Researcher says new iPad's charging math is 'wrong'

A report from DisplayMate Technologies yesterday noted that Apple's latest iPad is not fully charged when it shows that it's 100 percent juiced up, and today the creator of that report says the problem is due to bad math.

In a note sent to CNET and other media outlets this morning, DisplayMate President Ray Soneira said the iPad's behavior here can be attributed to the calculations that go into determining how far along the recharging process is based on battery health and other factors:

The charge indicator on all mobile devices is based on a mathematical model of the charge rates, discharge rates, and recent discharge history of the battery. It uses this information to estimate how much running time is left. It's actually rather difficult to do because most batteries degrade slowly as they discharge and then tend to surprise with a precipitous decline near the end.

So there is something wrong with the iPad's mathematical model for recharging. It should not say 100 percent charged until it stops recharging and goes from the full recharging rate of about 10 watts to a trickle charging rate of about 1 watt. Otherwise the user will not get the maximum running time that the iPad is capable of delivering, which is listed in my article.

Apple did not immediately respond to a request for comment.

DisplayMate's original report -- based on its own testing -- found that the full power draw while charging Apple's latest iPad continued for about an hour after the on-screen display said the battery was at 100 percent. That meant those who unplugged the device at that point were potentially getting less of a charge than if they let the device keep charging, the firm said.

After additional testing, Soneira said it takes an additional 2 hours and 10 minutes after the battery reports 100 percent for the recharging cycle to "fully" terminate.

Apple's latest iPad has a considerably more powerful battery than its predecessors, jumping from a 25-watt-hour lithium-ion battery to a 42.5-watt-hour battery. That change came in order to power a display with four times the number of pixels as previous generations, a dual-core processor with a quad-core graphics chip, and 4G LTE wireless networking on some models. All told, that's led to battery life that's about the same as the iPad 2, while adding extra weight and thickness to the unit itself.

Apple has tweaked the mathematic formulas behind its on-screen indicators before, primarily the one that told users what kind of reception they had on the iPhone 4. Following its 2010 press conference centering on the performance of the antenna on that device, Apple said it was adopting a new formula for calculating how many bars were displayed on screen based on signal strength. That change came in the form of an iOS software update, which also made the first three bars of that signal range taller.