I bought a small 10 W solar panel which is rated to deliver 570 mA at its peak, according to the specifications:
- Power: 10 Watt
- Max. Voltage: 18 Volt
- Max. Current (Imp) : 570 mA
So I bought a step-down converter that drops the voltage from 18V to 5V. Then I connected a USB port to it to charge the phone. Now, the problem is that it takes days to charge the phone battery, which is rated at 7.22 Wh.
So with a maximum power of 7 W from the panel (let's say that it will never reach 10 W), it should charge the phone in roughly one hour. Well, this is not happening and I would like to know why…
I initially thought it is the converter, so I have tried also with a 5V Voltage Regulator (the well known LM7805), and it has the same problem. Moreover, when I use an LM7805 and connect the phone, the voltage drops to 3V, so I suspect that there is not enough current from the panel.
The Step-down converter is this one, which has the specifications:
- Rectification: non-synchronous rectification
- Input voltage: 7V-35V
- Output voltage: 1.25V-30V
- Output current: adjustable maximum 3A
- Conversion efficiency: 92% (the highest)
So I have these two voltage regulators, and it takes days to charge a phone. Now, the question is: am I doing/understanding something wrong or can the solar panel not handle this? Is there any way to check this out?
After seeing the comments and the answer, I tried to measure the current in full sun. I get this:
This mean that the current is 0.7 mA, right? It's the first time I'm doing this and I followed this sketch to measure the current with a multimeter:
In my case, the battery is the solar panel and the light bulb is the phone.
I hope the measurement is correct.