Last week, Dr. Raymond Soneira of DisplayMate Technologies discovered that when he charged his new iPad, it continued to draw current long after iOS reported its lithium-ion polymer battery as being 100% charged.

Doing some experiments, Dr. Soneira discovered that if allowed to charge until the point where the 10W charger stopped drawing full current from the mains, his iPad could last 11.6 hours on a single charge, compared to just 10.4 hours if he unplugged it the second it reached 100%.

Why does the new iPad do this? Dr. Soneira believes that it’s a bug in the way the new iPad reports its battery charge. Apple has since spoken out and called it a “great feature” in iOS. But what the heck is really going on?

The truth is more complicated. Apple’s being disingenuous calling this a “feature” of iOS. In fact, technically it harms your new iPad’s battery. That said, it’s certainly not a bug, as Dr. Soneira emphatically suggests. Rather, this is the way all gadget batteries charge. To understand why this is, and how you can maximize your device’s battery life and longevity, you first need to understand a little bit about how batteries charge.

How Your iPad Determines How Charged The Battery Is

The first thing to know is that measuring how much charge is in a battery is not an easy task. It’s not like looking at the fuel gauge on your car, seeing where the needle is, and knowing how much fuel is in the tank.

“Battery state of charge cannot be accurately measured,” says battery expert Isidor Buchmann of Cadex Electronics, who runs an informative site on understanding battery technology called Battery University. “There is no completely accurate way to tell exactly how charged a battery is at any given time.”

Because of this, gadget makers tend to use how much voltage it takes to put current into the battery as an indicator as to how fully charged it is. As a battery’s state of charge increases, it takes increasingly more voltage to charge it up more. For the new iPad’s lithium-ion polymer battery, this means that once the system measures that the battery is charging at 4.2 volts, iOS says that as far as it can accurately measure, the battery is fully charged. But it might not actually be full.

Why? Here’s an analogy to make things easier to understand.

Let’s imagine charging a battery is like using a pump to blow up an inflatable mattress. As the mattress gets more full of air, it becomes increasingly difficult to put another pump of air into it. In other words, it takes increasingly more air pressure (voltage) to fill the mattress as you go along, because the air already inside the mattress is trying to escape. The point where you stop pumping comes when the pressure inside the mattress exceeds what you can easily apply with the pump, but that’s not to say that the mattress can’t take more air. It just requires a lot more finesse.

Why New iPad Batteries Charge After Reaching 100%

The “finesse” part of charging comes after a new iPad detects that the battery is taking 4.2 volts to charge. This mode is called trickle charging mode, and it does what it says on the tin: it trickles electricity into the battery cautiously until no more current can be accepted.

It’s this part of the charging cycle that Dr. Soneira was so worried about. According to Apple’s statement to AllThingsD:

Apple does in fact display the iPad (and iPhone and iPod touch) as 100 percent charged just before a device reaches a completely charged state. At that point, it will continue charging to 100 percent, then discharge a bit and charge back up to 100 percent, repeating that process until the device is unplugged. Doing so allows devices to maintain an optimum charge, Apple VP Michael Tchao told AllThingsD today. “That circuitry is designed so you can keep your device plugged in as long as you would like,” Tchao said. “It’s a great feature that’s always been in iOS.”

Got that? It’s a great feature of iOS! Except for one problem…

Trickle Mode Is Harmful To Your Battery, But The Alternatives Are Worse

Have you ever noticed that when you charge your iPhone, your iPad or your MacBook, the battery charges from 0% to 70-80% at a much faster rate than it charges from 70-80% to 100%? There’s a reason for that. Let’s revisit our inflatable mattress analogy again to understand why.

Let’s assume we’re pumping our mattress up full of air. As we begin to pump, there’s lot of empty, unpressurized space inside of the mattress, so pumping is easy. At a certain point, though, pressure inside the mattress makes it extremely difficult to put more air inside, and results in having to apply an increasing amount of pressure to get more air in. At this point, the fuller you fill the mattress, the more likely it is that the mattress will pop.

This analogy is a very simple way of looking at batteries and shouldn’t be extended too far, but for our purposes, it works. The fuller you charge any battery, the more stress you put on it. The more stress you put on a battery, the less capable it is of holding a full charge over time. Pretty simple.

Charging stress is a very real problem for gadget makers and consumers alike. This unfortunate but unavoidable aspect of how lithium-ion batteries degrade over time is what causes our laptops to need to be recharged more often the older they get, and for our smartphones to have to have their batteries replaced every couple of years.

What’s the solution? Many battery experts advise users to never charge their batteries more than 70-80% in order to keep their batteries healthy. In fact, some companies like Samsung ship products with a so-called “Battery Saver” mode that will stop a device from charging after it reaches 80% for just this reason.

But never allowing a battery to have more than an 80% charge kind of misses the point. All batteries degrade over time. To never charge your battery more than 80%, you’re essentially permanently sacrificing twenty percent of your battery’s permanent runtime in order to… prevent your battery from having less battery life over time. Madness!

What’s The Takeaway?

Lithium-ion and lithium-ion polymer batteries degrade over time, and there’s nothing that can be done about that, by Apple or anyone. You can technically prevent a battery from degrading as quickly by not allowing it to charge past the point where it is 80% full, but at the expense of permanently sacrificing battery life all around. And they still degrade. May as well fill that battery up as much as you can.

As for “BatteryGate,” while Apple’s being cheeky claiming that the way iOS measures a 100% charge is a “feature,” this is standard charging behavior of all gadgets containing lithium-ion batteries, according to Buchmann. Gadgets measure how much voltage is going into the battery up until it reaches 4.2 volts, then they trickle electricity in until the current turns off… which could take another hour or more, in the case of the new iPad’s massive battery.

“Yes, Apple’s cheating when they report the battery being 100%, but so is everyone else!” laughs Buchmann. “Every gadget maker has been cheating all this time, but no one ever noticed until the new iPad!”

Could there be a more convincing testament to Apple’s role in the tech landscape than that?