Solar Phone Charger Questions

[AndrewGee]

Hello. I’m new to solar energy and would like to build my own solar powered phone charger. I have tried to learn as much as I can from these forums as well as the wider internet, but please forgive any naivety. My main concern is cost, although I would set myself a target of charging a 3.7V, 1420mAh battery in under 10 hours. I will use this example battery throughout this post. Firstly, I’m looking for clarification that my calculations are correct: if I have a panel outputting 100mA at 3.7V, would this mean it would take me 1420/100 = 14.2 hours to charge said battery, assuming the phone was not consuming any electricity during the charging period?

I’ve purchased a Powerfilm mini panel which outputs 100mA at 3.6V, just to run some tests.

http://www.selectsolar.co.uk/prod/264/powerfilm-mpt36150-100ma-36v-mini-solar-panel

This is a 0.36W panel, but the voltage of the panel (3.6V) and the battery (3.7V) don’t match. Since P=IV, does this mean that said panel would output a current of 0.36/3.7=97.3mA rather than 100mA? I basically don’t understand why panels usually have a voltage and current rating given rather than just a power rating – if a panel is rated at 142mA and 37V (P=IV=5.25W), does this mean I could use such a panel to charge the example battery in just 1 hour? I have a sneaky suspicion that the answer is no, but I couldn’t explain why.

I’ve seen a number of solar products which take something like 8-12 hours to charge a phone, and to me this seems rather slow (compared to wall chargers). I understand that there is a limit to how much current we can put in, as this would generate excessive heat and cause damage to the battery, but could I, assuming I had the world’s best heatsink, charge a battery quickly by using 10A? How do I calculate the practical ampere limits for charging?

Can I charge a phone battery directly from the solar panel, using an LM317T voltage regulator combined with a resistor to regulate current, and a Zener diode with resistor to act as an automatic cutoff switch when the battery is charged? For a low power application such as a phone, is MPPT necessary and are its efficiency gains significantly greater than the power it consumes?

Again, I am a solar newbie, so thanks for reading and I look forward to hearing your answers.

[Mike90250]

you need to have a charge controller circuit designed for the battery you are trying to charge. Otherwise, you will soon get results you will not like, such as Damaged battery, undercharged battery, exploded battery. The odds that you will spontaneously find a panel that will charge a sealed battery properly, are close to zero.

Most likely you can find a charging circuit to work from a 5V or 12V source. Then you use a large enough solar panel, that it can power the charger without requiring another battery.

[PNjunction]

Andrew - the basic problem here is that your cellphone already incorporates an internal charger, which wants to see a stable source of 5v. The voltage from the wallwart of the phone is not directly applied to the batteries as there is a regulator / charger and possibly balancing circuit inside.

In solar, you have 3 major components: Solar Panel > Charge Controller > Battery. With the setup you propose, it won't work as the charge controller is not designed to power yet another charger directly internal to the phone. The wallwart typically only supplies a steady 5v at the required current, and the panel being unstable is not suited for the job, even if you use a simple 3 terminal regulator rather than a dedicated charge controller. Yes, you can if you want to go ugly and suffer phone resets and the like as clouds pass.

What you can do going solar, is charge up an external battery, which is then attached to the phone through your typical mobile 12v > 5v usb adapter, or your own regulator circuit. Boutique-panels, like Goal Zero adopt this approach - instead of a real solar charge controller, they have a 5v regulator output as well as an UNREGULATED 12v output which I find a bit less than useful.

In this case, you would do well by using a 7ah agm battery, like a Powersonic, UBS, or the like as your portable battery source. That, along with say a 20-30 watt NOMINAL 12v panel, (typically 18v ocv) and a charge controller like a Morningstar 4.5 Sunguard will do the trick charging the external agm battery. Or just bite the bullet and get a GZ setup for the phone with a dedicated 5v output.