Didier Stevens

In a chat with my colleagues, we were joking about charging smartphones with a lemon battery.

And I actually wanted to know what magnitude of electrical energy we were talking about.

So I connected a lemon battery to an electronic load:

I took a lemon, inserted a zinc and copper piece of metal (a couple centimeters deep) and connected an electronic load to draw 1 mA of current.

I let it run for a couple of hours until no more measurable current flowed.

The electronic load dissipated 0,034 Wh of electrical energy over that period. Hence, we can assume that the lemon battery delivered 0,034 Wh.

I’m sure the lemon battery could deliver more energy, by “resetting” it: cleaning the electrodes, inserting them in another place in the lemon, …

After a bit of searching through the web, I’m going to assume that a typical smartphone nowadays has a battery of 10 Wh. So we would need 294 times (10 Wh / 0,034 Wh) the electrical energy delivered by my lemon battery to charge a smartphone.

Except that, the 0,9 V that the lemon battery does deliver, is by far not enough to be able to charge via the USB interface. We need 5V, so, 5,555… lemon batteries connected in series.

On the screenshot above, you can also see that 37 mAh was measured. Notice that you can not compare this to the mAh rating of a (smartphone) battery, because both values involve different voltages.

Comparing this to a button cell like a CR2032 (Dutch Wikipedia article, because there’s no English Wikipedia article): the CR2032 has a 225 mAh electrical charge (on average) and a 2.0 discharge voltage. That’s 225 mAh * 2.0 V = 450 mWh. Or 13 times more than my lemon battery (34 mWh).

Here are more pictures of the lemon after the experiment (one week later):

In my blog post “Quickpost: Standby Power Consumption Of My USB Chargers (120V vs 230V)“, I looked at the power consumption of several of my USB chargers in standby mode (e.g., not connected to a device to be charged).

These are switched-mode power supplies.

They consume considerably less standby power than linear power supplies, like this one:

These contain a transformer to go from a high voltage (AC) to a low voltage (AC), and then contain some electronic components, for example a diode bridge and capacitors, to convert the low voltage AC electricity into DC.

I tested this old power supply I had lying around, and it consumed 1.6836 Wh when tested with my power meter during one hour:

That’s 14,75 kWh for a year. Which is about 10 times more than my worst switched power supply tested here.

So, if you are planning to follow the advice of energy experts here in Europe (and watch out, quite a few are not experts at all, just echo chambers) to reduce your electric energy consumption and save money, consider the following points (their idea is to unplug chargers you don’t use).

1. Start with your linear power supplies, they consume the most (a tip to recognize them: they are heavy compared to the switched-mode ones, because of the transformer; and they are old)
2. If you are going to do this daily, take into account mechanical wear and tear. Like on the pins of the power plug, the cables …
3. To avoid that extra wear and tear, you can plug your power supplies into a power-strip with a switch
4. I have a laptop power brick that regularly cause the power plug to spark when I plug it into a socket. That’s also something you want to avoid.

TLDR: reducing the sound volume level of our TV has no (significant) impact on its electric energy consumption, but reducing the back-lighting does.

Here in Belgium, mainstream media is full of news with tips to reduce energy consumption.

Some good tips, some bad tips … That’s mainstream media for you 🙂

Recently, there was an article with the following tip: “reduce the sound volume level of your TV to save energy” … (I’m not linking to this article).

It is true that a speaker (and the audio amplifier) requires power. And that there is a positive correlation between electric energy consumption and sound volume level. Large speakers can draw quite some amps…

But I was a little doubtful that lowering the sound volume level of our TV with a view clicks, would have a significant/measurable impact. Because some time ago, I already made measurements, and our TV drew 120 Watt maximum. So I did not expect a big impact.

Anyways, one has to make measurements to know if there is a (significant) impact or not.

We have a 55 inch QLED Samsung TV from 2018. The test protocol I worked out is the following: start to play a long movie (LoTR) and measure the electric energy consumption during one hour exactly (with a GW Instek GPM-8310 digital power meter). Don’t touch the TV or remote while testing is going on, and make sure that no dynamic settings are enabled that can influence the electric energy consumption (like ambient light based brightness control).

I measured at 3 sound volume levels: 20, 19 and muted. And I did this twice.

Here are the results:

For our TV, there’s no difference between a sound volume level of 20 and 19.

And by completely muting the TV, we save 0,08 Watts. That’s a very small amount. To put that in perspective, we would have to watch 125 hours of muted TV to power a 10 Watt LED light-bulb for 1 hour.

Of course, that’s for our TV. If you have a TV with a powerful soundbar and extra speakers, your measurements will be totally different.

While going through all the settings of our TV, there is one thing I noticed: the back-lighting setting was set to its maximum (20).

I reduced the back-lighting to 10 and measured again. That made a significant change: 77,666 Wh in stead of 117,74 Wh (both at sound volume level 20, our usual setting). That’s a 34% reduction in electric energy consumption. That’s a significant reduction, but …, don’t forget that the back-lighting setting happened to be at its maximum.

We will keep it like that for the moment, and see if we still enjoy watching TV.