What can happen, however, is that the phone itself loses track of how much capacity its battery has: Every battery gradually loses capacity over time as you use and recharge it, and the phone’s software isn’t always good at accounting for this capacity change. By periodically (once every couple of months) fully charging the phone and then using it until it dies, your phone’s software will determine the battery’s current capacity and thus let the phone better estimate how long it will last on a charge. In other words, the battery won’t last any longer, but the phone’s battery meter will be more accurate. If you find that your phone claims you have 80 percent of a charge left, but it dies a few hours later, you should try this procedure.

If all else fails …

If you have tried all the above and still struggle to get through the day with your battery, consider buying an external battery. These accessories — which can take the form of a bulky case with a built-in battery that you wear on the phone, or a separate battery pack that connects to your phone with a cable — can provide power to last an additional few hours at the end of the day, or even to fully charge your phone’s battery.

The Wirecutter tested more than 100 external batteries for dozens of hours to pick a few favorites. Its favorite battery case for the iPhone 6 and 6s is Anker’s Ultra Slim Extended Battery Case, which has enough power to fully charge a dead iPhone and then some, more than doubling the phone’s battery life. For larger iPhones — the 6 Plus and 6s Plus — the Wirecutter prefers Tylt’s Energi Sliding Power Case because the case’s two-piece design makes it appealing.

For Android phones, an external battery pack is a good option. The AmazonBasics Portable Power Bank with Micro USB Cable 2,000 mAh is the best pack the Wirecutter tested that will fit in your pocket with your phone, and it’s less than $10. A great battery pack for the iPhone is the $29 TravelCard, which is almost thin enough to fit in a wallet — with a built-in Lightning-connector cable to charge your phone.

For days of smartphone power, the Anker PowerCore 15600 is the Wirecutter’s pick if you need to charge a phone repeatedly or keep a full-size tablet and phone topped up through a busy week. It has enough available power to charge a typical smartphone five times or to fill a large tablet such as an iPad Air almost twice — for under $40.

After her stressful day at Disneyland, Ms. Temeña bought an external Amazon battery pack. She said the pack could fully charge her phone six times, but it wasn’t ideal because of its bulk. Ultimately, she wishes her iPhone had a better battery.

“I don’t understand why a battery wouldn’t be able to keep up with all the other advances they’re putting into phones now,” she said.