Hello! Long time no see.
This little project of mine was idle for a while, because I had to wait about 4 weeks for the step-up converters (
1 |
2) to arrive and then I needed a free day with sunshine to experiment. Luckily, today everything came together. Please allow me to summarize my findings.
Test subjects:
Nokia 1112,
Nokia X1-01,
Samsung GT-E2120 and
Apple iPhone 3GS.
Main components used: 12V/300mA solar array, LM340T-5 voltage regulator,
cable with female USB sockets and
another cable with a male USB plug on the input side and 10 different plugs on the output side for various mobile phones.
Prior to this solar charge test I tried to charge all four mobile phones with the USB cable from eBay plugged into my laptop's USB port (which we know that supplies stable 5 Volts and 500 mA). All of them were charging fine except for the Nokia 1112; I'm guessing that 5 Volts was too high for him. Also, all tests were conducted with bright sunlight reaching the solar cells.
Results without step-up converters (only voltage regulator with 4.99 Volts stable output)- Nokia 1112: didn't accept a charge at all; I couldn't make it to charge in any configuration so I won't talk about it anymore in the following paragraphs.
- Nokia X1-01: accepted a charge, but voltage was oscillating; it was dropping every second to 3.6 Volts which was too low for the phone and stopped the charging process; then it came back to 4.99 Volts for another second, started to charge, but it dropped to 3.6 Volts again and the process was in a loop.
- Samsung GT-E2120: accepted a charge and it was steadily charging at around 3.9-4 Volts.
- Apple iPhone 3GS: at first it accepted a charge and it was steadily charging at around 4.5 Volts, then I unplugged it to try another phone and when I plugged it back in it wouldn't accept a charge; it was weird, but I tried it a third time too and after plugging it in I got the message that 'Charging is not supported with this accessory'. I gave it a try with the original USB cable which came with the phone, but no luck there either. A bit later I plugged it again and it was accepting a charge.
Results with step-up convertersI kept the voltage regulator in the circuit to keep the step-up converters safe. The converter with the blue PCB (made by
LC Technologies) put out 4.93 Volts open circuit, the converter with the green PCB (no manufacturer) put out 5.35 Volts.
Basically I got the same results as without the step-up converters with a slight difference in the case of the Apple iPhone 3GS:
- Nokia X1-01: oscillating charge, 4.93 Volts dropping to 3.53 Volts with the blue converter and 5.35 Volts dropping to 3.55 Volts with the green converter.
- Samsung GT-E2120: steadily charging at around 3.88 Volts with the blue converter and 3.85 Volts with the green converter.
- Apple iPhone 3GS: didn't accept a charge with the blue converter at 4.36 Volts and accepted a steady charge at 4.80-5.08 Volts with the green converter (4.56 with the screen on).
Conclusions based on individual mobile phones:
- Nokia 1112: needs more research to find out how much voltage it needs to accept a charge; this value is most likely around the 3.7 Volt mark since the original Nokia charger puts out that much.
- Nokia X1-01: needs more research to find out why the voltage keeps dropping and what can be done to avoid this.
- Samsung GT-E2120: very easy to charge since it's accepting a relatively wide range of input voltages.
- Apple iPhone 3GS: relatively easy to charge, but with my current configuration and current sunlight conditions it was on the edge between accepting a charge or not accepting a charge.
Conclusions valid to all mobile phones:
- I had the feeling that with good sunlight charging was worse with the step-up converters compared to using just the regulator. The main reason I wanted a step-up converter was to be able to charge mobile phones in low light conditions too, but in low light conditions this configuration wouldn't work because this particular regulator would supply too low voltage to the converters. In that case these converters would work if instead of a voltage regulator I used a component that regulates only above 5 Volts; under 5 Volts this component should turn off and allow all the voltage and current to pass through and reach the converter freely. Questions is: which electronic component can do this?
- There are huge differences in voltage across different components of the circuit when under load and open circuit and this confuses me the most. I'm aware of the fact that the under load voltages count only; somehow these should be stabilized because charging any mobile phone can be achieved only with stable DC.
To-do list:
- Determining the minimum voltage at which mobile phones still accept a charge.
- It would be great to somehow determine the amount of current that my solar charger can put out while charging different mobile phones, because in this case I only measured the voltages. That way I could learn about the relationship between volts and amps.
- The input voltage of the step-up converters (received from the 5V regulator) was varying greatly. I guess that the converters act as a load too (to a certain degree) and the regulator can't put out a stable 5 Volts as it does without the converters in the circuit. It would be nice to log for a while and calculate min, max and average.
Also, I would appreciate any feedback from you guys who are more experienced to improve this little solar mobile phone charger. I would like to keep it as simple as possible, but at this stage it clearly needs some improvement. Just a side note here: I made a few test with a capacitor (4700uF/50V) and the Nokia X1-01 to check if the oscillation stops, but it didn't help; in some cases the voltage even dropped with the capacitor in the circuit. Maybe a super or hybrid capacitor would do a better job and then my to-do list would be irrelevant because there would probably be no need to have regulators and step-up converters in the circuit.
