[RE-wrenches] 24V charging with 60 cell modules
Ray Walters
ray at solarray.com
Tue Jun 4 15:05:05 PDT 2013
I agree with Allan and Bob-O that overall this is not going to work so
well. It will probably take you a set of batteries that fail in 3 years
instead of five to come to this same conclusion as the guys that have
been wrenching for a long time. Its going to be a chronic, long term
issue, not an immediate "its going to fry" problem. The chance of
success improves in the following "Rosy" scenario: a higher voltage 60
cell module than the Kyocera, a small gel cell battery (lower charge
voltage) Warm battery (with temp compensation) Cold module, cold
climate, and virtually no load during absorption charge.
Then it might actually get normal battery life, but that's just too many
ifs for any of my off grid work.
I'd go with 2 modules in series with MPPT, or drop to a 12 v battery
with MPPT.
Solar Converters makes a controller that can mate low voltage arrays to
higher voltage batteries, but as our fine rep from BZ Products noted:
the efficiency isn't so good.
R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760
On 6/4/2013 3:17 PM, Allan Sindelar wrote:
> Eric,
> Thanks for sticking with me on this. OK, that makes more sense. You're
> not saying that it's an unregulated float, but only that float will
> allow load to pass through as needed to maintain float voltage, up to
> the limit of either the available array or the controller's capacity,
> whichever is less.
>
> I still wonder whether a modest load on the charge during bulk would
> likely prevent the controller from ever reaching the bulk voltage,
> given that the module would be operating above its MPP knee, and thus
> with greatly reduced current (depending on temperature and other
> factors). If that occurred, would the controller just stay in bulk
> mode indefinitely?
> Allan
>
> *Allan Sindelar*
> _Allan at positiveenergysolar.com_ <mailto:Allan at positiveenergysolar.com>
> NABCEP Certified Photovoltaic Installer
> NABCEP Certified Technical Sales Professional
> New Mexico EE98J Journeyman Electrician
> Founder and Chief Technology Officer
> *Positive Energy, Inc.*
> 3209 Richards Lane (note new address)
> Santa Fe, New Mexico 87507
> *505 424-1112*
> _www.positiveenergysolar.com_ <http://www.positiveenergysolar.com/>
>
> *
> *
>
>
> On 6/4/2013 3:02 PM, Eric.Bentsen at schneider-electric.com wrote:
>>
>> Allen,
>> The C-series will provide 100% duty cycle until the bulk voltage is
>> reached. Assuming no load, duty cycle will be reduced
>> thru the absorption stage to hold the battery at the bulk level
>> (this occurs due to the internal resistance of the battery increasing,
>> requiring less and less current to hold the battery at bulk). After
>> 1 hour, the C-series will reduce the duty cycle even more to allow
>> the battery to drop to a float level. Duty cycle will increase up to
>> 100% while in float if load demand if present. If load exceeds solar
>> watts, the voltage in the battery will drop. Once.5V below bulk
>> setting (24V system), the C-series starts a new charge cycle.
>> We are assuming NOTC of 20C (68F), so there will be some reduction of
>> output at the bulk level. Considering AGM and gel batteries
>> would bulk around 28.2VDC, efficiency losses should not be too
>> extreme. Also, when using battery temp sensor, the charger will
>> target lower charge voltage as temp increases.
>> Obviously the MPPT would be optimal, with 2 or 3 modules in series.
>> Eric Bentsen
>> _____________________________________________________________________________________
>> *
>> Eric Bentsen* | *Schneider Electric ** | Solar Business* |
>> *UNITED STATES* | *Technical Support Representative* *
>> Phone:* +(650) 351-8237 ext. 001# | *
>> Email:* _eric.bentsen at schneider-electric.com_
>> <mailto:eric.bentsen at schneider-electric.com> |
>> *Site:*_www.schneider-electric.com/solar_
>> <http://www.schneider-electric.com/solar> | *Address:* 250 South
>> Vasco Rd., Livermore, CA 94551
>> _
>> _<www.schneider-electric.com/solar>
>> *** Please consider the environment before printing this e-mail
>>
>>
>>
>> From: Allan Sindelar <allan at positiveenergysolar.com>
>> To: Eric.Bentsen at schneider-electric.com
>> Date: 06/04/2013 12:57 PM
>> Subject: Re: [RE-wrenches] 24V charging with 60 cell modules
>>
>>
>> ------------------------------------------------------------------------
>>
>>
>>
>> Eric,
>> I understand what you write about PWM being able to do what MPPT
>> can't because of the buck function, but not the rest. What I
>> understand you're saying is that float mode on the C-series
>> controller is unregulated; that is, rather than hold the battery to
>> float voltage, it will pass module current through until Voc is
>> reached, or until the battery rises as high as it is able given the
>> available current. If so, that goes against all I have observed, and
>> even against the concept of a regulated float voltage.
