# DIY charger: solar panels in series or in parallel?

I am working on a DIY project where I am thinking of using a small solar panel to charge my iPhone.

I know iPhone chargers usually are 5 W, 5V at 1 A.

I can see two small, portable, and easily solderable solar panels:

Questions:

1. I cannot connect >5V to my phone as I risk damaging the electronics. But would connecting lower voltages still work? Slower charging, sure, but what's the difference between a high current at low voltage, and a low current at higher voltage?

2. Would it better to use 2x (or more) of the 5V solar panels in parallel, so as to keep the voltage at 5V but increasing the current, or to use 3x of the 1.5 V ones in series, to as to have high current and to reach 5V?

• Buy a solar cell with a built-in regulator and USB power output. Why risk frying your phone because of your inexperience? Commented Apr 4, 2021 at 23:03
• It's for learning. Hence the questions before I actually fry my phone. Commented Apr 4, 2021 at 23:25
• You can't connect solar cells directly to a phone, you need a regulator/converter. circuitdigest.com/electronic-circuits/… Commented Apr 4, 2021 at 23:45
• Thanks that basically answers my question. Commented Apr 5, 2021 at 0:16
• Because you're planning to use a panel well below the input power capability of your phone, you should use a minimum voltage cutout or better yet MPPT.
– K H
Commented Apr 5, 2021 at 2:15

The 5V 100 mA panel probably WILL work correctly with some phones and more in parallel will work better.
Some phones will not accept too low a wattage.

You can add a simple clamp regulator to stop the voltage rising above 5V.
A 4.7V zener diode across the parallel cells (or several zeners in parallel for more wattage) will probably work. Place the arrangement in full sun without the phone connected and measure Vout with only the zener as load.

5V 100 mA = 500 mW so say 1 Watt of zener diode per PV cell will clamp the panel at whatever Vmax you measure. SOME phones will tolerate 5.5V or even more BUT some will not so aim at say 5.1V absolute maximum.

________________________________________

So how low can I generally go with voltage for the phone to still charge as if it were seeing 5V?

Maybe 4.5V - perhaps less, but depends on phone manufacturer, and close to 5V is desirable.

A PV panel that is genuinely rated at 5V loaded will give 6 + V open circuit in good sun, so a clamp zener will be needed to keep it "safe" when the phone stops drawing current. If the wattage available is less than the phone "wants" it will load the panel down below 5V. Some systems will run at a range of wattages - others may not charge at all if available wattage is too low.

You can test this with a variable voltage & variable current power supply.
Set supply to 5V and current limit to max current.
Gradually reduce current limit and see where charging stops.

• Thanks. So how low can I generally go with voltage for the phone to still charge as if it were seeing 5V? Commented Apr 5, 2021 at 5:46
• @SuperCiocia Maybe 4.5V - but depends on phone , and close to 5V is desirable. A PV panel that is genuinely rated at 5V loaded will give 6 + V open circuit, so a clamp zener will keep it low when the phone stops drawing current. If the wattage available is less than the phone "wants" it will load the panel down below 5V. Some systems will run at a range of wattages - others may not charge at all if available wattage is too low. || You can test this with a variable voltage variable current power supply. Set supply to 5V and max current. Reduce current limit and see where charging stops. Commented Apr 5, 2021 at 7:18
• I still wouldn't suggest this, as most well designed smartphones not only check the voltage but also appear to monitor for fluctuations, and will drop charging if the voltage fluctuates up or down too much as that's a sign of a failing or damaged charger Commented Apr 5, 2021 at 8:14
• @Pecacheu Yes/no :-). My simple power supply test will allow how a given phone responds with little risk. There is a very small chance that a phone may be damaged by operation below minimum charge energy, but no competent designer would allow this. Incompetent designers exist :-) :-(. Commented Apr 5, 2021 at 9:18

I'm afraid that with that dinky little 100mA panel you will be sorely disappointed with the charging speed.

I've tried similar experiments with solar panels and you probably won't break your phone but it will probably refuse the charge altogether and give you something along the lines of a "charging was disabled to prevent damage to your device" error. Which is just a generic error your phone gives from over-voltage, under-voltage, or under-current.

For one, you need a 5V voltage regulator. Step down is usually more efficient than step up, so I'd go with series panels for 10V or 12V then step it down.

The second issue is you won't have enough power to charge a phone with just 100mA. The phone's charger isn't just a basic regulator. It's smart, and if you don't give it enough juice, it won't change slowly, it will straight up refuse to charge.

I'd suggest at least 5 to 10W of panels to just BARRRELY get any juice. Is it big? Yes. But it's also realistic from my testing tbh.

You will get more like a puny 1-2W out of quote on quote "5W" panel in direct sunlight in any realistic average conditions (so literally anything that's not an absolutely 100% perfectly clear sky, face meltingly hot summer day). Because even high efficiency monocrystaline solar panels kind of suck.

Also from my testing poly panels are even further off their rating and basically useless for anything other than a novelty/gimmick, or powering solar calculators. So I'd 100% recommend you go mono panels, despite the slight cost increase. And go for 5 to 10W total, you can do a combo of parallel and series in fact, to get the target voltage you want. Though keep in mind that one panel not getting as much sun can sort of "pull the others down"