I have a unused utility window in my apartment which is very narrow and faces in an odd direction, so I figured instead of just boarding it up I'd build a small system in it and see how it goes.

WIP

I don't really have a serious load in mind. As of current, I am powering a lamp with the system and use it to charge my phone sometimes.

I bought the only 18 V panels on AliExpress that I was able to find that would fit the window, though they do almost perfectly:

https://www.aliexpress.com/item/1005003123354268.html

I have 6 of them and they all provide 18 V at 5 W (I = P / V = 5 / 18 = .278 A) and are wired in parallel so the whole array produces 18 V 1.7 A in theory.

I am using a PWM charge controller as MPPT seems like a complete overkill for this use-case. Also, I read online that PWM is good for purely parallel installations, which I am not sure is true but this one happens to be one such so if it is, great.

https://www.alza.cz/bygd-solarni-regulator-nabijeni-pv2420u-d6959817.htm

The controller charges this little 12 V 7.2 Ah SLA battery:

https://www.alza.cz/auto/csb-evx1272-baterie-12v-7-2ah-d5597727.htm

In theory, the system should make 18 W 1 A, so in 4 hours of peak sun, 72 Wh. The battery is 7.2 Ah at 12 V, that's 86 Wh.

So we should be just shy of cycling the battery every day with this system.

But in practice, due to the I guess side shade of the window frame or the bad orientation of the panels, I never see more than 13 V hit the charge controller and it seems to be able to top up the battery to 75 % going from 50 % charge.

So I think I can cycle 25 % of the battery capacity every day.

I also have another SLA battery at hand which I am not using for anything else but without better panels or better wiring, there is no point in using it:

https://www.alza.cz/auto/goowei-energy-6-dzm-20-baterie-12v-24ah-electric-vehicle-d6217380.htm

I also have this LFP battery on order so that I can discharge the battery more deeply if needed, but it is unlikely to top up much more than the SLA one.

https://www.aliexpress.com/item/1005002405783446.html

25 % of the usable (cyclable) capacity that I have is around 22 Wh. I am powering an LED lamp off this system sometimes. It eats up 5 W so it could run for 4 hours off this battery's 25 % capacity.

I am also thinking of powering a Pi Pico W off this system continuously. I found online that it eats 1 mA in sleep and 100 mA while running. It operates at either 3 or 5 V or I think even up to 9 A on the power pin? I will need a buck converter anyway or I might just run it off the USB port on the solar charge controller.

Let's pretend .1 A at 12 V (if I used a buck converter), so that's 1.2 Wh. 25 % of the battery capacity was 22 Wh so 22 Wh / 1.2 Wh is around 18 hours of operation.

That's non-ideal as it would drain more of the battery than the system can replenish over the course of the day.

I might be able to utilize sleep mode to bring the power down or use better panels to get more solar power or use 2s3p over 1s6p wiring to maybe be more resistant to shade and in turn also generate more power?

Possible candidate solar panels:

https://www.aliexpress.com/item/1005003956761039.html

The are a worse fit for the window and would be permanently covered by the window frame at their horizontal edges but it might works out better overall? 120 Wh as opposed to the current 72 Wh if parallel.

I bought this battery gauge:

https://www.aliexpress.com/item/1005001763596519.html

It shows 100 % no matter what. The voltage is displayed correctly.

I think the problem might be with it not being set up for SLA?

I have not found a way to configure it in this listing, but I found it listed by another vendor here:

https://www.aliexpress.com/item/4000448925124.html

Here are the instructions:

1. Disconnect the module power supply, press and hold the setting button until powering the power module. Display shows the current power setting parameters
2. Press the setting button to select P / L / F ,(P = 12V lead acid, L = 3.7V lithium battery, F = 3.2V lithium iron phosphate battery)
3. Press the function key to set the number of the battery string, according to the function keys and set the key up or down to choose the number of batteries in series
4. Press the function key for a long time to enter the advanced setting interface, use setting key to select S1-S5, enter key to select or cancel the current mode
5. Reenergize the power after setting. The power automatically saves setting data.

E.g. 10 lithium battery power, turn on power percentage and voltage automatic rotation display (S1 mode)

Operation: Disconnect display module power, press and hold the setting button, power the power module, press the setting button for a short time to select L, press the function key to select 10, press the function key for a long time to select S1 and re-power.

At some point I will figure this out and fix it.