It’s a universal truth that battery life on phones is bad. Even with the giant batteries and external battery packs we tote around, most modern smartphones struggle to make it through a full day of use, while the best devices barely scrape through two.

But it didn’t use to be this way: back in the (relatively) old days, phones had (relatively) fantastic battery life, lasting for multiple days at a time without needing to be charged. And, yes, our phones today are vastly more powerful than, say, a Nokia 3310, but why haven’t batteries kept up with the pace of progress?

Why haven’t batteries kept up with the pace of progress?

According to Venkat Srinivasan, director of the Argonne Collaborative Center for Energy Storage Science and an expert in battery technology, the core of the issue is simple: Moore’s Law has simply outpaced battery technology, meaning that our phones have gotten better — and demanded more power — at a much faster rate than advancements in batteries have.

It’s not that there haven’t been any improvements: we’ve been able to steadily increase energy density over the past few years by shrinking down internal components. But according to Srinivasan, “Five years ago, it became clear we couldn’t remove any more things, there were fires. We’ve reached a stage where new improvements in energy density are going to come from changing battery materials, and new materials are always slower compared to what I would call engineering advances.”

That’s because today’s rechargeable batteries in phones are based on lithium cobalt, a battery technology we’ve been using since the early ‘90s, and we’ve largely reached the limit of how much power we can squeeze out of it. There is hope for the future. Researchers are already investigating new battery technology, like solid-state batteries, that could open the door to more energy-dense materials that could offer more power for future devices.

There is a catch, though: by the time those new batteries roll around, our phones may be even more advanced and need even more power, which could leave us right back with the same, one-day battery life that we started with.