#### How Much Juice?

In our brains, electrical signals are propagated through neurons following action potentials—brief pulses in which the charge gradient across the cell membrane flips. Here’s what that looks like:

An action potential pulsing across a neuron leads to the flow of electric current. Image via Wikipedia

The average mammalian neuron has a ‘resting potential’ of about -65 millivolts (mV). After being stimulated, the cell membrane depolarizes (the charge difference becomes closer to zero.) If the stimulus raises the membrane potential above -55 mV (call the threshold potential), an action potential is triggered. The membrane potential shoots up to a peak of approximately 40 mV and quickly falls again. When this happens, electric current is propagated forward.

So, every time one of our neurons fires, it produces a charge gradient of about 105 mV (the net change between resting potential and peak.) This tiny change in voltage causes an even tinier amount of current to flow — about 1 nanoamp, or 0.000000001 amps, according to biophysicist Bertil Hille of the University of Washington.

You may be thinking these numbers sound pretty puny. But lucky for us, the human brain contains roughly 80 billion neurons. Of course, not all of them are lighting up at once. In fact, recent research suggests that only about one percent of the neurons in our brains are active at any given moment. Still, 800 million neurons is nothing to sniff at — it might just be enough to produce a meaningful amount of power. Let’s do the math and see.

#### Given: 105 mV per action potential, 1 nanoamp of current per neuron, and 800 million neurons firing at once, how long would it take my brain’s electricity to charge an iPhone?

First, we’ll convert everything to fundamental units:

1 nanoamp / neuron = 1.0 x 10^-9 amperes / neuron

105 mV / action potential = 0.105 V / action potential

Next, we’ll calculate the amount of power generated when a single neuron fires:

0.105 V x (1.0 x 10^-9 A) = 1.05 x 10^-10 W / neuron

Scaling up to an entire human brain:

(1.05 x 10^-10 W / neuron) x (800 million neurons / brain) = 0.085 Watts / brain

Now we’re getting somewhere! On average, at any given moment, your brain’s electricity is outputting roughly 0.085 Watts of power. Your brain is basically an energy saving LED.

I have an iPhone 5C, whose battery claims to hold a charge of 5.74 Watt hours. So, how much time would it take my brain’s electricity to charge my phone?

5.74 Watt hours / 0.085 Watts = 68.33 hours

There you have it, folks: If you could somehow divert every single biological wire in your skull to your iPhone’s battery, you’d be fully charged in just under seventy hours! Huh! That doesn’t sound particularly efficient, especially when compared to a standard wall charger, which usually does the trick in under an seventy minutes.