“It takes a relatively long time to recharge the massive battery in the new iPad – over 5½ hours when it is fully discharged. That’s only if the new iPad is off or in sleep mode – if you try recharging while using the new iPad I estimate that it will take about 20 hours if the display is set to Maximum Brightness,” Dr. Raymond M. Soneira reports for DisplayMate. “But when is the battery fully charged?”
“While preparing to measure the battery running time to evaluate the Retina Display’s load on the iPad battery I noticed that the power meter indicated that the iPad AC charger was still continuing to deliver close to the full 10 watt recharging power long after the screen indicator showed that the battery was 100% charged – the additional time is slightly over 1 hour if the new iPad is off or in sleep mode, and slightly over 2 hours if it is on. Since I wanted to measure the proper maximum running time for a fully charged battery I waited until the recharging power dropped, indicating that the battery was actually full and the charging cycle was completed,” Soneira reports. “But if you stop charging the iPad when the battery indicator says 100% you won’t get the maximum running time – something that is very important to many people. Let’s see how much less…”
New iPad running time based on a fully charged battery is 11.6 hours
“Starting with a truly fully charged battery as described above, the battery running time for the new iPad at the Middle Brightness Slider setting is 11.6 hours. That is in Airplane Mode with no WiFi, with no activity or running Apps of any sort, and with Auto Brightness Off – the same running conditions specified on the Apple website,” Soneira reports. “Note that you’ll get only about half that running time at the Maximum Brightness Slider setting, but it delivers 2.8 times the Brightness (Luminance) of the Middle Brightness Slider setting – see the Running Time on Battery measurement results above.”
New iPad running time based on the 100% battery indicator is 10.4 hours
“If you stop charging the iPad when the battery indicator says 100% you won’t get the maximum running time – something that is very important to many people. I repeated the Battery Running Time measurements exactly as above, but stopped the battery charging when the battery indicator reached 100%. For the new iPad at the Middle Brightness Slider setting the Running Time decreased by 1.2 hours to 10.4 hours (10 percent),” Soneira reports. “While at first sight this appears consistent with Apple’s own “up to 10 hours running time” my tests were in Airplane Mode with no WiFi and no activity or running Apps of any sort – just a static display. The 11.6 hour running time above for a fully charged battery would most likely deliver a real use running time of over 10 hours as indicated by Apple, but the 10.4 hour time would most likely not.”
What’s the matter with the battery indicator?
“The battery 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. Note that batteries are based on complex chemistry so there is no practical way to measure the charge level ‘in hardware,'” Soneira reports. “So there is something wrong with the battery charge mathematical model on the iPad. It should not say 100% until it actually 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.”
Conclusion – the battery is only 90% charged when it says 100%
“So, when the battery indicator first says 100% the battery is actually only 90% charged and you get 1.2 hours less running time,” Soneira reports. “However, anyone that recharges their iPad unattended (and off or in sleep mode), especially overnight, will get the necessary extra charging time and get the full running time indicated above.”
Tons more about the new iPad, especially everything you’d ever want to know and more about the Retina display, in the full article here.
UPDATE: 3:58pm EDT: Apple says the battery indicator is working as designed. More info here.