Why does the new iPad continue charging after it hits 100 percent?

Since the launch of the third-generation iPad, Dr. Raymond Soneira, president of DisplayMate Technologies Corporation, has put the device through its paces testing just about every aspect of its “resolutionary” new Retina display. The lengthy report provided a detailed comparison of the new iPad’s display versus the iPad 2 and iPhone 4.

In a running time time of the new iPad’s battery, Dr. Soneira found its “batteries do not actually reach full charge when 100% is shown,” and noted up to an extra hour of charging is required before reaching full capacity. We already discovered that the third-generation iPad’s new 42.5-watt-hour battery takes considerably longer to charge than the iPad 2—up to several hours. One explanation suggested is that Apple is trying to shorten the amount of time people think it takes to charge to line up with iPad 2 expectations. Soneira found the new iPad running no applications at maximum brightness lasted for 5.8 hours in comparison to the iPad 2 at 7.2 hours.

He did not go into detail about his findings in the report, but Soneira provided an explanation of the charging issue below:

Here is my explanation of the reason why the new iPad keeps charging for up to an hour after it says 100% charged.

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 battery charge mathematical model on the iPad. It should not say 100% 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.

Ray Soneira

President of DisplayMate Technologies Corporation