Why do smart phone / laptop batteries still suck after all these years, and when can we expect an improvement?
Adam Rifkin stashed this in History of Tech!
Best answer on explainlikeimfive subreddit is from KippTheKidd:
On one hand, battery technologies have improved a bit - about 4-fold, since laptops were first invented. The very first portable computers ran on lead-acid batteries. Now we have lithium ion batteries, which have about 4 times the capacity (by weight).
Other improvements have helped too - we have made much smaller transistors, which consume a lot less power... An "old" portable computer could run for maybe an hour. Your "new" cell phone can do an hour's-worth of the same computations in less than a second now. The battery-to-work ratio is probably hundreds of thousands of times better than it was in the 90s.
The competing factor, the thing that "fights" our battery power, is customer expectations. Your computer has numerous extra features that kill battery life, from a display that is has 16 times as many pixels with 4 times more colors (that's 64 times more computation), to multiple modes of wireless connectivity.
There are computers out there that have a battery that will 20 hours or more - but people rarely buy them. They are hundreds of times more powerful than early computers, but still not "modern enough".
So, customers have struck a balance. The most popular products have reasonably good computation power, as well as a battery life that is good but not awesome or impractical.
There are some "next big steps" in battery life, including lower-power processors and high-capacity / quick-charge batteries. They will probably be appearing in laptops in the next 3 or 4 years.
Addendum: Per comments, lithium batteries have been around for a long time, but didn't regularly appear in laptops until around 2000. Also, computation, while it does consume power, is usually not the major consumer of battery power; nothing is - it's a combination of backlights, fans, computations, wireless radios, I/O, etc.