Off grid solar project

[mcart117]

In the context of my last post, this has become quite an interesting chart; at least for me it has.


EVE in their product specs state that the maximum voltage for these LFP cells is 3.65V and the minimum voltage is 2.5V. They also say you can extend the life of the cells by limiting the depth of discharge (DoD) to 80%.

I used charts published by the OFG to estimate the voltages associated with an 80% DoD. Essentially I followed the discharge curve to just before the inflection point to gauge my cell under voltage setting and the charging curve to just before its inflection point for guidance on my cell over voltage setting. I didn’t create my own curves because I don’t think these stress tests are good for the cells and I didn’t see any need to replicate the work of others.

Besides, I can tell from my own live working data, if I get it slightly wrong, because one or more runners will appear if I get too close to the inflection points. For example I originally set my cell over voltage to 3.45V, but one cell started running close to 50 mV ahead of the rest, so I reduced the cell over voltage to 3.425V. I also reduced the threshold for balancing from 3.4V to 3.3V on a difference of 10mV.

On the chart above, the orange lines represent the EVE max and min voltages, and the yellow lines represent my own cell over and under voltage levels. And you can see from 4 weeks of data the cell voltages have mostly remained well within that range.

According to the OFG video and other posts, that should give me an 80% DoD, which on 16 cells with a nominal capacity of 1KWh each should give a usable capacity of 12.8 KWh. But you can see from the chart, my actual usable capacity is getting very close to the nominal capacity of 16KWh. Of course the figures come from the BMSs and they could be wrong. But I don’t think they are, because the battery is easily running for 15 hours a night now, from 4:30 p.m. to 7:30 a.m., and the capacity figures are consistent with our measured usage over that period.

I should add that I sourced these cells from Amy Wan of Shenzhen Luyuan, and I am pretty pleased with them so far.

 

[Hedges]

Are you saying battery and therefore cell voltage in graph is an estimate based on SoC (reported by BMS, I assume)?

I noticed cell voltage seemed to bounce up and down without much of a trend, while SoC in kWh has its good and bad days. Some are 7 kWh, some almost 16 kWh.

You also show energy getting close to "0", but I don't think cell voltage is at the lower knee.

So I wondered if that's because voltage doesn't reveal much. Also, if your operating voltage was high enough to accurately reset SoC or to maintain balance.

 

[mcart117]

Are you saying battery and therefore cell voltage in graph is an estimate based on SoC (reported by BMS, I assume)?

I noticed cell voltage seemed to bounce up and down without much of a trend, while SoC in kWh has its good and bad days. Some are 7 kWh, some almost 16 kWh.

You also show energy getting close to "0", but I don't think cell voltage is at the lower knee.

So I wondered if that's because voltage doesn't reveal much. Also, if your operating voltage was high enough to accurately reset SoC or to maintain balance.

I am showing mean cell voltage - ie battery voltage divided by 16, because it would take too long to record the voltage for every cell. But the difference between the voltages of all the cells is usually less than 10mV, so that average figure is pretty close to the mark.

The dodgy number is remaining capacity, because sometimes it shows as being zero, when from the cell voltages it clearly isn't. But assuming it accurately records the amps being pumped in, the Ah, which I convert to KWh, should be a reasonable estimate at the top end.

So the chart may have some flaws, but I shared it, because I thought it provided food for thought.

 

[Hedges]

It appears your total rarely if ever hits either 100% of 0%, like if consumption and production are fairly balanced for now. Will that drift to bouncing off 0% SoC during the winter, with system shutting down and eliminating consumption? Or is something else going on to keep it running (like generator, since you're off grid?)

I would expect an off-grid system to have excess generation (and frequently hit 100% SoC), or else shed loads so system never hits 0%.

 

[mcart117]

It appears your total rarely if ever hits either 100% of 0%, like if consumption and production are fairly balanced for now. Will that drift to bouncing off 0% SoC during the winter, with system shutting down and eliminating consumption? Or is something else going on to keep it running (like generator, since you're off grid?)

I have just bought some extra panels, to carry me through the night in winter or even after cloudy day in summer.

I would expect an off-grid system to have excess generation (and frequently hit 100% SoC), or else shed loads so system never hits 0%.

I have two circuits: an offgrid circuit with a battery for night time use, and an ongrid circuit with just panels and an inverter for daytime use. When the battery gets too low I have been switching back to the ongrid circuit in the middle of the night, but hopefully with the new panels I won't need to do that.

 

[mcart117]

One of the things I have learnt with this project is that (second hand) solar panels are very cheap, but everything associated with them – solar rails, clamps, cables, connectors, isolators and labour – are quite expensive. My main motivation at this stage is science: how much energy can I collect in my own back yard, and what can I do with it?

So I started out by simply leaning my second set of solar panels against the wall of the shed, as I had with the first.



But it was getting into the summer, and the sun no longer stayed to the north of the shed as it did during the winter. It is amazing how much the ancients knew about the path of the sun through the sky which is no longer taught in school today. I blame it on cultural genocide by the early Christians. But whatever the reason, I was taught about Stonehenge but nothing about why it was there, so I am having to relearn all this stuff from scratch.

Anyway, the bottom line is that as we pass the equinox, the sun starts to rise and set in the southern sky (in the southern hemisphere) and so my solar panels leaning against the northern wall of the shed were in the shade of the shed and the eves of the shed roof in the early morning and late afternoon. I therefore had to move them away from the shed wall.



They are now leaning on a set of old kitchen chairs. It looks ugly, but they trap a lot of energy. In fact I now had so much energy that I had to revisit a derivative of Ohm’s law in order to harvest it.

Someone needs to write a textbook on this stuff, with everything you need to know from soup to nuts. The YouTube vids are great but each one gives just a little bit of the picture and the ordering is more random than systematic. In some cases, such as the OFG, the poster posts whatever is in his mind at the time, which may be something he wants to learn more about, or simply something that arrived in the post that day.

My learning was also correspondingly haphazard. My first stop was to learn about batteries, because that was the driver for the whole project. I then watched a couple of vids on charge controllers, but those I stumbled across were for 12 volts camping systems and they were mostly talking about MPPT. I had some detailed discussion here about choosing an inverter while thinking about power and current and voltage, but I didn’t have a corresponding discussion about charge controllers. I went out and bought a 40 amp one, and I thought that would be fine because I had four pairs of solar panels producing 5 amps each. And when I added four more pairs, I thought it would still be fine, because that added up to 40 amps.

I didn’t realise, until I started researching why the extra solar panels weren’t giving me any more power, that the 40 amps applies to the other side – the side connected to my 26 volt battery. 40 amps x 26 volts = 1040 watts: and that’s what I was getting. So I had to go out and buy another charge controller, and now the system is sweet.



I am harvesting, and using, close to 16 kWh per day. This is higher than I estimated I would need, and probably reflects that were are more casual about electricity consumption, now that it is free. I used to turn off the air con as soon as the house cooled down, but now I let it run all night. And each of the 304 Ah cells I purchased from Shenzhen Luyuan seems to be storing and disgorging close to 1kWh, while the voltage remains within a 3 – 3.4 volt range.

So the project has met and exceeded my expectations so far. My power consumption from the PoCo is close to zero, and with the energy I export from the front of the house during the day, my last account was a credit to me.