PoC single solar panel system

cnrd

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May 13, 2019
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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 inthe 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 APCSUA1500RMI2U (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
 
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).
 
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.
 
@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.
WhatKorishan 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 Vocrange, if I'm reading the specs on the MakeSkyBluecorrectly I would need either 2 panels in series for 48v or one panel with a Vocabove 37V (Unless I can find a 60 or 72 Voc panel)
 
Also note that a Ups have high Idle draw. Easy 60w or more in losses running idle
 
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:

image_jqyvwn.jpg

I read this as:I would need to get a solar panel with 37+ Voc for a 24V system.
 
Korishan said:
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 at24V 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 20pit'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 24Vyou'd need min about 80p packs to get the current per celldown.

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.
 
Yeah, didn't account for the inverters draw and inefficiency. Thanks :)
 
I had several APC 1500(s) all over the house when I started. My original idea was to use 18650 as batteries and inverter aspect to provide 120v.

Turns out that 7s is compatible with APC 1500(s) as the APCs charge up to27.8v (3.97v/cell) and cutout at 21.6v (3.08v/cell). 7s7p in an 7x8 18650 cell holders fit nicely in place of original battery. You can stack 2 high if you want double capacity.

I did 7s7p in 7x8 leaving 1st empty row for wiring/fuse/balance cable.

image_izygli.jpg


The problem with APCs are
- They must be plugged into 120v AC source to 'activate' the charge cycle so they will supply power when disconnected. So you can't just use it in isolation to charge/discharge/charge/discharge. It just won't turn on after the discharge till you plug it in for a moment.
- They only go to 3.97v/cell - so you loose 4.0 -> 4.2 range of power if that is of concern.
- They draw 40-60w/hour just sitting there. That's alot of power burn. **In fact I had 6 of them around the house and was shocked to find out I was buring 6 x 60w = 360w per hour x 24hrs a day = 8.6Kwh/day in idle mode.

But hey - I confess I also thought it was a great starting place, so no judgement!

I consolidated but still use 2 of them downstream of the automatic transfer switch to provide 'solar circuits' in house for computer/tv areas so the power drop during automatic transfer of grid/PV does not affect computers AND use my original 7s7p 18650 batteries in place of AGMs, so it was not a waste.
 
OffGridInTheCity said:
The problem with APCs are
- They must be plugged into 120v AC source to 'activate' the charge cycle so they will supply power when disconnected. So you can't just use it in isolation to charge/discharge/charge/discharge. It just won't turn on after the discharge till you plug it in for a moment.

I've been able to turn mine on without being plugged into the AC. Maybe it depends on the model. I've had an APC300, 550, and SUA2000 SmartUPS. the 300 and 550 units are generic dumb models used for computers and other electronics that most standard people forget about until it beeps about a dead battery. With those, I had to turn the switch on, then hold the "test" button in for a couple seconds and it fired right up.
 
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