Steven J. Vaughan-Nichols, a blogger on ZDNet wrote on March 23rd about an issue supposedly present on the new 2012 model Apple iPad (aka iPad 3) where the battery status indicator on the iPad indicates that the battery is fully charged when in fact it is not. He says:
An expert finds that when your new iPad tells you that it’s fully-charged, it’s not even close to being charged up. Are we looking at an Apple iPad “Batterygate?”
The core issue they identified is that “the power actually drawn by the AC Adapter and first found that the new iPad continues to charge for up to 1 hour after it claims to reach 100%. This affects the battery run time if you stop charging when it says 100%.” The article goes on to discuss the issue and wonders why the new iPad lies about the charge status, when the battery is not yet fully charged.
Dr. Raymond Soneira, president of DisplayMate, the world’s leading display and display tuning company, theorizes that “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.”
After reading these blog posts and being involved in the RC hobby industry where we use Lithium Polymer batteries day in and day out, I felt I had to comment on this issue to explain a few things about Lithium battery technology. Although the new iPad has a 4.1V 11,560mAh Lithium-Ion battery and not a Lithium Polymer battery, the charging characteristics of these batteries are virtually identical. The primary difference is Li-Ion batteries have a lower overall voltage (4.1V fully charged versus 4.2V for Li-Poly) and are generally more stable for low current draw applications like phones and tablets. Li-Poly is generally used where you need high discharge rates, such as in RC helicopters, airplanes and cars.
So coming from the RC hobby industry, it is very well known that the charging cycle for these types of batteries is done in two phases. The first phase is the constant current (CC) phase, and is what is considered the ‘fast’ charging phase. During this time the battery is charged at the maximum current possible until it reaches the maximum voltage of the cells, which for Lithium-Ion batteries is 4.1V per cell. When this occurs the battery is considered at about 80% of the total charge capacity, but it has in fact already reached 100% of the maximum voltage. If you put a volt meter across the battery at this point in the charging cycle, with the charger still attached, it would read 4.1V.
After the first phase is complete, the charger changes to a constant voltage (CV) phase where the current tapers off until it effectively gets to zero, and the battery is now fully charged. During the entire time the battery is being charged in the CV phase, the battery voltage remains at 4.1V per cell but the current tapers off. In general this last 20% of battery capacity takes a LONG time to complete, and since the current tapers off it actually goes from 80% to 90% capacity in a relatively short period of time. Then it takes it takes longer and longer to get to 95%, then to 97.5% etc due to the fact that the charge current is tapering off.
Now the catch in all this is that in order to measure the effective capacity left in the battery when the battery is NOT on the charger, you need to look at the voltage of the battery and run some complex math based on the discharge curve of the battery to estimate how much capacity is really left. Now most Lithium batteries under a small load will not have much voltage drop so they will measure pretty close to the maximum voltage when they come right off the charger (and compared to RC models which can run a battery flat in 5 minutes, 10+ hour discharge times is a SMALL load!). Now when the iPad is attached to the charger, once the charger changes to the CV phase at the 80% charge point, the voltage of the batteries will ALWAYS show full 4.1V per cell because the charger is currently pegging the battery voltage at that value. So for that reason, once the battery reaches 80% of its maximum capacity, the battery meter on the iPad is going to always show 100%! However if you remove the iPad from the charger as soon as it first reads 100% on the battery meter, it would not actually have reached full capacity yet so the voltage would immediately drop.
For the most part the battery meter is really designed to let you know and approximate capacity left when the unit is NOT ON THE CHARGER! So it will always give some odd results when the battery is being charged and is not under load. So it seems to me this is all a lot of hoopla about nothing, since what is being described is pretty standard stuff for charging Lithium batteries, so I hardly think we have a ‘Batterygate’ or ‘Chargergate’ type situation on our hands. I have no doubt most (if not all) other tablets and phones that all use Li-Ion batteries do exactly the same thing.
Talk back below and let me know what you think!