Most helpful critical review
35 of 41 people found the following review helpful
Works well, but inadequate capacity
on June 23, 2012
I purchased the Anker Astro 3 10 A-hr battery to use as a backup for my iPad 3, which I am using as an "electronic flight bag" in an airplane. As in all aviation-related areas, a backup for critical items is necessary. I am, unfortunately, disappointed in the capacity of the battery, which seems to be well short of its specification.
The battery in the iPad has a 42.5 W-hr capacity and the Anker battery, which has a claimed capacity of 37 W-hr, should charge the iPad to 87% of its full charge. Instead, starting with 10% charge, the external battery charges the iPad to 73%, which suggests that it has a capacity of 27 W-hr. Of course, the percent scales on the iPad are probably not calibrated very well, but the telling point for me was that, while lightly using the iPad, it actually slowly lost charge when plugged into the external battery! It is almost certainly not providing the claimed 2.1 A output -- the Apple charger (also rated at 2.1 A output) fairly rapidly charges the iPad during use. All of these tests were done after about 4 complete charge/discharge cycles and the last test was done while the Anker was showing 2 leds (about half charged).
I am returning the unit for a replacement and will upgrade my rating if the claimed capacity is met. In all other respects, the unit is fine.
Update: The second unit has the same problem: much less than expected capacity and inability to effectively charge an iPad 3 during use. I will return the battery and find one with greater capacity.
Here are my two objections to the unit:
1. As the vendor admitted, they are excessively optimistic about the capacity and don't include the inefficiency of the additional circuitry. Since all the battery makers do this, I suppose I could live with this.
2. Their claim of 2 A output current can't possibly be accurate. The iPad 3 has a 42 W-hr battery and a useful operation time (which I have confirmed) of 10 hours or so. This means it consumes about 4.2 W or a little less than 1 A from a 5 V battery. If the Anker battery supplied 2 A, it would definitely have enough left over to charge the ipad battery. Neither of my two units were capable of charging the battery with the iPad running in idle. With it sleeping, it took 8 hours or so to charge the iPad battery by 60%. Supplying 25 W-hrs (60% of 42) in 8 hours (3 W) suggests about .6 amps at 5 V. Again, much less than the spec. It seems to be providing <1 A from the USB port. A voltage much less than 5 V might also result in slow charging. (I have no idea how good the output regulation is -- they might simply use the output from the bare cell, which would vary from 4.2 to 3.0 V as the battery discharged and would probably result in inadequate charging current.)