[RE-wrenches] 24 volt Battery Bank comparison

Thu Jun 2 07:34:07 PDT 2011

Wrenches:

This is a really great thread. Here’s my two cents…

I started selling and configuring battery systems when I got out of the Army back in 1967 I’m a marine electrician. In the boat environment everybody tries to make a big battery out of a bunch of small batteries, Over the years this is what I’ve witnessed; All else being equal.

3 parallel strings = 4 ½  to 5 ½ years Max

2 parallel strings = 5 ½ to 6 ½ years Max

1 series string = 8 to 12 years Max

The above seems to hold true if the charge settings, Bulk, Absorb time and Voltage are correctly set. But as we all know 85% of the time this is not the case.

So my conclusion is that parallel configured banks just about cut the life in half. 

And there is always the stories about the guy that drank a quart of Bourbon a day and smoked cigars chasing women until he was 102 years old. The same is true with batteries… the exception but not the rule.

  Tom Duffy

  Systems Design Engineer

     tom at thesolar.biz

       575-539-2111 X 122

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From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Allan Sindelar
Sent: Wednesday, June 01, 2011 11:07 PM
To: re-wrenches at lists.re-wrenches.org
Subject: Re: [RE-wrenches] 24 volt Battery Bank comparison

I had done this a few times in the distant past, and also had done it in recent years on my own previous battery bank (four strings of S-530s - yes, I know; it was in anticipation of future plans to upgrade to 48V, and two strings wouldn't have been enough).

It worked fine - and that turned out to be a problem. I had noticed reduced capacity, but assumed foolishly that it was simply to age and cycle life (going on six years and three teenagers). Found out I had two failed cells in two different strings. The parallel connections probably - I'm not sure here - allowed the bank to perform more robustly and less obviously weakened. It might have even allowed the bank to perform as if both failed cells had been in the same string. But it also allowed me to not pay enough attention to a potentially serious problem by disguising it.

I blame myself more than the parallel connectors (just some #2 interconnects I had lying around). I'm just not sure I would use it on a customer's system.

Allan Sindelar
 <mailto:Allan at positiveenergysolar.com> Allan at positiveenergysolar.com
NABCEP Certified Photovoltaic Installer
NABCEP Certified Technical Sales Professional
New Mexico EE98J Journeyman Electrician
Positive Energy, Inc.
3201 Calle Marie
Santa Fe, New Mexico 87507
505 424-1112
www.positiveenergysolar.com <http://www.positiveenergysolar.com/>  

On 6/1/2011 9:20 AM, toddcory at finestplanet.com wrote: 

Does it make any difference to do as boB recently suggested and inter-connect the strings? So in other words, if you have two parallel strings, what happens if you parallel each battery in one string with the battery in the other string? Has anyone tried this? It might help insure more balance between each series string.

Todd

On Wednesday, June 1, 2011 3:06am, "James Surrette"  <mailto:james at surrette.com> <james at surrette.com> said:

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  <mailto:boB at midnitesolar.com> <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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