updated 11:30 pm EDT, Fri March 23, 2012
Testing points to bad calculation in algorithm
A leading researcher examining the third-generation iPad has discovered through testing and actual voltage readings that the latest iPad requires around two hours of additional charging beyond the point where the indicator reads "100%" in order to truly achieve maximum charge. Initial testing indicates that the problem lies only in the accuracy of the on-screen battery charge indicator rather than a hardware fault, but could lead to consumer confusion on performance.
Dr. Ray Soneira, president of DisplayMate and best known for running down the science behind the term "Retina Display" as used with iPhones and iPads, has continued his analysis of the latest model iPad and says that the tablet continues to draw a significant amount of power for up to two hours after "prematurely" displaying a full battery level. A faulty mathematical charge rate model could be behind the bad readings, Soneira hypothesizes.
Because of the new iPad's longer charging cycle, many users may rely mostly on charging the unit overnight or for longer-than-necessary times, since the unit will automatically reduce its power draw to nearly nothing once the unit is fully charged. The fact that it actually needs considerably more time than it appears could confuse customers who unplug the unit after it reaches 100 percent but don't get the longevity of battery life seen by those who leave the iPad on the charger until it reaches "trickle charge" levels and thus think their unit may be defective.
The charger normally draws 10 watts of power in order to recharge the battery as rapidly as possible, and in other iOS devices is often seen to draw the full 10 watts until it reaches an estimated 90 percent of full charge, then gradually reduces the draw for a short period until it finally drops to a one-watt "trickle charge" activated as the battery slowly degrades even when not in use.
It's possible that the mathematical model the indicator uses to guess at the battery's level hasn't been updated from the previous iPad, even though the new, larger battery offers 70 percent more capacity and thus requires a longer time to charge. According to Soneira, "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." If the formula used for the new iPad is incorrect, it would not affect the actual charging, but does prematurely show a full charge when in fact the device is still drawing power.
Some iPad owners have reported that normal charging appears to take around six hours, a considerable increase from the previous model. Many users will not be aware that the battery is physically larger than the previous model, being needed to drive the doubled resolution and more powerful processor and graphics. The latest iPad is only marginally thicker and slightly heavier than the previously model.
Soneira points out that most other mobile devices also "lie" about their true charging times, including both other brands and various Apple products. Largely this is less from malfeasance and more due to the way mobile devices calculate how long they ought to charge, which is what the indicator usually shows, rather than verifying that the battery is actually fully charged.
He calls for Apple to correct the issue, which can likely be mitigated through a software update. The fix may also determine if the battery indicator is inaccurate all the time, or just in the final stages of charging. The amount of time actually needed to charge the new iPad's battery is not the real issue, Soneira says -- but the indicator should not read 100 percent until the point where the charger drops to a trickle charge.