Topic: WH41 and a Remote Solar Panel Install

[jwboyd52]

Hello,
I am using a WH41 that I mounted under the eves on the North side of my house. It cannot see direct sunlight only reflected and as expected the batteries needed recharging in about 3 weeks. What a hassle.  I decided to add an external solar panel to the sensor that I could mount where it could have direct sunlight on it. I used a panel I got from Amazon. 136mm x 110mm, 6V @330ma. I could use a smaller panel, but, this is what I had. I soldered the power leads in parallel with the existing panel and installed a small connector to allow disconnecting the panel if I needed to remove the sensor. I installed the panel in early April 2021 and as of October 2021 I have never had to charge the batteries. In early morning I do see the battery level has dropped by 1 bar, but, once the sun is up the battery is always showing full bars.

Cheers,
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[davidmc36]

Nice job. I love constructing things.

https://www.wxforum.net/index.php?topic=42801.msg436757#msg436757

I paralleled the daytime solar panel to keep supercap well charged on rainy winter spells.

[Rover1822]

jwboyd52

I am curious if you measured the open circuit voltage (Voc) of the existing panel from WH41 in the sun?

I am also curious if your external panel has a blocking diode which would prevent batteries from draining or discharging back through the Solar panel at night. I was thinking of your 1 bar drop overnight.

[jwboyd52]

Rover1822,

Hello. I made some measurements today on my external solar panel setup today. I did this @noon time so I was seeing the max sunlight exposure at the time. All measurements were made at the PM sensor solar panel leads, unless otherwise indicated.

Also, I believe the PM sensor is mostly asleep as it only does an air quality check every 5 or 10 minutes?
So my readings are probably not showing that much of a load, except maybe from the batteries.
Also the batteries were showing a full charge.
I may try and disconnect the external panel and wait for the batteries to discharge down by a couple of bars and then do the measurements again.

- Both panels in parallel, with batteries installed: 6.02V
- External panel, disconnected, no load, measured at External panel: 6.68V
- PM sensor mounted under eves, no external panel, no batteries: 2.58V
- PM sensor in full sunlight, no external panel, no batteries: 6.06V
- PM sensor in full sunlight, no external panel, batteries installed: 3.0V
- PM sensor mounted under eves, no external panel, with batteries: 2.85V
- I am not using a blocking diode as I assume the existing circuit has one. I have seen comments on the web where it is
   recommended to install a diode between each panel when used in parallel. However, this was on high capacity panels and I am
   not certain this is needed on these small low power panels.
- I also need to clarify that in the morning I sometimes see the battery level is down by one bar, but, this is not always the case. In
   fact for the last three days the battery level has shown a full charge.

Cheers,
jwboyd52

[Rover1822]

Thanks

I just ordered a 5V panel, and will mimic your work.

I find the "- PM sensor in full sunlight, no external panel, no batteries: 6.06V" interesting. Must be a real low current panel on the PM sensor. I say this as the panel is so small.

- Rover

[Rover1822]

"PM sensor in full sunlight, no external panel, batteries installed: 3.0V"

This kind of points (for me) to a direct panel connection to the series batteries are bringing down the voltage. This is not uncommon as long as the output of the solar panels is small enough not to require a controller and can be handled by the batteries. I had a small 12V nominal panel connected without a charge controller to maintain a boat battery.

I'm also making the assumption that you were measuring at the solder parallel joints for the panels for all tests.

[jwboyd52]

Rover1822
My measurements were done at the small external connector I added which is directly connected to the built-in solar panel solder points on the PM sensor.

At one time I was thinking about disconnecting the built-in panel and just operating off of the external panel. However, I wanted to try and keep everything working as I may move the sensor to an area that can see direct sunlight.
I really wish I knew more about the charging circuit the PM sensor is using. Too much guessing is going on here right now!

Ecowitt could make an improvement on the sensor by removing the built-in solar panel and just use an external panel instead. This would provide a lot more flexibility in sensor placement.

Cheers,

jwboyd52

[Phil23]

My D-Fars runs 24/7 with a 7Ah SLA battery charged by a 20 Watt panel. (Was 10W until a few weeks ago).
An adjustable Buck regulator drops the 12V to 2.2V for the fan......

What my next plan is, is to upgrade the battery to a 12Ah, add a second Reg to get 5V and then feed that to the USB port of the WH41 that's mounted beside the VP2.

Advantage I see with providing the power @ 5V via the USB is that the WH41 will then take samples at 1 minute intervals.
Pretty sure I've read that mentioned somewhere in the specs.

Cheers.

[Rover1822]

I received a couple of cheap panels, testing  Voc and Isc on them now. What I do believe is I don't want to exceed the charging rate or C rate (Ok the actual charging C rate is actually much higher , but needs to be disconnected when charge completed) for the NiMH cells . So basically, try not to exceed 150 mA on what I believe is a non regulated connection from the integral solar panel.

I do find it amusing how they market these, but I guess for the price, can't expect real specs. For instance I ordered a 6V panel, that goes 7V in sun, and I actually expect that as that falls inline with the "nominal" voltage rating of a panel. I also have a 5 volt panel that I bought, and it can barely make 5. (All Voc), where I would have expected near 6V.  Since I work with much larger panels normally , in the multi 100W range, I know that getting to voltage is usually not that hard and then the current climbs giving you the Watt value these things usually get rated at.


Fun Stuff, just to add power to  little sensor