Elon Musk just explained how Tesla's future really comes down to its battery technology

Batteries aren't exactly the most interesting thing to talk about, but Tesla CEO Elon Musk put into perspective the important role they will play in Tesla's future.

Speaking at Vox Media's Code Conference Wednesday night, Musk said the energy density of its batteries is increasing on the order of nearly 5% per year, which ends up being a "significant number" when added up over a number of years.

Energy density is a measure of how much energy a battery can hold. So having a higher energy density means having a longer battery run time.

"A lot of people think we cobbled together some laptop batteries and made a car, but it's a lot harder than that," Musk said. "If it was that easy, I think we would have quite a few competitors who did the same thing."

But Musk said energy density really isn't really an issue for Tesla. He's so confident in that he said Tesla could make a car with a 400-mile range today.

What Musk really wants to do through his massive battery plant, the Gigafactory, is decrease the cost per unit of energy of battery packs. Put more simply, decreasing the cost of Tesla's batteries will make its cars more affordable.

This isn't the first time Musk said his real goal is on making more affordable electric cars. The Model 3 is Tesla's first stab at affordability with a price tag of $35,000 before federal tax exemptions. And in a recent conversation with Norway's Minister of Transport and Communications, Musk said "there will be future cars that are even more affordable down the road."

Tesla is putting its energy (yes, pun intended) in batteries so it can make the electric vehicle of the future: a car with a long range at an affordable cost.

He likened it to cell phones, which have gone through so many design iterations and have such a large economy of scale that everyone can "have a supercomputer in their pocket."

Battery breakthroughs could be what makes Tesla the iPhone of electric cars.