0.5w solar panel on rak19003?


I’ve been working with the RAK19003 and RAK4631 for a while now and have two of such devices with a GPS module on there as well.
Both have a solar panel. The one is 1w, the other is 0.5w.

The one with the 1w solar panel updates every 3 minutes, while the 0.5w solar panel is updating every 5 minutes.

They have been located in my garden about 30 centimetres apart, so the sun exposure is the same.
The software on these devices is the same and i report the voltage via chirpstack into influx.

I’ve noticed that the 0.5w panel doesn’t charge as fast - that’s logical, nothing special, but i see an odd behaviour where it seems to discharge faster in the beginning?

Here’s the plot for the few hours this morning:

I’ve seen the same pattern on other days:

I’m just wondering if the charging circuit on the RAK19003 should work properly with 0.5w panel, and/or if at a certain input voltage it’s just not as efficient. There’s very little known about the charging circuit, so i’m primarily looking to understand it fully.

The IC used for the solar charging section of WisBlock baseboard doesn’t have MPPT capabilities. This means the efficiency of amount of energy that you can transfer to the battery is highly impacted by solar panel’s electrical output (in terms of voltage and current). It can’t track properly these parameters so 1W and 0.5W can have huge difference. Unless we have voltage and current comparison (ideally in or out) on the charger IC, it is hard to determine the difference and how much.

As with discharging faster in the beginning, in theory, standard li-ion cells has this characteristic and usually settle around 3.7v. But this is highly related as well on the state of charge of the battery and the discharge current (not mentioning temperature). Without enough data, it can only assume that maybe the circuitry wasn’t able to utilize the delivered power (due to lower voltage/current) or could be that the cells do not deliver actual 1W or 0.5W.


I can’t measure that in detail unfortunately (don’t have the tools for it)…

Just to follow up for others here:

  • It seems the 0.5w panel just keeps my voltage around 4v. It’s as if that never produces enough power to get the battery to 4.2v
  • the 1w solar panel sort of sinks to 4v before it charges to 4.2v again.

This in itself is fine, my power usage is low enough to never really go below 3.8v on a day when there’s a few hours of sun.