>>
>> David Katz wrote: "You can do it if you don't need much power. That
>> module will float and equalize the battery but as the voltage rises,
>> the current will drop significantly. I used one 230 watt module for
>> a year to charge a 24 volt battery with an 11 watt load on it and it
>> worked fine. It is probably cheaper than buying two 50 watt 36 cell
>> modules and putting them in series." I accept that logic, although it
>> breaks all the old rules. A 230W 20V module is cheaper than two 50W
>> 12V modules - I get to explain that wackiness to customers on a
>> regular basis. But the kicker is the 11W load - even if continuous,
>> it's only 264 watt-hours/day, less than two hours' output of the
>> oversized module. My opinion is that if the load is much larger than
>> that, the bulk setpoint, which triggers the one-hour absorption and
>> transition to float, will never be reached.
>>
>> Brian's excellent post really captures the principles involved well.
>> As battery voltage rises above Vmp, current drops sharply. It'll
>> work, sort of, but is poor design.
>>
>> Also, you wrote "The question was making a single 24V module work
>> with a 24V battery." I assume that you meant "The question was making
>> a single 20V module work with a 24V battery.", as that's the original
>> issue, and this was just a typo. If not, and you meant what you
>> wrote, we need to start over with this discussion...
>> Thank you.,
>> Allan
>>
>> *Allan Sindelar*_
>> __Allan at positiveenergysolar.com_ <mailto:Allan at positiveenergysolar.com>
>> NABCEP Certified Photovoltaic Installer
>> NABCEP Certified Technical Sales Professional
>> New Mexico EE98J Journeyman Electrician
>> Founder and Chief Technology Officer*
>> Positive Energy, Inc.*
>> 3209 Richards Lane (note new address)
>> Santa Fe, New Mexico 87507*
>> 505 424-1112*_
>> __www.positiveenergysolar.com_ <http://www.positiveenergysolar.com/>
>>
>>
>> On 6/4/2013 12:31 PM, _Eric.Bentsen at schneider-electric.com_
>> <mailto:Eric.Bentsen at schneider-electric.com>wrote:
>>
>> Hi Allan,
>>
>> The C-series behaves in the same way as if you were to connect the
>> module directly to the battery,
>> then watch a DC volt meter and disconnect/reconnect when the bulk
>> voltage is reached to "hold" the battery
>> at that level. It will hold at the bulk level for 1 hour, then
>> transition to float.
>> If left connected without disconnecting, the voltage would eventually
>> climb to Voc (approx 36VDC). So, while it is not
>> going to be as efficient as the MPPT controller in the
>> bulk/absorption stages, once the PWM controller transitions to float,
>> it will operate right about the sweet spot (Vmp). The question was
>> making a single 24V module work with a 24V battery.
>> PWM is the only way to do it, as the MPPT requires a higher PV
>> voltage due to the use of a buck transformer.
>> I will certainly pass on any recommendations for improvement to our
>> engineering team, and
>> I appreciate the feedback.
>>
>> Eric Bentsen
>>
>> _____________________________________________________________________________________*
>>
>> Eric Bentsen* | *Schneider Electric ** | Solar Business* |
>> *UNITED STATES* | *Technical Support Representative* *
>> Phone:* +(650) 351-8237 ext. 001# | *
>> Email:* _eric.bentsen at schneider-electric.com_
>> <mailto:eric.bentsen at schneider-electric.com> |
>> *Site:*_www.schneider-electric.com/solar_
>> <http://www.schneider-electric.com/solar> | *Address:* 250 South
>> Vasco Rd., Livermore, CA 94551 _
>> __
>> _<www.schneider-electric.com/solar>
>> *** Please consider the environment before printing this e-mail
>>
>>
>> From: Allan Sindelar _<allan at positiveenergysolar.com>_
>> <mailto:allan at positiveenergysolar.com>
>> To: RE-wrenches _<re-wrenches at lists.re-wrenches.org>_
>> <mailto:re-wrenches at lists.re-wrenches.org>
>> Date: 06/04/2013 10:35 AM
>> Subject: Re: [RE-wrenches] 24V charging with 60 cell modules
>> Sent by: _re-wrenches-bounces at lists.re-wrenches.org_
>> <mailto:re-wrenches-bounces at lists.re-wrenches.org>
>>
>>
>>
>> ------------------------------------------------------------------------
>>
>>
>>
>> Eric,
>> Please explain further, as what you wrote makes no sense to me. Mac
>> didn't indicate whether he wanted to charge a flooded battery
>> (~29.2V) or sealed (~28.4V). The C-series of PWM controllers has a
>> fixed two-hour absorption, which it only counts down once the bulk
>> voltage is reached. If the bulk voltage setpoint is above the MPP of
>> the module, as it is in this case, how can it ever reach float, as
>> you suggest?
