Hey, I aam new to solar panels and wanted to ask a few small questions. I have a system that is running 3.3V and 0.15A continuously and needs to run 24/7.
I calculated the power at roughly 0.5W and 12Wh (please correct me if I am wrong).
I am looking at this battery to power the system as it has a bit more energy in it than I need.
Luke
Refer the linked solar panel.
Please note that while the actual panel max output is 10W (6V @ 1700mA) the output device as a whole is nothing like that as noted (I assume by Core after measuring) in the Core description “NOTE: While this product is rated at 10W we found the actual max output to be around 3W (5V at 600mA).”
You use 12W over 24hrs so this has to be replaced. Remember you only get anything like max from a solar panel for something like 2hrs tapering off as the sun moves from optimum with useful output for only 5 or 6hrs max. This is your charging time so you would probably need a minimum input to your battery for charging of 6W. Allow for efficiency losses for charging and any other conversion processes and you would be looking at 10W++ minimum. If it were me I would consider 20W minimum, but I am pretty much keen on plenty of headroom.
Cheers Bob
This all looks correct to me. You seem to have your head around it pretty well but I’ve found this video is an excellent summary of how to use and convert AmpHours, WattHours, and a quick bit on C ratings.
A solar panel will have an output voltage and current that varies over quite a wide range when coupled with different loads and light conditions so they are almost always combined with some sort of solar power manager, that may or may not implement MPPT (a system for maximising the power generated from the panel).
Some products do this well and include all the recommended circuit protection like the PiJuice HAT, unfortunately, we aren’t like to get more stock of the PiJuice until sometime next year.
There are some more basic solar chargers out there, but many of them are designed to be battery chargers, not a power supply and charger all at once.
One of DFRobots Sunflower boards could possibly be a part of your design, but would need a 3.3V regulator coupled with it, which will reduce efficiency.
Hi Luke
If considering the Sunflower unit as mentioned by Trent above you would not combine that with the Solar panel / charger you mentioned. That panel has some electronics built in which supposedly outputs 5V and you would have 2 units adding to the efficiency losses. The Sunflower device has the charging circuitry on board so you would use a bare Solar panel with it. Beware this unit has a maximum input voltage of 6.5V and appears to be a pretty sound device. Max charging current is 900mA which should be enough to keep charge with just a few respectable charging hours. Downside it outputs 5V and unless your device can be configured for a supply of 5V you will need a converter to reduce to 3.3V. Check on this as some devices have an on board regulator to provide the option for 5V input. A converter will add to efficiency losses. The Sunflower converts from 3.7V to 5V and another one from 5V to 3.3V, both conversions have efficiency losses which could add up to something like 30%.
One other point. Your battery must be capable of a charge current of 900mA. Anything less and the battery could be damaged and if you cut down on charging current you run the risk of not getting enough into the battery in the very few hours of charging time available to operate your system 24/7.
Cheers Bob
Thanks for sending that video through. It was helpful and helped me go through things to make sure I understand them! I am checking out the charger you recommended now too. Thanks so much.
Hey, thanks for all the help. I am thinking that the sunflower unit mentioned above is a good idea. I was already looking at that, but I will try to ensure I account for the potential losses.
Oh good catch! I saw the typical voltages for various loads at the bottom of that page and assume it was just a bare panel. It is a little unconventional to list the panel output specs if it’s going to be passed through a buck converter after that, but it does allow you to determine the Power Curve of that particular panel.
A bare panel will quite often give somewhere between 3 and 18 volts output when in sunlight and the maximum power output occurs at the Maximum Power Point of the PV curve (which looks a bit like a knee when you graph it). If your panel has some sort of regulator built-in great! If it doesn’t use a power manager, avoid doubling up on converters wherever possible as every conversion loses efficiency.
Hi Leslie
Don’t forget to convert to watts and do all your calculations with that. That battery you linked quotes 4400 mAh but that is at 3.7V. Entirely different at 5V, 3256mAh.
Do all your calculations and estimates in watts and W/hrs then you can convert back to mA and mAh at the end of the line. You may be surprised.
Cheers Bob
