Electronic – Takes days to charge phone from 10W solar panel

battery-chargingchargingdc/dc convertersolar cellvoltage-regulator

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?

EDIT:
After seeing the comments and the answer, I tried to measure the current in full sun. I get this:

Photo of digital ammeter reading 0.7mA

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:

hand drawing of circuit to measure current

In my case, the battery is the solar panel and the light bulb is the phone.

I hope the measurement is correct.

Best Answer

First make sure everything is working as intended. Point the solar panel at full sun and see what kind of current you can get at 5 V. Try a 5 Ω power resistor. That should draw 1 A, which takes 5 W of power at 5 V. You should be able to get at least that according to your specs.

If that works, it may be that the phone is expecting a "smart" charger. It uses the USB charging protocol to communicate with the charger to find how much current it can draw. If it can't talk to the charger, the phone falls back to a rather low current.

Measure the current with the phone connected, first making sure the output really is at 5 V.