You need to do some basic sums to check whether you are able to charge the batteries fast enough for the task .
In essence if you can’t charge the flat battery faster than the fresh one is discharged you will end up with two flat batteries . If this is “ portable” I would guess you need a big panel . Do the sums before you buy !!
Many of the 5v packs have led indicators for state of charge you sense one of those to show when the battery is discharged .
Be ware that most packs cannot be charged whilst feeding a load and the voltage input is often inverted compared to the output ( ie the 0v connection is not the same on input as output , this can make issues when switching packs .
Finally … a 10w panel will only give 10w if correctly aligned in bright sunlight , on average it may be far less.
Search for pass through power supply and then you only need one pack and no switching gubbins .
It's the battery cells themselves that have to go in parallel - not the boosted outputs.
Equivalent to indeed buying a single battery double the size.
That instructable looks reasonable to me at first glance, not digging into it deep. Your main problem now seems to be that for whatever reason you're totally dead set on using ready-made power banks rather than a set of battery cells of the capacity you need and adding the other components as needed to get to what you actually want.
Post a schematic of how you see this connecting together. It doesn't have to be perfect, just legible. Give us something to work with. Identify the components you already have on-hand. Don't buy anything else until we sort this out.
Is it too late to return what you already purchased? The 10W panel is worthless.
The instructable you provided is what I had in mind, using a 4S rather than a 3S. Plus the 50W and 100W 4S I pointed to has an internal BMS provided.
Might be good if you posted a diagram of how you connected your packs to load.
A copy of your circuit, a picture of a hand drawn circuit in jpg, png?
Hand drawn and photographed is perfectly acceptable.
Please include ALL hardware, power supplies, component names and pin labels.
I don't have an exact idea, the nearest thing to a schematic is what I found in the instructable
I will use the battery packs internal bms as bms, since those packs already come with a bms.
Next step would be to add alternative types of charge to the contraption so that I will have something that will give me a bit of energy in absence of solar light/while the panels are charging.
I was thinking for example to use the pressure under my feet while walking as well as wind and water but that is the next step of the experiment (I am all ears if you have suggestions ).
OK, maybe I shouldn't have used the term schematic. On a piece of paper, draw a diagram of the components you have and how you THINK they should be connected together. The components can be simple squares or rectangles. Identify the connections as its voltage. Draw a line from connection to connection showing how you envision it should be connected. Show how the solar panel connects. Are you using one or two panels? Take a picture of this drawing and post it.
It doesn't have to be pretty, or accurate. Just show us on paper what you have and how you think is should be connected. Does your wifi need 12V or 5V?
Tell us what parts you have in your possession right now. What have you already bought?
for now I haven't bought anything in regards the switch pcb. I like your idea of timing the power consumption with the contraption under stress (simulating a worse case scenario where the device is sucking up electricity like there's no tomorrow) and I would use the internal arduino uno hardware clock, or something like this
You keep referring to your project as the contraption. It's still unclear to me what the contraption is. Put It On Paper. Draw out what this contraption is, identifying its major components, connect the components.
You posted Amazon links to some components. No datasheets, user manuals, operator manuals, etc. I have no idea the details of each or how you intend to connect these components together. I don't even know if your battery bank can be charged by a solar panel. You are heading for another disaster.
Tell us everything you know about each component before we move forward.
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