What’s your favorite thing about your smartphone? Is it its minimalist design? Its pretty interface? How about the fantastic HD display, the apps, 4G networking, or great cloud backups? Or maybe it’s all of those things? If someone were to ask me what I like about my phone—my year-and-a-half old iPhone 4—I’d likely say some combination of these factors. I like the OS, I like the display, and I like that there are a whole lot of apps in the store to help me goof off.

But if I said any of that, I’d be wrong. Because if I honestly examined how I use my phone every day, I’d have to say that what I like best about it is that it’s right there, always available to do whatever I want it to. Or, almost always. My phone is right there until late in the day, after which its battery indicator begins to issue increasingly desperate warnings of imminent death. The thing I like best about my smartphone is the absence of the thing I like least about it: I just want it to work, dammit, and when it doesn’t—when I need to recharge it and I can’t—it’s worse than useless to me.

This sounds obvious, right? Whatever other features it may have, a phone’s battery is its limiting factor. If the battery doesn’t work, nothing else does. This week J.D. Power and Associates put out its 2012 smartphone customer satisfaction survey, and the results bear this out. The study shows that battery life is one of the most important factors in determining whether people love or hate their phones. Owners of 4G phones were less happy with their devices’ batteries than owners of 3G phones, mainly because 4G phones don’t live as long as 3G ones. What’s more, among people with 4G phones, battery life was the deciding factor in whether or not you’d be willing to buy the same brand of phone again. If you give your phone a 10 out of 10 for battery life, you’re definitely buying that same phone next time. If you give it anything less, you’re going to look elsewhere for your next device.

That underlines a looming problem in the smartphone business, one that will haunt every manufacturer and may undermine the post-PC revolution over the next few years: Every year, everything about phones keeps getting better—except the battery. In fact, that’s kind of the problem. Because manufacturers keep adding extra features to phones—especially more powerful processors, displays and networking—battery life remains stuck.

The move to 4G LTE phones will put this problem into stark relief. If you’re currently using a 3G phone or a dumbphone, you’re bound to notice a significant drop in battery life when you move 4G. Will you consider the drop a worthy trade—will you live with shorter battery life for better networking? The people in J.D. Power’s survey didn’t think so. I’m right there with them. If LTE means my phone needs to be charged an hour earlier than before, I’m going to stick with 3G.

The trouble with batteries, as everyone who makes phones will tell you, is that they don’t follow Moore’s Law. Batteries are an ancient technology that depend on chemistry that scientists have already pretty much optimized. Improvements in batteries are incremental—and, as the writer Seth Fletcher describes in his fantastic book Bottled Lightning: Superbatteries, Electric Cars, and the New Lithium Economy, most of the innovation in the industry is occurring in the huge batteries meant to power electric cars (and there’s not much hope for those, either).

Improvements in smartphone battery life are mainly the result of power-saving techniques in the processors and operating systems that power our devices. But there’s only so much efficiency you can squeeze out of the phone itself. Manufacturers can’t keep making faster phones with better displays that we use to do more stuff while keeping battery life constant. Something’s going to give. And we’re not going to like the results.

So what should phone makers do? They can’t skip 4G. Even though none of us wants to sacrifice battery life, faster networking is your new bicycle; everyone wants it, or at least thinks they want it, and wireless carriers and phone makers are determined to give it to us. John Gruber says it’s possible that Apple may hold off LTE in the next iPhone if it can’t figure out a way to maintain current battery life, but I really doubt it can risk delaying 4G any longer, given all the competition. If any company can figure out how to make an LTE phone without ruining battery life, it’s Apple, which has invested more than any of its rivals in power-consumption techniques. Notice also that the new LTE-enabled iPad doesn’t sacrifice battery life compared to its 3G predecessor.

The question now is whether Apple can pull off the same miracle on a device with a much smaller battery, and one whose users are much more likely to be hogging cellular data. If it can, its transition to LTE will be seamless. If it can’t—and chemistry is running against it—the iPhone 4S may look much better than the 5.

It’s likely that over time, as LTE coverage expands and as 4G cellular chips improve, the new networks won’t lead to as much battery drain as they do now. So 4G phones will eventually last as long as 3G phones. But simply maintaining our currently dismal battery life won’t solve the problem. J.D. Power’s survey shows that battery life is the least satisfying part of our phones, one of the few areas in its study that declined from last year. That suggests there’s room in the industry for some kind of giant leap forward.

The first company—whether it’s an incumbent phone maker or Ph.D.-laden start-up in a garage—that figures out how to solve the smartphone battery problem will see enormous gains. Batteries are the main hurdle to a ubiquitously mobile digital future. Are they a permanent hurdle? Let’s hope not.