Noob/beginner question - Circuit board design & production

Thread Starter


Joined Nov 13, 2022
Hi all,

I'm new to the Forum & to circuit boards. My apologies upfront if I post this question under the wrong section.

I'm into product development and my most recent finding requires a not-so-traditional circuit board design. The idea is to have a 4000mah battery & small solar module attached to it. It will serve as a led lantern & power bank, that can be charged via solar or USB.

I'm trying to figure out, what would be the best way to design the circuit board to get some prototypes manufactured. Any pointers in the right direction are highly appreciated.



Joined Jun 5, 2013
Have you calculated the charge time from the battery size and solar panel output? My quick calculation comes to about 2 sunny days.


Joined Jun 5, 2013
That is more like 4-5 sunny days. The best you are going to get is the equivalent if 6 hours of full sun in a good day.


Joined Dec 23, 2011
If that is a lithium ion-esque battery, you probably know this, but it will run need charged in constant-current mode until a particular voltage is reached, followed by constant voltage, where the battery will then absorb the Coulombs it needs in its own time to finish charging.

Let's assume the 4,000 mAH rating means that from full charge (4.2V or so) to 3.2V it can be used and can deliver a total of 4,000 mAH of charge in that 1V window. Typical constant current charge rates are some fraction of output rate, probably around 100-150mA for this battery, but I would refer to spec sheet to be sure. If this number is close to accurate, that's 26 2/3 hours, which is right in line with your guys' estimates...but that's only constant-current mode, it takes several more hours for that last 10-20% in constant-voltage mode. And when it's 120F outside or worse in your car, or very cold, this all changes quite a bit...which is why batteries typically have a wire to give temperature information.

If the specification for that battery allows more current in constant-current mode than the above estimates, that would help bring it down into the 8-10 hour range perhaps. The only way you'll make this practical with a solar panel is if it outputs a higher voltage to begin with and you buck it down to the levels needed for your charging purposes. As buck regulators are effectively DC transformers (as an analogy), you may be able to generate comparable USB charging currents, but I doubt it if the solar panel is small. They use this trick in those e-cig things; lithium-ion battery voltages stepped-down to very low voltage but high enough current to run a filament to heat the liquid...I think, anyway.

My suspicion is that the solar panel would output AT MOST 100mA at around 2V, which means that you'd be actually boosting voltage and halving current, which would cause this thing to take just shy of an eternity to charge on the sun alone. I'd ditch the panel, myself, unless there is information you know that I do not that makes it practical...

As an aside, I work at a company that uses lithium-ion and lithium iron phosphate batteries in practically everything we make. I am often in these conversations and this is how I can share some information on charging, rates, real-world "gotchas", and practical numbers.

Best of luck,