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How to prevent voltage drop on solar phone charger? Answered

Hello folks,
I made a solar phone charger - cell (12V 400mA - at peak 5W)  and step down DC/DC converter R-785.0-1.0 (5V up to 1000mA). and I am using it as direct charger for my phone (no batteries). When the sun is bright, everything is fine and phone is being charged almost by 1A. When the dark comes, everything is fine - no charging but also no discharging of the phone to a solar cell.

Problem is in the middle, sunlight but not so much. Voltage on cell drops to 6,5 V because converter wants more power and cell cant support that (output is still 5V but almost no Amps). Problem is, that charging circuit in phone is constantly switching on and off(as the load is pluged unpluged) and that can run down fully charged battery within 10 minutes. Is there any solution how to prevent that?- for example some kind of minimum current limiter? Or I saw using 555 timer as a charger controler, but not sure how to use it. Main goal is to prevent that oscilation.

Many thanks in advance.       

Discussions

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-max-

2 years ago

This is actually a common problem a solar engineer would face. The problem is that the output impedance of the solar panel changes with the amount of sunlight hitting it. You will notice that for a given amount of light, there is a particular ballence between voltage and current that leads to the most power. If you do not connect anything to the solar panel, you will notice that the output voltage reaches it's maximum pontenial. However if you connect a heavy current sucking load to it or straight-up short the output, you will have lots of current but a tiny voltage (or 0 volts with a dead short) So the output power in both of these cases is actually ZERO!!!!

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A while ago I have plotted this data in excell for my panels, and it looks sorta like a gaussian or parabolic curve. The problem is that this optimal voltage/current ballence will shift with changing sunlight and obstructions. so you need a smarter charging circuit that can control how fast it charges the phone intelligently to maximize the use of the solar panel. Unfortunately the phone tries to set and control how much current it accepts and the USB standard is only 4.8v to 5.5v (with the extension of fast charge technologies which push 9v or even 12v down a 5V usb cable). So you cannot precisely control the charge current. That is why a lot of solar chargers also include a large lithium ion battery the circuit is designed to charge that instead. This is like a UPS for the phone. When the sun gets too dim the internal circuit can switch over to the battery, and if there is a lot of sun it can charge both batteries at the same time.

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icengiceng

Answer 2 years ago

Oh when the sun fades the solar panel turns into a resistor that the diode prevents from reverse biasing your down converter and attempting to discharge you phone..

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HanoCZiceng

Answer 2 years ago

I think that phone is the sucker, because with no sunlight and everything connected battery is discharging as usual. I did a test with bench power supply instead of solar cell. Setting 12V and very limited current and after I connected phone, voltage dropped to 6,5V (minimum for converter) for a second, so phone stopped charging, after that voltage again rised to 12V, phone started charging what caused voltage drop again and so on. Tic-tac-tic-tac. Analog voltmeter is like metronom. Maybe making larger interval between,somehow, dont know.

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icengHanoCZ

Answer 2 years ago

Seems like the phone plug is keeping the phone more awake by virtue of recognizing the two or more pins.

The Only Idea left is a servo Mechanism that pulls the plug out and pushes it back which may use more energy then you can spare...

Ultimately you may be looking at a larger array and a super cap on best deal either side of the converter to fill in for cloud block periods...

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HanoCZiceng

Answer 2 years ago

I am affraid that this is going to do no difference because phone is not discharging to cell but to itself. I tested that with multimetr and there is no backward current.

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icengHanoCZ

Answer 2 years ago

Do you think the down converter is pulling a stay alive current ?

Or is the phone the battery sucker ?

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Yonatan24

2 years ago

Find a bigger solar power that will be able to supply more amps/current at low sunlight.

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Downunder35m

2 years ago

You can't compensate for non existing power levels.
Best way would be to have a voltage check that disconnects the phone when the supply voltage goes under what the converter needs.