Charging iPAD Battery from External Battery

+"I have seen documentation that says, if the iPad battery is dead (less than 10%), then the external battery will charge the iPad to a "certain level", and then the iPad will run from the external battery."+
This information is incorrect. Can you identify this documentation? A link, perhaps?

An external battery functions just like a charger. You are always charging the battery and, if the iPad is turned on, you are always draining from the battery. HOWEVER, an important factor is the current capacity of the charger. The one that came with your Mac puts out 2A at 5V (10W), which is more current than the iPad uses so the battery will continue to be charged when the iPad is turned on. If you attach a charger that puts out less, it may not be sufficient to power the iPad and the iPad's battery will drain.

Yes the iPad expects a constant voltage input of 5 Volt. The maximum charge current is 2.1 Ampere. The internal battery is charged by an intelligent charger, there is no direct connection between the external power and the iPad battery. Charging Lithium based batteries isn't trivial. Below a certain voltage the charge current has to be pretty low. When the battery is completely drained the charger will start with a lower rate. Once it reaches a certain voltage the full 2.1 Amp are charging the battery. As the internal battery is 'filling up' the voltage goes up. Once it reaches a certain level (4.2 or something around that, depending on temerature) the charger has to reduce the current (Ampere) gradually to make sure the internal battery's voltage never goes beyond that level. Letting it go higher is what makes those batteries explode.

All this is completely separate and independent of the external power supply. The iPad expects a constant 5 Volt and it will draw as much current as it needs or the supply allows. All external batteries that are sold as i-device chargers have a regulated output. The output voltage is constant (5 Volt) regardless of the battery level or voltage. The majority of these have a 500 mA or 1 A limit, only a few newer models are now equipped with regulators that are capable of 2 Amp. So only those will allow the iPad to charge as fast is it's possible. But again, the output is a regulated constant voltage.

I'm pretty sure the iPad would not even start charging if the external power is an unregulated source like a direct connection to a battery.

Phil, what is said is true for constant voltage source such as a AC charger. However, a battery is not a constant voltage. As the battery drains the battery voltage goes down. Thus, if the iPad's batter is almost fully drained an external battery will tend to charge the iPad's battery until the voltage differential between the iPad's and charging is zero. If you use the iPad, the iPad's battery voltage will go down and will be charged a little from the external battery. hat id not know id how the iPad controls to prevent overcharging since that may effect how a battery charges an iPad. charging..

I agree completely. However, the practical impact is impossible to assess without detailed information regarding the external battery. We know nothing about the voltage curve during discharge.

There are also a few unknowns about the iPad. You had mentioned the overcharging control. I'm also thinking about the presence/absence of a diode to ensure that current is in only one direction. If there is no diode, I can imagine a weak external battery actually draining the internal battery!

+"When the battery is completely drained the charger will start with a lower rate."+
On the surface, that sounds very logical. However, look at the chart on the link below. It says quite the opposite.

http://www.apple.com/batteries/

Illaass...
Look at this. It's not 100% clear but it gives a hint regarding the overcharge control.
http://www.apple.com/batteries/