Friday, July 27, 2007

Carbon Footprint: Cell Phone Chargers

Sorry for the skipped entry in the Carbon Footprint series. My footprint totally died and I had to recharge it for, like, two weeks.

Which brings us to this week's recommendation: battery chargers. Did you know that chargers use power even when the electronic device is not plugged in? In fact, 95% of the energy used by mobile phone chargers is wasted. Why is this? Most electronics these days use batteries, and batteries need direct current for charging. That little black box that blocks the other outlet whenever it's plugged in does not 'know' if the phone is plugged in at the other end; it just goes about its business converting alternating current from the socket into low voltage direct current for your battery. This activity is called a phantom load. This kind of appliance is known in green circles as a vampire.

My AC adapter is rated at 4.8 volts and a 350 mAmp current. That means that fully loaded the phone's battery resistance is almost 14 Ohms. Ohm, my! When the phone is not plugged in, the resistance has to be much lower, especially since the voltage from the wall is 120V, and according to some engineers at Cambridge, a lonely charger sucks a miniscule 0.472 Watts. Even so, according to a CNN article, chargers of all kinds make up fully 5% of your electricity bill every month. Unplugging them is an easy way to cut your bill and your carbon use. Plus, it will be easier to use other devices in the outlet without that bulky plug in the way.

Update 8/19/07: I have altered this column slightly to address concerns of accuracy brought up in the comments below.


Anonymous said...

I would be very curios to see the 4th grade math you used to get to the 14 ohm conclusion. I am not surprised to see CNN as your source of info, seems to go hand in hand with uninformed, naive, gullible people such as yourself.

Larry Brewer

thomas said...

This comment perplexes me. What is it that makes you so angry about saving just a little energy. You are right that my "4th grade" math is quite simple - it is based on one of the most accepted principles in all of physics: Ohm's Law.

V=IR; or R=V/I

I used the information on my phones AC converter label:

R= 4.8 Volts / 0.35 Amps = 13.7 Ohm

This is the rating for a charger in use, so these are all estimates, but I would not expect the physical property of resistance to be so off to invalidate the estimate. Is there something intrinsic in the circuitry of chargers that would change this calculation? I am happy to alter my calculations if you provide me with more information about the AC converter's internal circuitry. In absence of that, I will take the CNN, BBC and other reputable outlets' word on this.

Speaking of taking people's word, Professor McKay's website offered that the scientists at Cambridge found (I assume in a bomb calorimeter) that a charger without a load generates 0.472 Watts. This is a measure of the amount of heat that the empty charger emits. (Have you ever felt a phantom charger - they get warm!) So a charger just like McKay's in the Cambridge lab emits 0.5 W, yet his power meter needs six devices to register 1W!!???

This home experiment is cute, but why Dr. McKay did not account for one important factor: Time.

Watts are a measurement of power. (Newton-meter/second) The reason your power bill uses units of kWh (h for hour) is that the power company charges you for the total power consumed over time. Not only is the power meter used in David McKay's website not designed to measure heavy loads (I wouldn't trust any reading under 2 W), you both overlook the fact that (empty) chargers are plugged in all the time.

It seems reasonable to me that such a phantom drain is, much like your leaky kitchen faucet, be a pernicious drip, drip, drip energy sink.

Finally, the suggestions I make on this site range from small to large; easy to hard. The time it takes to argue for your right to keep a cell phone charger plugged in when you are not using it might have better been spent unplugging some unused appliances. I know you inspired me to recheck all of my plugs!

Anonymous said...

I don’t have a problem saving a little energy. It’s just how little are we talking about is the problem. Your numbers are wrong.

The information on your cell phone converter typically represents its max output capacity. If the phone is not plugged in that information means nothing. Even if the phone is plugged in the current draw will be less than the max capacity of the charger.

Problem is, you are looking at the wrong end of the device we are discussing. The voltage provided is 120 volts AC. If the device had a resistance of 13.7 ohms, as you stated, we would see a current of 8.75 amps. (I=E/R) . Power (watts) = voltage x current, or 120 volts x 8.75 amps = 1051 watts. At this point you should realize that you have missed something. The real load is the primary winding of the wall transformer. The "unknown" is the current traveling through the primary. If there is no load on the secondary (cell phone unplugged) the current thru the primary is almost zero. Zero with a cap Z.

BTW. Just checked my cell phone charger, plugged in overnight. It is not warm, not even luke warm, like dead cold would be a better way to describe it. Check yours.

