Like so many else trying to charge a power bank from a solar panel:
I've got a solar panel claiming 18V and 280 mA, so 5W and a 2800 mAh power bank expecting USB in so 5V 1A. The simple "circuit" I've made is simply a 5V, 2.5A step-down voltage regulator connected to the power bank and the panel.
My problem is that the voltage drops to 0.8-1V when in direct sunlight and connected to the power bank. I don't know what charging circuit the bank has (a cheap super market bought one) has but it seems like it's trying to take as much as it can up to 1A and that brings the panel on its knees. The 1V I get is too low to make it charge.
Is there something I can put in between the panel and the bank to either limit the maximum current the bank gets or something charging up and letting out when it's enough to power the bank?
I know the regulator is completely over dimensioned but I had one at hand I assume it won't be the problem. I'm also hoping to get my hands on a more suitable charging IC and battery but before that I'd like to get this prototype working.
*edit:What I know about the power bank is that it's got USB in and USB out. It says 3.7V and 2800 mAh so I guess it's a single cell li-ion in there. I don't know the tolerance of the bank, I've only got access to 5V and the panel. So I know it charges at 5V and doesn't at 1V. I'm getting a lab power supply this weekend so then I can figure that out.