Nokia: the Sun Can't Charge Your Phone

itwbennett writes "Nokia's research into solar-powered cell phones ended with a (barely audible) thud. Under the best of conditions researchers were able 'to harvest enough energy to keep the phone on standby mode but with a very restricted amount of talk time,' Nokia wrote in a blog post on Tuesday. Not surprisingly, the prototype phone, which had a solar panel on the back cover, performed better in Kenya than in other testing locations, like southern Sweden and the Arctic Circle."

Re:Why did they think this would work?

[viperidaenz]

You can get 0.5W panels about the size of a smart phone for $2.00 [seeedstudio.com]. considering they only have a ~5w/hr battery it should be possible to get an 80% charge in 10 hours. The problem being that solar power drops significantly when not in direct sunlight, partially covered, through glass, not perpendicular... etc.

My cocktail napkin agrees

[__aaltlg1547]

You can't get more than 100mA of charging current out of a collector on the back of a cell phone.

With a typical battery capacity of 2700 mAh, that means it would take 27 hours of vertically incident sunlight to charge your battery.

Good luck with that.

Re:But does it extend battery?

[sfm]

Unfortunately, letting the phone sit on the dash of your car while
charging causes it to heat up, significantly reducuing the life
of your lithium battery. A better choice is to use an external
solar panel to ship power ot your phone (which is tucked safely
away, out of direct sunlight). So have we come full circle on this ?

Sound waves don't carry enough power

[Anonymous Coward]

A 100dB sound pressure at 10cm (lets assume you can achieve that by screaming very, very loudly into the phone, say when you're talking to your boss ;-) will have a sound power of maybe around 90dB (sound power & sound pressure are two different things).

As sound power is referenced to a level of 1 picowatt, 90dB represents an actual acoustic power of 0.001 watts. This is how much power you're putting into that scream. The phone only sees a small part of it, the rest 'leaks' into the surrounds (letting the neighbours three doors down overhear your latest 'performance review').

I can't see that charging a phone any time soon. Even microphones, which are specifically designed to be as efficient as possible in converting sound waves into electrical signals, usually require pre-amplification before you can do anything useful with the signal.

As an aside, the very low power levels associated with actual sound waves is why most stereos / home theatre setups are grossly overpowered. I have a 65w per channel amplifier, and with some custom-built high-sensitivity speakers, I've never turned it up much above -20dB, and that's painfully loud. That's less than 5w per channel...
(Note: really low-frequency *does* require a lot of power, as it needs to move a lot of air to get the same sound pressure level, which is why subwoofer amplifiers are often rated at 5-10 times the main amp - my sub has a 450w amp in it, for example)

Re:Why did they think this would work?

[Anonymous Coward]

They killed it because it didn't work. Didn't you even read the summary or the article title? Although they should have been able to figure out that it wouldn't have worked based off calculations before they left the office. I'm cynical so rather than give them credit for testing prototypes, I think they must have known it wasn't going to work, but tested it anyway as a PR stunt.

Re:Why did they think this would work?

[quacking duck]

Don't know about other phone chargers, but my iPhone USB charger block registers 0W on my watt meter when the cable is plugged in but no iPhone is attached.

Re:Why did they think this would work?

[nedlohs]

Over 3.5 years for me (by the numbers in that link) - my phone does a discharge/charge cycle once a day basically.

It's hardly suspiciously low.

My phone has a battery in it with 3.7V and 1300mah. My old high school memories tell me that P=IV, so 1300mah*3.7V = 4.8Wh. My electricity bill says that the distribution charge is $0.0654531250/kWh and the energy charge is $0.113187500/kWh (more expensive than the original calc used, and by lord how many decimal places do they want to use...)

So one charge would cost if that was the price of abstract electricity:
(0.0654531250+0.113187500)/1000*4.8 = $0.00086

So $1 gets me 1166 charges, at once a day that's 3 years (Wh are likely higher due to the voltage actually being higher when fully charged, but we have enough slop over 3 years to cover that).

The charging efficiency while not 100% is high enough that it is more than covered by the fact my phone doesn't actually get to 0 charge each day - it tends to have 25% or so left.