# How does the NerveGear fallback battery work?

On the first episode of Sword Art Online we know about a microwaves device installed on the helmet. We learn that this device can damage to death the user's brain removing the limiter. Unplugging the device causes the battery to activate as fallback, using the remaining power on the battery to damage the brain of the user.

The main device of a today microwave oven is magnetron, I cannot find any example of a magnetron consuming less than 1kWh as now (I accept correction), emitting more than 600W in microwaves. As of current technology, can a portable battery provide enough power to burn a brain? Are there more details about this issue described later in the anime or in the novels?

• Just to state that SAO is futuristic, and as such battery power densities are probably better than current (possibly by orders of magnitude).
– Oded
May 28 '13 at 14:07
• Right, SAO is set in 2022. Still, power consumption for a magnetron is very high for a portable battery, so I'm interested about how the writer address this issue and if it's feasible even providing some technological advancements (the last question). May 28 '13 at 14:26
• There was a Nerve Gear. No such thing exists is present time. It doesn't make sense to compare the batteries and power consumtion based on today's facts and limitations. A few decades ago, Super Computers required hell lot of energy, times where a thing as a laptop would be unthinkable. May 30 '13 at 10:50

Lets say the magnetron indeed has a power requirement of 1kW (I never tried, so I might be off with this value and 1kW is not enough to burn a brain. On the other hand, I guess it also depends on the duration the magnetron is turned on.)

Lets also say a mobile(-ish) battery of 50V is used (I do have to pick some value, right).

P = V * I , so it would draw 1000W / 50V = 20A.

Now, lets say a brain is fried in 1 minute (maybe this assumption is too high: if it takes too long the 'immediate dying' effect would be gone? Although some agony is a pro in the eyes of an evil mastermind)

So this battery for example (disclaimer: first one I found; I do not own stock for this company) seems to be sufficient. As it should be able to power it for about 1 hour (19.5Ah)... It is a bit heavy with its 8.8kg, but we do probably not need power for one hours, so a smaller battery should be sufficient, weighing a lot less.

So in short the answer is (with these crude estimates and calculations): Yes it is possible to power a (mobile-ish, the magnetron you linked to did not look too mobile as well) device with our current technology.

I did not read the manga or anime, but I guess the complete system needs to be scaled down to match the description and fit in a helmet...

• The one instance I know of of a 'resurrection' item from SAO required use within 10 seconds. Thus, we could probably assume fatal results soon after that time. What does that do for the calculations? May 29 '13 at 21:23
• Using my guessed values, the total 'required'/dissipated energy is 1kW*1/10h = 100Wh (or 360kJ). If the amount of time this energy needs to be dissipated in, reduced from 60s to 10s (factor 6), the power is increased to 100Wh*(1/60s) = 6kW (same factor 6 compared to the 1kW). Assuming the battery still provides a 50V voltage, it needs a discharge current of 6kW/50V = 120A! (Which cannot be delivered by the battery I linked to) But if the battery is able to provide a higher voltage, the required discharge current drops. But the required 10 seconds seems too much with our current technology... May 29 '13 at 22:17
• (continued) But then again, I do not know anything about frying brains, so my guessed/used values might be completely wrong! :) May 29 '13 at 22:18
• @Clockwork-Muse the assumption might be wrong too. maybe there is just a delay of 10 seconds until the magnetron is turned on ;) Jun 1 '13 at 13:40
• @Veger please keep in mind, that the brain will not necessarily absorb all the power "provided" by the magnetron. also before you fried the whole brain, the water inside it will have evaporated, thus making the whole skull explode... you could ad an alternative approach with the energy needed to boil a liter of water (which would be more than enough to kill in seconds ;)) Jun 1 '13 at 13:43

In light novels there is no "infinite batteries". Kayaba says "You will die if you're unplugged from the electricity or the internet for more than 10 minutes". Infinite battery thing is made by anime productors.