If you have an iPhone 6 or later that really isn't holding a charge, it's necessary to update the battery.

 If you have an iPhone 6 or later and are having battery problems, now is the time to go to an Apple Store and have your battery replaced.

Apple admitted in December that it had been secretly "throttling," or lowering, the output of older iPhones, which iPhone owners had suspected for some time.

It stated that the aim was to maintain battery life on older phones and prevent them from shutting down suddenly, but consumers thought that Apple delivered this message too late, as many had come to assume that iPhones were intentionally becoming slower in order to entice people to upgrade to newer models.

Apple discussed iPhone battery and efficiency in an open letter to consumers later that month, following widespread consumer uproar.

The most valuable aspect of Apple's explanatory letter was a deal at the end: the price of an out-of-warranty iPhone battery replacement will be reduced to $29 from $79 "for everyone with an iPhone 6 or later whose battery needs to be replaced, available worldwide through December 2018."

So, if you have an iPhone 6, iPhone 6s, iPhone 7, or some other phone released after that and are having battery problems — maybe it's draining faster than normal — visit an Apple Store before December 31.

Anecdote: My wife has an iPhone 6s and has been having battery problems for months. She'd have to continuously charge her phone at work and at home, and she didn't remember it being this bad before. So we went to an Apple Store a few months ago, where an employee measured her phone's battery life and discovered that it had degraded by around 83 percent.

Apple claims it would repair batteries after they have degraded by 80%, but the employee offered my wife the option of replacing it right then and there for $29. So we did that and while we waited, we walked around the mall for a few hours.

It was well worth that wait: her phone's battery life has improved since that visit, because she no longer wants to charge it during the day.

If you're having problems like this, go to an Apple Store or mail your computer in before December 31 to get your battery replaced for $30. If you hesitate, you'll end up paying more.