Simple solar cell phone charger

25 Jan 2010
Posted by Laptop Junction

Cell phone batteries can be charged with a simple solar panel and USB Car cigarette charger. But what happens on a cloudy day? Find out here – based on measurements results in southern Europe in winter time.

Simple solar powered cell phone charger

There’s a lot to say about charging batteries with solar power. Information on the internet is enough for a life time of reading. So here’s the other end of the spectrum, a real example of charging a cell phone battery using a solar cell phone charger in winter time. Not the perfect conditions – but well - stuff happens when traveling.

Solar Cell Phone Charger Measurement Setup with USB Cigarette Charger. Solar Cell Phone charger measurement setup

The solar panel is a 12 Volt – 200 mA – branded OASI - produced in 2003 by SOCO for Thuraya – a satellite phone company. It has 2 solar cell strips connected in series underneath a glass plate with many small bubble lenses.

The USB cell phone charger is a cheap, a “made in china” 12 - 30 volt to 5 volt USB charger. It runs on a 1 chip DC-DC step-down converter. The same already used with a Nokia N95 8Gb smart phone.

The solar panel power source

So let’s have a look at the solar power source before staring charging the cell phone battery.  A few ratings

  • 2.54 Watt OASI Solar panel  (12.7 Volts – 200 mA)
  • 5 Watt USB cigarette lighter charger adapter (5 Volt – 1000 mA)
  • 3.77 Watt Nokia BL-5C battery (3.7 Volts 1020 mA)

The Nokia AC mains cell  phone charger has a power rating of  4 Watt (5v – 800 mA). It takes about 66 minutes to charge a Nokia BL-5C battery. During charging the AC power meter measured about 5 Wh with 6.4 Watt peak power. Efficiency rating = 3.77 Wh / 5Wh = 75 %. So 25 % is lost in heat and power needed for the AC / DC conversion.

Based on voltage measurements with different solar panel angulations, the small bubble lenses really work. It doesn’t matter that much if the solar panel is 20-30 degrees off from being prefect perpendicular towards the sun.

Solar cell phone charging test run

So what happens when the solar power panel is taken outside and a USB car charger is connected? Here’s the answer – measured with a Fluke 123 Scope-Meter in trend plot mode.

Connecting a USB car charger and Nokia N3110 Classic to a Solar Power panel
Free Link TF C501000 USB cigarette lighter adpater car charger Solar Cell Phone Charger Voltage Output Trend on a Cloudy Day

The solar panel outputs about 12.5 Volt on a cloudy day. Connecting the USB car charger with 5 Volt USB output drops the output voltage to about 11,5 Volt. The voltage drop is caused by the 1 chip DC-DC step-down converter alone. Next the Nokia 3110 Classic is connected. Now the solar panel output voltage drops to 5.8 volt level and the Nokia shows it’s charging the cell phone battery.

So what causes the voltage drops seen at the output of this 12 volt solar panel?

In simple terms – the solar power panel can not deliver enough power (Amps) to sustains it’s 12 volt output. The voltage drops to the input voltage required by the 1 chip DC-DC converter.  Consequently, the current to charge the battery is low(er) compared to when the solar panel sits in full sunshine. Thus, charging the cell phone battery will take much longer – assuming if it charges at all. However there’s more to it.

Solar Cell Phone Charger Voltage Output Trend with cut outs ton a Cloudy Day

Nokia N3110 Classic cuts out and battery charging stops

The voltage trend shows spikes – meaning the Nokia 3110 Classic stops charging for a fraction of a second and then continues. The small current is apparently below the charge threshold so it cuts out shortly – then continuous again because the charge voltage is still there. Then the sun suddenly started to shine and solar panel voltage jumps to about 13 volt with the Nokia 3110 connected.

The Nokia 3110 still shows it’s charging the whole time. But was there any effect? Checking the battery icon shows no difference at all. Hence the charge indicator of the Nokia 3110 seems unreliable on cloudy days. When power is removed – the charging indicator keeps moving for a few seconds. This confirms the Nokia N3110 Classic charge indicator is unreliable in cloudy weather.

During the test run the Nokia 3310 was switched on and connected to a cell phone network all the time. Switching it off while charging on a cloudy day most likely leaves more charge current to go to the battery.

Simple Solar cell phone charger in action

So here’s the 5.5 hour trend of the solar panel output voltage while charging the Nokia 3110 cell phone battery on a cloudy afternoon. The phone is switched off during the charge cycle. The battery was on low power – a message that appears when it’s switched on.

Solar panel voltage trend while charging a cell phone battery on a cloudy day

Solar panel voltage trend during 5.5 hours of charging

The cell phone battery did not charge during part of the measurement. Clouds passed over and the voltage output of the solar panel dropped below the charge threshold. There was full bright sun on the solar panel during 30 minutes – the voltage jumped to a maximum of 14.9 V. Based on the 30 minutes of sun, at least there should be more power in the battery.

Switching the Nokia 3110 Classic confirmed the battery was charged. The "power low" message did not appear. Best case the battery was charged for 30 minutes at 200 mA - the max Amp rating of the solar panel. That's the equivalent of 100 mAh @ 12 volts = 240mAh @ 5volts. The BL5-C battery is rated at 1020 mAh - so at best 30 minutes of full sun added 23 % more. That sounds like brochure talk.

The BL5-C battery wasn't new, the USB charger need power including the LED and it's winter time.. Most likely if the battery gained 10% more power it's already too good to be true. Anyhow - it made a difference.
So even on partly cloudy days a solar cell phone charger does work - if the phone is switched off.

Simple Solar cell phone charger results

Results of the solar cell phone charger measurement:

1.    Charging on cloudy days only makes sense when the cell phone is off
2.    Forget about a full cell phone charge on cloudy days.
3.    More portable solar power would help
4.    Nokia 3110 charge indicator is unreliable for charging on cloudy days.
5.    Battery charging starts when solar power exceeds the charger voltage. (> 5.6 V)

New solar battery charging technology called MPPT increases solar panel output upto 30%.

Read about Powering a laptop using solar, wind or human muscle power

Other findings

1.    The charger LED indicating there’s output power is very helpful
2.    Solar panels with bubble lenses prevents constant solar panel angle adjustments

Cell phone battery charging makes sense on sunny days with clouds. On full overcast skies battery charging is minimal to none.

Cell phone features that would help

3.    Reliable and instant charge current indication
4.    Powering a cell phone directly on solar power without a battery

Battery charger features that would help

5.    Standalone battery charging
6.    A switch on the USB power LED preserves a few mA of solar power
7.    AC mains to DC 12 volt step down converter
8.    MPPT battery charging feature
An impression of a cheap 180 Watt solar panel made in China.

Measurement details

Nokia 3110c – RM237
Version 07.21
Date 05-11-08