[RE-wrenches] 24 volt Battery Bank comparison

Wed Jun 1 03:06:27 PDT 2011

Hi boB, 

I would say maintenance is a large issue - especially when dealing with L-16's or the like, as the number of cells & connections gets large, i.e. 1000-1200AH @ 48VDC is three (3) strings, 24 batteries and 72 cells.  Will a client water 24 cells...maybe - 72?? 

However, from Tech Support days, imbalance is the larger problem.  It seems to take 18 months - 3 years and then you would get SG readings where one (1) bank was perfect and the other(s) is significantly lower......which is hard to overcome without effort (either break the banks & charge independently or hammer the banks and allow the high string to get even higher).  

One of the key take aways that often gets overlooked is the proper initial activation of the strings.  It is impossible for all cells to have identical resting voltage.  During commissioning, each string should be charged to 2.45-2.5VPC and held until all cells read the same voltage.  If running multiple strings, then connect the strings and run the entire bank at 2.45-2.5VPC until all cells measure the same voltage.  Now the bank is well balanced and the chances of a major imbalance are greatly reduced.  However, even after this effort, if you use an amp clamp, you will notice slight differences in the amount of current being accepted by each bank. 

Best, 

Jamie 

>>> boB Gudgel <boB at midnitesolar.com> 6/1/2011 5:00 AM >>>
On 5/31/2011 11:25 PM, Tom Elliot wrote: 

Bob, 

I totally understand the concern about multiple strings *if the strings are connected across the battery bank* as had always been the tradition in off-grid installations.  My own experience with such a bank in my system  showed why when I had a single cell get to near meltdown.  I was lucky to find it.

This might be a good reason for an installer to have one of those FLIR (or similar) thermal imager cameras.   They're a bit on the expensive side, but could
really be helpful for so many things.   Even just to know if you have left a nut loose (under load of course)

boB

Maintenance is another reason, such banks are far more difficult to maintain.  But I think those concerns are ameliorated a great deal with a buss bar installation and once I did that I never had another problem and found the battery bank much easier to maintain because the strings no longer had to be right next to each other.  Heck, it would be possible to put each series string in its own box for that matter.  And with the inclusion of string fuses that reduces problems even further.  

When living with an off-grid system my biggest concern was system reliability and redundancy and the idea of a single string failure was a big worry which is why I never would consider it. 

Tom 

From: boB Gudgel ( mailto:boB at midnitesolar.com ) 

Sent: Tuesday, May 31, 2011 8:16 PM 

To: RE-wrenches ( mailto:re-wrenches at lists.re-wrenches.org ) 

Subject: Re: [RE-wrenches] 24 volt Battery Bank comparison 

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

On May 31, 2011, at 8:23 PM, "Wayne Irwin" <wayne at pureenergysolar.com> wrote:

I think the verdict is 12 - Rolls 2-YS-31 2430 AH @ 20 HR (one string) and one dry spare in storage is the best case scenario!

Wayne Irwin, EE
Director of Engineering
Pure Energy Solar International Inc. 
State Licensed Solar Contractor
License # CVC56695 
Wayne at PureEnergySolar.com
http://pureenergysolar.com
352 377-6527 Office
352 336-3299 Fax
352 316-1637 Cell 

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