How to use a 49Voc solar panel for USB charging after a calamity (without using proper solar charge controllers)?

Question

Some background first, actual question at the bottom:

Last year December my area in the Philippines was directly hit by a super typhoon, international name: Typhoon Rai.

For three weeks after that happened, we basically had no electricity. That means no phone/laptop charging if we didn't go to malls that had generators. There were other serious problems that came with that calamity, but for this post I'm only focusing on basic USB charging.

Three days after that typhoon, a local solar panel supplier reopened business and yes, all the solar charge controllers were out of stock because it was in extreme demand. For the same price I had the choice between two 150 watt (21 Voc) panels and a 400 W (49 Voc) solar panel. I went with the 150 watt panels because I luckily started messing around with Arduino months before and had several usb buck converters on hand that could handle up to 24volts. I didn't have any 12 V batteries or solar charge controllers at the time so it was the best I could do and that choice did put my family in a more comfortable spot compared to most other people in the area.

I also had the usual bunch of electronics components that come with the Arduino hobby, such as capacitors, diodes, transistors and the like - but they didn't see much use at the time. I didn't have any buck converter that could handle 50 V and if I did I wouldn't be asking this question, haha.

The actual question:

If I went with the 400 W (49 Voc) solar panel, would I have been be able to use it for USB charging?

• apparently we can't just connect two USB buck converters in series. (I don't understand why.)
• I had a small kit of 1/4 watt resistors at the time, would connecting those in parallel, then connecting that to USB buck converters in series work fine even if inefficient?
• Other possible ideas that might be feasible for an Arduino hobbyist? - Without proper solar charge controllers?

I'm mainly asking so if someone I know ends up in a similar situation I might be able to provide some help even when directly sending electronic components can't be an option. (I now have a proper off-grid solar setup for future emergencies.)

Answer 1

One extremely inefficient way to do this would be to build a high-power voltage divider with the resistors in parallel. Drop the voltage down to 1/3 of its original value, then run that to the USB converter. Or you could make an improvised (and far more efficient) switching voltage dropper. Make an Arduino switch a MOSFET on and off really fast, carrying the voltage through. Just PWM it until its on a third of the time and off the other two thirds of the time. All you'll need after that is some capacitors to smooth out the voltage spikes.

Answer 2

In Star Trek, one of the alien races jokes "where are your Starfleet engineers who can turn rocks into replicators?" Yeah, you can't turn rocks into replicators. "Pulling a MacGyver" has its limits. You did as well as could possibly have been done without being Ukrainian lol.

You have no reason whatsoever to second-guess your choices there. I've built high power voltage dropper resistors, and there is no way you could build a viable dropper out of 1/4 watt resistors.

Now, the key is to be prepared for the next one. Tsunamis, earthquakes and volcanic eruptions are part of life on the Pacific Rim. For people who don't like that sort of thing, there's Ohio, but then it's 500km to the nearest Jolibee.

Answer 3

If you can find a way to load the solar panel down until its voltage is low enough for your buck converter, you can then connect the buck converter in parallel. Hope your multimeter battery is okay.

One idea is to connect it to a bunch of resistors but since you only have small resistors you may need a lot of them.

Another idea is to use whatever partially conductive material you can find. A bucket of water should do. Tape the wires down so they don't move. You can add salt to increase the conductivity if it's too low. Don't drink it afterwards, because electrolysis will cause some of the wire to dissolve into the water. Another partially conductive material you might have lying around is graphite pencil leads.

Another idea is to put a partial shadow on the solar panel. You'll need to sit there and watch the multimeter and adjust the shadow, because the voltage will change as the sun moves.

Actually you will probably need someone to keep checking the multimeter with any of these ideas. The amount of sunlight falling on the panel will change its voltage/current curve.

If your Arduino kit includes some nichrome wire, that's a good option. Make your own resistor by using some of the wire. Adjust how much is used (slide an alligator clip along the wire) until you get the resistance you need.

If you have an electric stove, you might try unplugging one of the elements and using it as a big resistor. Or any other heater, hairdryer, or vacuum cleaner. Don't expect it to turn on properly as it won't get mains voltage. If using a heater with a fan, or a vacuum cleaner, beware there's a chance it could overheat if the heater ends up working more strongly than the fan.

Obviously, all of these ideas are pretty terrible. The nichrome one is possibly the least terrible, if you have it.