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From: boB <boB@K7IQ.com>
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Subject: Re: Solar panels
Date: Sat, 01 Jun 2024 18:29:20 -0700
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On Fri, 31 May 2024 22:42:59 -0700, Don Y
<blockedofcourse@foo.invalid> wrote:

>On 5/31/2024 6:18 PM, KevinJ93 wrote:
>>> I count ~72 wafers on a panel I have here.  How does *it* get to a 48V
>>> nominal output?
>> 
>> Are you sure that isn't the open-circuit voltage(Voc)?
>> 
>> For the couple of 72 cell panels I looked Voc is 49.0V +/-
>> 
>> The peak power voltage is about 41.0V.
>> 
>> Each cell has a peak voltage of ~0.7V and a peak power voltage of ~0.56V.
>> 
>> That voltage is not enough to charge a nominal 48V battery which probably needs 
>> up to ~54V.
>
>Actual voltages don't matter.  Its the design topology that's important.
>Delivering an output that can be simply summed among panels and avoiding
>a many kilowatt "low voltage power supply" following that.
>
>> Interestingly I did find out that the 400W Q-Cell panels are in fact 144 cell. 
>> They look they are configured as two 72 cell strings in parallel. There isn't 
>> any provision for reconfiguring as far as I could see.
>
>>>>> I.e., absorbing the cost of the conversion inefficiency in return
>>>>> for being able to eek a bit of extra power out of an underperforming
>>>>> panel?
>>>>
>>>> Residential installations commonly use micro-inverters with one per panel. 
>>>> This minimizes issues with individual panels being shaded or being placed on 
>>>> different facets of a roof.
>>>
>>> Yes.  Or "power optimizers" in lieu of inverters.
>> 
>> Some inverter vendors (in particular SolarEdge) use optimizers with each panel 
>> - to perform part of the MPPT function. As far as I know they only work with 
>> the same vendor's inverter.
>
>Yes, but, again, you're assuming a "domestic power" application.  If all you
>want it for the panel to track all the other panels that it is in parallel
>with, you don't care about an inverter.
>
>Get the panel to operate at its maximum power point.  Then, follow that with a
>"48V" converter.  The panel just contributes whatever current it can, at that
>voltage, /while operating at its MPP/.
>
>>>> Electrical code in the US requires that where panels are placed on a 
>>>> residence that there be no more than 80V DC present when disabled.
>>>
>>> Local electronics at the panel ensure that -- whether microinverter or power 
>>> optimizer.
>> 
>> Or a Mid-Circuit Interrupter (MCI) as in the case of systems such as Tesla.
>
>Output During Standby (when disconnected from inverter or inverter off):  1 Vdc
>
>(read that as "when load disconnected", in my case)
>
>Absolute Maximum Input Voltage (Voc):  75 Vdc
>MPPT Operating Range:    5 - 75 Vdc
>Maximum Input Current:   10 Adc
>Maximum Output Current:  15 Adc
>Operating Output Voltage:  5 - 60 Vdc
>Maximum Efficiency:      99.5 %
>European Weighted Efficiency:   98.8 %
>
>>>> Even batteries for residential are commonly AC-in/AC-out with their own 
>>>> bidirectional inverters. (eg Tesla Powerwall and Enphase)
>>>
>>> But, those try to back up the entire array's capability.  Give me 3-4KW
>>> during daylight and I can get by on < 100W for the rest of the day (night).
>>> So, a modest battery can carry the dark load and act to bridge small
>>> variations in output during full illumination.  Adjusting the load
>>> accomplishes the rest.
>> 
>> There are many "Solar Charge Controllers" on Amazon that could do the voltage 
>> conversion and MPPT to charge a 48V battery at up to 60A (~3kW).
>> 
>> They seem to want a string voltage of up to 100V or 150V which would equate to 
>> 2-3 panels in series. Provided all panels have the same aspect and sun exposure 
>> you could put strings in parallel to get the power level you need.
>
>Yes, but now you're up at those "lethal" voltages, again.  If you think of it
>as a ~48V source, then it contributes just a few amps (~6).  You rely on having
>many of them working alongside each other to get the ampacity needed.
>
>> Not sure how to avoid excessive charge currents into the battery if it can't 
>> take the full solar output.
>
>I think you have to be able to "talk" to the module to tell it how you would
>like it to behave, NOW.
>


The MPPT controller simply raises the PV operating voltage above
Vmp...  At Voc, the current becomes zero. Whatever Vpv gives the
correct controller output voltage to the battery it is charging.

boB