>>
>> And further, this is spec-sheet calculation. In all but the coldest
>> winter weather, the Vmp of the module will be lower still as the
>> module temperature rises above 25ºC, and is likely to be even below
>> the float voltage in hot weather. And in the winter, if the battery
>> is also cold, it will need a higher voltage still to get fully charged.
>>
>> So I'm with Bob-O on this one. You can't charge a 24V battery with a
>> 20V module. Please tell us how I'm wrong.
>> Thanks, Allan
>>
>> P.S. - And while I'm on a virtual roll, please tell your engineering
>> folks to build a charge controller with a higher voltage input
>> window. I don't mean 600 volts at $1,500 or whatever it is; just
>> higher than 150V. On a 48V system, we're stuck with series strings of
>> three modules if we use the lowest-cost-per-watt 60-cell modules.
>> That means that systems have to be designed with arrays in ~750-watt
>> increments. Give us another 50-100V or so of headroom, so we can
>> design with strings in multiples of 3, 4, or 5. Outback, are you
>> listening too? The Midnite Classic does this for us... Thanks.
>> *
>> Allan Sindelar*_
>> __Allan at positiveenergysolar.com_ <mailto:Allan at positiveenergysolar.com>
>> NABCEP Certified Photovoltaic Installer
>> NABCEP Certified Technical Sales Professional
>> New Mexico EE98J Journeyman Electrician
>> Founder and Chief Technology Officer*
>> Positive Energy, Inc.*
>> 3209 Richards Lane (note new address)
>> Santa Fe, New Mexico 87507*
>> 505 424-1112*_
>> __www.positiveenergysolar.com_ <http://www.positiveenergysolar.com/>
>>
>>
>> On 6/4/2013 10:43 AM, _Eric.Bentsen at schneider-electric.com_
>> <mailto:Eric.Bentsen at schneider-electric.com>wrote:
>>
>> Hi Mac,
>>
>> A PWM charge controller, such as the C35, will work in this
>> application, because it will
>> act like a switch that connects the panel to the battery. While the
>> bulk voltage will likely be slightly
>> higher than Vmp, once the controller transitions to float, which is
>> approx 27V for most battery types,
>> it should allow the module to operate very close to Vmp.
>> Typically, MPPT controllers require a higher Vmp than the target
>> charge voltage.
>>
>> Eric Bentsen
>> _____________________________________________________________________________________*
>>
>> Eric Bentsen* | *Schneider Electric ** | Solar Business* |
>> *UNITED STATES* | *Technical Support Representative* *
>> Phone:* +(650) 351-8237 ext. 001# | *
>> Email:* _eric.bentsen at schneider-electric.com_
>> <mailto:eric.bentsen at schneider-electric.com> |
>> *Site:*_www.schneider-electric.com/solar_
>> <http://www.schneider-electric.com/solar> | *Address:* 250 South
>> Vasco Rd., Livermore, CA 94551 _
>>
>> _<www.schneider-electric.com/solar>
>> *** Please consider the environment before printing this e-mail
>>
>> From: Mac Lewis _<maclewis1 at gmail.com>_ <mailto:maclewis1 at gmail.com>
>> To: RE-wrenches _<re-wrenches at lists.re-wrenches.org>_
>> <mailto:re-wrenches at lists.re-wrenches.org>
>> Date: 06/04/2013 08:05 AM
>> Subject: [RE-wrenches] 24V charging with 60 cell modules
>> Sent by: _re-wrenches-bounces at lists.re-wrenches.org_
>> <mailto:re-wrenches-bounces at lists.re-wrenches.org>
>>
>>
>>
>> ------------------------------------------------------------------------
>>
>>
>>
>> Hello wrenches,
>>
>> Is there a good method to charge 24V battery bank with single 60 cell
>> modules? Ideally, I'd like to charge a 24 V bank with a Kyocera
>> 245GX-LFB. However, at NOTC the Vmp is 26.8V. Any good way to do this?
>>
>> Thanks in advance.
>> --
>>
>>
>>
>> Mac Lewis
>>
>> "Yo solo sé que no sé nada." -Sócrates
>>
>>
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