Dr. McKay’s experiment need not mention time. 1 watt is just that, 1 watt. Plug in 1,000 one watt devices and you’re using 1 kilowatt. Leave them on for an hour and you’ve used 1 KWh. He left nothing out in that regard.

Your drip, drip, drip sorta makes sense unless the drip is so small that three years of dripping wasts less than even 1 flush of a low flow tiolet.. See where I’m going with this.

Why would Dr. McKay use a watt meter designed to measure heavy loads? Don’t understand that one. If anything the load is too small to be measured accurately with his device. Which is my point. The power wasted by a cell phone charger over a year’s time probably wouldn’t lift a garage door. Make that 5 year’s time.

Finally. Outside your house or apartment is a transformer provided by the utility company. The primary voltage is as much as 25,000. The secondary 110 – 120. If you disconnect everything in your house and even throw the main breaker on your electrical box, the power to that transformer’s primary is still connected. The utimate phantom appliance. This would dwarf anything going on in your home.

Michael said...

To find out how many watts were being consumed by my cell phone charger (the one that came with my low-end Verizon Samsung phone), I measured the AC current out of the wall (115VAC) when the cell phone was not connected. The AC current was 0.42milliamp (0.00042 amp). This corresponds to a power consumption of 115 x 0.00042 = 0.048 watt (watts = volts x amps).
When the cell phone was plugged in, the current was 29.4milliamp, for a power consumption of 3.4 watt—about 71 times more than when the phone was not plugged in. If you charged for 8 hours once every three days, leaving your charger plugged in all the time, your charging operation would consume 27.2 watt-hr (0.027 kw-hr) and the energy wasted by leaving the charger plugged in would be 3.1 watt-hr (0.003 kw-hr).
I also checked other types of battery chargers (for example, the chargers used to charge digital camera batteries); the worst of these consumed ½ as much power with no load as they did with the battery plugged in and charging.
Bottom line: my cell phone charger definitely consumed power when it was plugged in and not charging a cell phone, but the power was only 0.048 watt. Leaving the charger plugged in all the time increased my power consumption by just over 10%, relative to unplugging it when it wasn’t in use.

Anonymous said...

Michael, your reasonably accurate calculation equates to 0.365 kWh per year. In contrast, I wonder how much energy has been used for the design, production, distribution, and broadcast of the TV commercials telling people to unplug their cell phone chargers? Probably a whole lot more than 0.365 kWh times however many people actually listen to that drivel. A single studio light can use 500 watts or more.

Paul Edney said...

I read this thread with great interest and am appreciative of the efforts gone into getting the data correct. I was quite disappointed to read Larry's initial comments which I felt were quite insulting and disrespectful. Correcting people is a great thing to do, deriding them while you do it is not.

While 0.365 kWh is a small amount of power, if we assume that the ad ran on 3 networks once at prime time, it would hit well over 3 million viewers.

I realise this is crass math but let's assume for arguments sake half the people watching leave their cells charging all the time ... the resulting savings if 10% of them follow the advice is 54,750 kWh ... well worth it ... even it it's 1%, at 5,470 that's as much as the average house uses in a year.

That said, I wanted to add that this is not about saving a little power. This is about shifting attitudes and behaviour. If you can convince someone to unplug their cell charger they will start thinking about their power usage in general and begin to explore other ways they can take positive environmental and social action.

The longest journey starts with a single step.

Thanks for this Thomas.


P.S. I tried leaving the cell charger in without a phone plugged in and within minutes it was warm. (I then unplugged it :-)

Anonymous said...

Hi, here's just a few questiona. What is the average device wattage for those cell phone chargers? Or, atleast an average cell ohone charger. Someone please answer in the comments.
:) Thanks.

Dory said...

Hey Anonymous you are 1000000% right, this was a good point too (wonder how much energy has been used for the design, production, distribution, and broadcast of the TV commercials telling people to unplug their cell phone chargers...)

Is like government is taking a piss with us and people like Thomas and Paul listen to them thats why they don't make sense at all man, Paul's charger got really warm ohhh bless him lol and even if it did for unknown reason i can send u the money to keep 4 chargers in your house for the next year mate for free!! And i'm not even a rich guy or something, so is even pointless for us to talk about this anyway, coz from your point of view u could be wasting electricity reading this comment now, look i can write a bit more and i waste more electricity to you guys im so sorry lol bloody hell what is this world coming to man.