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Is it safe to connect a small 5W 5V solar panel directly into a portable power bank?
(self.solardiy)
Can I connect a 5w 5w solar panel directly into a 10k mAh portable battery bank safely via USB 2.0, or does it need some sort of protection in-between like a solar charge controller or a schottky diode?
Example of battery bank (except one with usb 2.0 input): https://www.anker.com/ca/products/a1229
Example of solar panel: https://www.aliexpress.com/item/1005006012344670.html
It might be, or it might not be. It depends on several factors.
The first thing you're going to want to do is check the output voltage of the solar panel. You can either use a multimeter or a dedicated USB tester. You're looking for a stable voltage of 5V +- 0.2v. If your voltage is outside of that range, you're going to want to throw a voltage regulator in between it and the battery.
While the battery might be able to handle a larger range, there's no guarantee that it will, so you could very well end up frying the circuitry, or it just never charging because you're outside of the range.
You can get solar charge controllers for under $20. That's just an example of what to look for.