[RE-wrenches] Low Voc for effective string?

Wed Sep 12 21:55:35 EDT 2018

Bill and all,

To elaborate a bit on Ray's accurate suggestion with a bit of history... 
in the early days the general consensus developed that for general use, 
36 cells (given that one cell produces just shy of 1/2 volt, regardless 
of cell size) was the standard cell count for battery charging. This was 
based on the understanding that a typical Vmp of 17.1-17.6 for a 36 cell 
module was necessary to equalize a 12V FLA battery to ~15.5 VDC in hot 
ambient daytime temperatures after all system losses due to various 
resistance in wire, connections, controllers, etc., were accounted for. 
You'll occasionally see early modules with 35, 33, 32 and even 30 cells, 
tried as "self-regulating" modules - didn't work, as it couldn't account 
for voltage variations due to temperature. Anyway, 36 cells is the 
default standard to charge a 12V battery. Extrapolating from this, 
there's a direct correlation between cell count and voltage in most 
applications: as 36 cells > 12Vnom, 72 cells > 24V and 60 cells > 
20Vnom. Go outside of these rules of thumb only with clear understanding 
of these limitations.

Three 60 cell modules is a nominal 80V; ideal for a 48 Vnom battery 
bank, while still below the 150V limit of many standard controllers in 
record cold temperatures. Three 72 cell modules, or 72 Vnom, is on the 
ragged edge of voltage compliance in cold winter climates.

Allan

On 9/12/2018 10:30 AM, Ray wrote:
>
> A 2nd option: A pair of 72 cell modules should be enough.
>
> Ray Walters
> Remote Solar
> 303 505-8760
> On 9/12/18 9:21 AM, Mac Lewis wrote:
>> Hi Bill,
>>
>> I agree with Allen. We get similar peak temps here and two 60 cell 
>> modules doesn't charge a 48V battery bank adequately. This effect 
>> would become worse over time with voltage degradation. I try to be at 
>> least 15% above highest expected battery voltage with Vmp of the 
>> strings. With 150V input limit on the Outback controllers, that 
>> leaves you with only one option, strings of 3, with our low temps.
>>
>> On Tue, Sep 11, 2018, 4:24 PM Sindelar Solar <Allan at sindelarsolar.com 
>> <mailto:Allan at sindelarsolar.com>> wrote:
>>
>>     Bill, I wouldn't, especially with FLAs. I have seen this
>>     situation here, where the summer highs are similar, with a few
>>     homeowner/hack installations and no, 60-cell modules will not
>>     finish the charge in hot weather. Strings of three work just
>>     about perfect with modern charge controllers, especially in
>>     winter, as they range high enough but well below 150VDC limits.
>>
>>     Allan
>>
>>     On 9/11/2018 4:04 PM, Bill Battagin wrote:
>>>
>>>     I've been on hold with OB for about 40 minutes now and I need to
>>>     know if 2 SW PVs with a Voc of 39.9 and a Vmpp of 31.3 will do
>>>     an effective job of charging a 48 volt LA battery especially
>>>     with temps of 95 degrees.  Anybody have experience with this or
>>>     similar,  Appreciate the feedback.  It just seems I'm a little
>>>     low in Vmpp if I want to equalize when its hot.  Will it be even
>>>     more questionable in 10 years as PVs age?
>>>
>>>     OB did just pickup only to tell me I need Java for the string
>>>     sizing tool and he can't help me with this configuration.
>>>
>>>     Bill
>>>
>>>     -- 
>>
>>     *Allan Sindelar*
>>     allan at sindelarsolar.com <mailto:allan at sindelarsolar.com>
>>     NABCEP Certified PV Installation Professional
>>     New Mexico EE98J Journeyman Electrician
>>     Founder (Retired), Positive Energy, Inc.
>>     *505 780-2738 cell*
>>
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-- 

*Allan Sindelar*
allan at sindelarsolar.com <mailto:allan at sindelarsolar.com>
NABCEP Certified PV Installation Professional
New Mexico EE98J Journeyman Electrician
Founder (Retired), Positive Energy, Inc.
*505 780-2738 cell*

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