Charging Hot iPad2 in Cockpit is Cool Idea

Diagram taken from Apple iPad2 User Manual

When the iPad is used in the cockpit as part of the navigational information system, such as in the iPad + Garmin GDL 39 configuration, the pilot must make sure that the iPad does not get too hot, since it then turns itself off and does not start up again until 15-20 minutes later. According to Murphy's Law, if this event happens, it will likely occur in a critical phase of flight where a waiting time of 15-20 minutes will seem like eternity.

How can we avoid that troubling scenario? Here are obvious remedial steps.

1. If the iPad is enclosed in a case, remove it from the case. If you still want to protect the screen, use a light-colored cover such as Apple's iPad Smart Cover with light-gray color option.

2. Turn off the iPad temporarily and have an air vent blow cold air on the aluminum backside.

3. Turn down brightness and delete all processes that are not needed but are running in the background.

4. Shield the screen from direct sunlight.

Then there is a seemingly unorthodox idea.

Connect the iPad to the 12V USB charger.

Why does this make sense? Let's look at all operations that generate heat in the iPad.

First, running the iPad produces heat, due to the screen display, the computer running, and so on. This heat is produced no matter how the iPad is powered. Turning off the iPad is the only way to eliminate this heat source.

 Second, if the iPad runs on battery power, then the discharge process of the battery produces additional heat.

 Third, if the iPad is being charged, then the charging process of the battery generates additional heat.

 So, what operation would produce minimal heat, assuming that you must use the iPad? Yes, just supplying to the iPad enough current from an external source so that the battery is neither charged nor discharged.

 The iPad does not give us that least-heat-producing option, at least not explicitly. But when the iPad gets above a certain critical temperature that is only known to Apple and maybe aliens in a remote galaxy, then we can get the iPad to enter this desirable mode.

Here is how.

 Suppose the iPad is gradually heating up. We connect it to the 12V USB charger. One of two events must occur.

 1. The battery icon is replaced by the left icon in the diagram shown earlier and repeated here.
This means that the iPad is below the critical temperature and is being charged while it is running.

As we have seen above, this will accelerate the heating-up process, which is not desirable. But once the heating process has pushed the iPad beyond the temperature threshold that Apple will not tell us, then we enter the second case.

 2. The iPad displays the right-hand plug icon. According to the iPad2 manual, this means that the charging process has been completed.

 But lo and behold, this event may happen at any charge level of the battery! Actually, the icon tells that the iPad is no longer being charged. You can verify this, too, since the battery level no longer changes up or down.

 You may get upset, thinking that the battery has become defective and can no longer be charged. Could be. But when the iPad is quite warm to the touch, it is more likely that the iPad has ceased charging the battery to eliminate one source of heat! 

 This means that the iPad is running solely on the current supplied by the 12V USB charger. And that means that we have the above, ideal situation: All heat produced within the iPad is solely due to the running of the iPad.

We tested this on a recent flight. Yes, it works. Of course, it may still happen that the iPad becomes too hot just due to its running, in which case the only remaining choice is to turn it off during all non-critical phases of flight.

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