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PoC single solar panel system
#1
Hi

First of all this is very early stage planning and acquiring knowledge.

I am in the process of planning a proof of concept solar system, as I'm currently in an apartment but will be moving into a house in the next 2-4 years, at which point I will be setting up a larger system.

I currently have:
  • 140 lightly used cells (All have a capacity of about 2600 mAh).
  • An APC SUA1500RMI2U (Can this be used as an inverter?) (24V)
Planning to get:
  • Used solar panel (Anything specific to look for when buying a used panel?)
  • MakeSkyBlue charger (It wouldn't let me link to it)
I'm thinking that it would make the most sense to create a 7s20p battery for this system?
Or should I be creating a 14s10p and use a step-down converter between the battery and the inverter?
If I create a 14s10p battery then it does not seem like I can use the MakeSkyBlue charger with a single solar panel, as the VOC would be too low, is that correct?

Sorry if some of these questions are a bit basic, if anyone have some links on where I can read up on some of this, please post them.

Thanks
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#2
There's a FAQ somewhere that relates ...
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#3
If you're planning to go 48V longer term, anything you spend now on 24V gear will be less useful later.

It's not practical to try & run an inverter off a step down converter, inverters typically drawn quite high currents.

If you build your battery packs so they're more modular, you can connect as a 7s for now & then re-assemble into a 14s later.

Check that UPS does a true sine wave output (ie not a "modified sine"), it says it does but many lower end units don't.

re 2nd hand panel, look for ones from a house roof system, eg ones replaced during an upgrade, building work, etc, make sure no signs of physical damage, especially to back of cells & no "snail trails" (cracked cells) or hot spots evident. Most are 60 cell, but 72 cell might be better to play with (higher Voc voltage).
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#4
Make your string 7s20p, then you can put two strings together to make your 14s later on. I would go with 20p for the amp delivery.

Remember, the lower the voltage, the higher the amp draw. If you plan to use that UPS as an inverter, 980W @ 230V is 4A. Under surge conditions, that could be 8A (tho UPS's are usually not rated with a surge rating, so this is a guess).

With 10p, that would 0.8A per cell. For used cells, that's a bit high unless they are power tool high drain cells (which being 2600mAh I'm guessing they are not). The lower the amp draw on the cell, the longer it will last, both longevity and capacity. And that's the other reason for 20p vs 10p, as the battery bank will last twice as long as parallel determines ampacity and capacity. Series only determines voltage.
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#5
@Redpacket

Thanks

I don't think that there is anything on that list (Other than the UPS, but I got that for free) which would not be usable in a 48V system.

The UPS is a professional unit (Hopefully it won't mind a bit of abuse) and I'm 99% sure that it output a pure sine wave.

I'm thinking 7s10p*2, that would allow me to easily reassemble as 14s10p or should I just cut up the packs when the time comes and redo them? As the packs will be staying as is for at least a couple of years.
What Korishan is saying makes sense, as I should probably have more cells at that point.

I have also been thinking about getting some of Jehu's PCBs, as I don't currently have a spot-welder and it would be cheaper/easier to just grab 20 PCBs.

All of the solar panels I have found for sale here are in the 35-45 Voc range, if I'm reading the specs on the MakeSkyBlue correctly I would need either 2 panels in series for 48v or one panel with a Voc above 37V (Unless I can find a 60 or 72 Voc panel)
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#6
Also note that a Ups have high Idle draw. Easy 60w or more in losses running idle
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#7
The system is ultimately just going to be for learning purposes, so the UPS won't be connected at all times (I'm only using it because it was free), but thanks :-)

The thing I'm most confused about right now is how the specs of an MPPT charger correlates to the specs on a solar panel I'm guessing that the Voc from the solar panels must match the input on the MPPT charger:

I read this as: I would need to get a solar panel with 37+ Voc for a 24V system.
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#8
If you haven't seen/read it yet, here's some info on solar panel and mppt/pwm controllers: FAQ: What is MPPT and PWM on solar charge controllers and what is the difference?
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#9
(05-14-2019, 11:44 AM)Korishan Wrote: 980W @ 230V is 4A. Under surge conditions, that could be 8A .

With 10p, that would 0.8A per cell.

I think the numbers might be out here?
980W @230VAC is ~4.25A @230VAC.  
But at 24V with 90% inverter efficiency it's 230/24/0.9 x 4.25A = about 45.25A
So for a 10p pack that's ~4.5A per cell (way too high).
Even 20p it'll be ~2.25A/cell (too high). 

(Surge/motor start not considered)
Run time is going to be short at full load!  You might get some life with only a few 100W of load.
Like Korishan suggests, max recommended current per cell at full load is 0.5A if you want the cells to last.
ie at 24V you'd need min about 80p packs to get the current per cell down.

Re the solar panels, 60 cell panels usually have Vmp approx 32V (Voc=37V) on the label, 72 cell ones Vmp =~37V (Voc=45V).
Voc will start the charger, but you need to be in the Vmp range to get real power from the panels.
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#10
Yeah, didn't account for the inverters draw and inefficiency. Thanks Smile
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