Do It Yourself

You would not be able to accomplish this without an inverter setup. The only way those little powerbank things work is because those panels barely charge the bank, and the charging happens at a trickle rate that doesn't harm the cells with current variation.TLDR the power bank is just normal battery cells, the panels are just a bit gimmicky.

As long as you regulate the output of the solar panel down to 5V then it should be doable. If the power bank supports USB PD charging you can get a USB PD module or car charger that takes 12V in, regulate the panel voltage down to 12V and power the PD module.

That said, solar panels are not going to provide consistent voltage due to clouds and shadows and such. That's why pretty much every solar system has at least a small battery to buffer the output of the panels. Since you're just charging a power bank it's probably not a big deal.

Also, any large panel will be completely wasted charging a USB power bank, so you should just get a small 10-15W or so panel. A power bank won't charge much higher than that at 5V.

I'm in the middle of a solar install, and I was surprised upon getting into the weeds that you can't just go panel to battery. Not an expert by any means, but here's my takeaway for your question:

• the powerbank is designed to charge at a particular voltage (range) and current
• the panels will output whatever voltage and current they produce (this varies by panel and by sunlight)
• something needs to be between them to convert the solar output to a form the battery can accept without damage

If you're really intent on charging a battery via solar, there are loads of ways to do this, but a powerbank, even with a small panel attached, is not an intended use case for large solar panels.

I picked up this pair of 10,000mAh battery banks last year after a series of weather-related power outages made me start rethinking my emergency kit. If you look at the description, the input voltage and current are unspecified.

However, the manual says "Connect the bank and an adapter with a USB C cable to start charging. For the best performance, a 5V/2A adapter is recommended." V×A = W means the most power my battery banks can handle coming in is 10W. Completely ignoring the need to step voltage down and address current, a panel larger than 10W starts to cause issues; the solar power you're generating in a circuit needs to go somewhere, which is why when doing a solar install, the charge controller gets connected to the batteries before the solar is connected.

Now, here are the 100W panels I went with. Massively simplified, each 100W comes by way of 20.4V at 4.91A.

The spex for your battery bank may or may not be the same as my banks. It still comes down to a 100W panel needing to be connected to something that can handle 100W of input lest you have a bad time.

So the issue isn't that it can't be done with some form of charge controller because of the variation on both volts and amps throughout the day and year; rather, it's that it can't be done without wasting up to 90% of generated power, which is generally where one starts looking for a new solution, especially because you're at best getting in the mid-20%s for panel efficiency to start.

You have a good idea but the wrong parts and scale in mind to execute it.

Charge controllers under 60A are available for cheap, but you quickly run into diminishing returns in terms of bang for the buck below about 30A (I've done zero research at this level), which start around $25. A 30A controller at 12V can handle 360W of input, so you could get three 100W panels.

A 10Ah 12V LiFePO4 battery can be found around $40. Randomly clicking on one, I found the max charge rate to be 6A, so at 12V, now you're good on the panels and charge controller, but the battery can only handle 72W out of the 300W. Oh, and it's 2½ pounds and 6x4x2½ inches, has exposed terminals and has the same capacity as a 10,000mAh bank while giving up every advantage a powerbank brings to the table.

We're not even to the point that you can charge anything via USB, and you're needing to think about wire gauge for each connection so as not to start a fire.

To sum up: Big panels need big batteries. Small batteries are current-bound and thus can only accept small panels. Even changing the destination from a powerbank to a standalone battery of the same capacity does not alleviate this situation.

Wow thanks to all of you for the very detailed replies, especially you, @Powderhorn@beehaw.org !

Looks like I'll save your answers and keep them in mind for when I have the nerve to deal with all that effort (we had the idea of installing solar panels on our car roof and buffer it to a second car battery for extended camping trips)