Trading in your phone? This one rule can help you maximize the value of your trade

If you're looking to trade in your phone, BankMyCell is a website that will show you which buy-back outfit will pay you the most money for your used handset. Just as a new automobile starts to depreciate the moment it is driven off the lot, a new smartphone loses value the moment it arrives at your home. That can be illustrated by a chart created by BankMyCell (via Android Authority) that shows the decline in value of the Samsung Galaxy S9 and Galaxy S9+ from last year's launch to the end of the year.

The data, based on the average price of all storage sizes of the Galaxy S9 and Galaxy S9+, resembles the chart of a stock before and during the Great Crash of 1929. The 64GB Galaxy S9 launched in March 2018 for $740 and was worth only $290 at the end of the year for a 60% decline. Following the unveiling of the 2018 Apple iPhone models in September, the decline in value of the Galaxy S9 and Galaxy S9+ accelerated.

The Galaxy S9 also depreciated twice as fast as the iPhone X did over the first 9 months following the launch of each model. The Galaxy S9, as we pointed out, was priced at $740 and dropped to $290 by the end of 2018. Again, that was roughly a 60% decline. The iPhone X was launched with a price at $999, and 9 months later it was valued at $690 for a 30.9% decline.

So BankMyCell has come up with some recommendations based on past price action in the smartphone trade-in market. Almost 70% of the total depreciation of the iPhone X's price was seen after Apple's September event. So if you plan on trading in a 2018 iPhone (iPhone XS, iPhone XS Max, iPhone XR) you need to commit to the transaction before Apple holds its annual September event. Following this one rule alone could earn you an additional $100 or more with your trade.

Another interesting trend is that once the calendar fourth quarter ends, and the New Year begins, iPhone prices tend to rise in value. Both the 2018 iPhones and 2017 iPhones (iPhone 8, iPhone 8 Plus and iPhone X) saw their trade-in value rise this past January.

BankMyCell also notes that the release of new Android models do not have as big of an impact on the trade-in market as the iPhone does. When the Galaxy S9/S9+ was released last March, older Samsung models like the Galaxy S6 through the Galaxy S8 saw their trade-in value drop by a small 5%-13% over the following weeks.

Looking at charts of past years, the trends seem to remain the same every year. Apple's September event brings on an acceleration in the depreciation of Samsung Galaxy S models and Apple's current and older iPhone models.

The company posted a chart of the ten smartphones that had the largest percentage decline in trade-in value by percentage and actual dollar amount. Percentage-wise, the 2015 Sony Xperia Z3+ topped the list as it lost nearly 58% of its value last year. Six of the ten phones on the list were iPhones led by the 51.67% decline in value of the iPhone 5s.

Another chart showed the top ten smartphones that had the largest dollar decline in trade-in value over calendar 2018. The Apple iPhone X was on top as it lost $190 of its value over the year. That was followed by the Apple iPhone 8 Plus, which suffered a $160 drop in value last year. Once again, six of the ten phones on the list were iPhones, three were Galaxy S handsets and the 10th model was the Pixel 2.

So if you've come away from this article learning anything, it is that if you have a phone that you want to trade-in, the best time to do so is before Apple's September event.