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Solar power in autumn - M0YNG.uk

Created 2021-11-02

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Note: This is not a full write-up of my solar power APRS radio, I will do that one day, honest!

I've been running my APRS RF iGate purely off solar power for a few months now, and we're at an interesting time of year for solar stuff, so I thought I'd quickly note down some observations as the seasons change.

wet solar panel basking under a rainbow [IMG]

Quick history

Some time ago I wondered if I could press a 20w solar panel and 7.5Ah SLAB into service doing APRS, the answer was YES - but only just.

I was happy for stuff to turn off when it runs out of power, reasoning that APRS traffic is light when there is least light so it wouldn't matter too much.

However, the 20w panel could only just keep up with the power demands of the system on a slightly dull summer day, and the 7.5Ah SLAB could only just keep it all running overnight in summer after a sunny day.

I could have left the experiment there, but I spotted a reduced-to-clear 100w panel on eBay and bought it, then bought a 55Ah battery on sale at the local motor factors (yea, I know it's the wrong type of battery.)

Since getting all that I've had everything running fine 24/7 from the moment I connected it all up.

Quick setup overview

It's not optimised, but it's also not fancy.

So, Autumn?

Up until now, even with three or four rainy days in a row the system has never run low enough to power off, until this last week. We had a few VERY dark days, which ran the battery down enough that the charge controller shut off the output. Even after a sunny day there isn't enough juice going into the battery to keep everything alive overnight in early November.

Let's look at a graph/chart, and see if you can spot what is going on (answers after the break!)

Chart of amps and volts, discussed in detail below [IMG]

This graph/chart is from today, so the data doesn't extend to midnight, because it isn't midnight yet. However, there is another big gap between around 0300h and 0900h where the system was offline. Which is fine, that's part of the design and "expected operation".

We can see that from some point in the morning the panel has been generating power and charging the battery, as we're at around 12.5v before we get any data (because the Pi is monitoring its own voltages) and it is coming it a respectable (if not amazing) 2 Amps. The panel is rated 100w at 5A, so we're running around 40% here, which for an autumn morning isn't too bad.

Can you guess what happens next though? From around 1000h to 1300h we have basically no solar generation. Any ideas why? Yup, my garden is north facing and even though I have the solar panel as far away from the house as possible there is still a big house shaped shadow that falls on the panel for these three hours. I suspect a lot of potential power is being lost here because it is the hight of the day and when the solar panel could be generating the most power.

This theory is supported by the nearly 4 Amps generated almost immediately the shadow moves off the panel. (The data is read every 30s but grouped up to 15 minutes for the chart.)

If we look at my historic solar data[1] we can see how much power is generated and used and stored every day since late June 2021. The highlights are:

1: gemini://m0yng.uk/historicSolarDataChart.gmi

This is because if we start with 0 power, and the next day also starts with 0 power, clearly the system is only able to use the power generated that day. On that point, it's also not that useful for days where there is lots of sun because the battery can only store solar power in the "space" made by using it overnight.

Compared with summer

Let's look at another graph/chart and see how this compares with a summer day, this is from September

Chart of amps and volts, it looks like you'd think it should! [IMG]

In this chart you can see the voltage is following the "proper" cycle of charging upto 14.25 volts (maybe there was a cloud around 1000?), then dropping off to hold at 13.8 volts for the rest of the day. We also start earlier with the voltage picking up from 0700 and dropping off again at 1700. These times are mostly dictated by neighbouring houses and the shadows they cast. The panel was also closer to my house at this point, but as the sun was higher in the sky it didn't matter, unlike now!

The raw power generated is actually less, but that's because we're just topping up what was used overnight, and providing some to run directly, rather than sucking up every single amp available.

A dark day

Chart of very few amps and volts [IMG]

If I Remember Correctly [IIRC] this was a dark day that rained a lot, amazingly the solar panel still managed to produce 2.77 Ah that day, but barely managed to prevent the battery voltage from ending the "sun lit day" lower than it started.

What am I going to do?

We're always going to struggle with the triangle of power, to keep this thing up overnight I need to do one or more of:

Decreasing usage isn't that easy, other than tuning the pi a little by turning off USB+Ethernet, turning off HDMI, and turning off LEDs that I'm not using. I could also underclock the CPU, but it is already running the `ondemand` governor which turns down the clock speed when it is idle. I can't reduce the frequency it does APRS stuff either, as that is the entire point of it existing.

Increasing production could be done by adding more panels, but that's overkill for most of the year and expensive. I am considering moving the panel again so it gets sun later in the morning, but isn't shadowed during the peak of the day. This should help boost production a little. But at the expense of a later "start up"? I *could* move the panel to the house roof, but a) I'm not climbing up there, b) I don't have any way to attach it, c) I don't have long enough cables, d) the roof faces East and West, not South so we'd still have the shadow problem.

Increasing storage probably ins't useful in this context, we're not making enough to keep going until the next sunrise so more storage wouldn't do much.

What was the point of all this?

Nothing really, I just thought it was interesting and I look at these graphs every day so wanted to share!

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