue.
>
> Battery balancing is a great idea. I would like to see an active 
> system for shunting current on a string to keep an equal current 
> across all strings. I know this is done in the EV business.
>
> Larry Crutcher
> Starlight Solar Power Systems
> 11871 S Fortuna Road, #210
> Yuma, AZ 85367
>
> larry at starlightsolar.com <mailto:larry at starlightsolar.com>
> (928) 342-9103
>
>
>
> On May 31, 2011, at 11:16 PM, boB Gudgel wrote:
>
>>
>>
>> OK, I've been hearing for years now that one battery string is best 
>> and maybe two but no more should be used.
>>
>> What is the reason for this fear ???    What is the experience with 
>> multiple strings in parallel ??
>>
>> If it is because of current sharing of the strings, that shouldn't be 
>> an insurmountable problem if the wiring is done right.
>>
>> If it has to do with the possibility of fire, which Darryl mentioned, 
>> seems that series string fuses could mitigate the possibility of that 
>> issue, if because of shorted cells, just like PV combiners.
>>
>> Or maybe it has to do with the trouble of getting into the middle of 
>> a pile of batteries ?  Use battery boxes ?
>>
>> The strings are in parallel and so they all get the same voltage, 
>> theoretically anyway if the connections are good.
>>
>> I would think that the more strings, the LESS stress on individual 
>> batteries, not more.  The current in each battery should be less than 
>> it would be I would think.
>>
>> So what is the real experience ??
>>
>> Is a battery balancer the answer ??
>>
>> Thanks,
>> boB
>
>
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