iPad Slow Charging

TIP: Slow iPad Charge? A Simple Fix with Cable Upgrade can Charge your iPad or iPhone 30% Faster

UPDATE: The same is true for iPad 2, iPhone 4S

Some people have noticed that the iPad will not charge or seems to take forever to charge on some after market cables, especially if they are longer than the standard charge and sync cable.  In fact I have seen this on standard length cables where the iPad battery drops even though I have it plugged into the charger it came with and it says charging.

The main reason for this is pretty simple.  USB charges around 5 volts and if you are expecting the over 1 amp ( 5 watts ) on the standard cable, it better not be very long.  This is because the size / gauge / AWG / diameter of the wires is too small.  By specifications, USB 2.0 cables can have wires with AWG as high ( small ) as 28. The result of this is that during the high speed charge phase ( usually the first 50% ) you are potentially losing 30% charge efficiency with the stock cable based on my multimeter tests. Not very green and very annoying that Apple cheaped out by a few pennies on this.

First a table of solid core wire AWG for power transmission ( most USB cables have stranded wire for more flexible cables ).  The following is a table that shows common iPhone or iPad charging methods, for nerds the values are approximate and designed to carry the 'concept' and length is length of cable but AWG for true 'round trip'.  Smaller AWG means a thicker wire.  Data from Powerstream calculator.

Note the larger ( thicker ) cables required for more power AND longer cables.  There are aftermarket cables in non apple USB sync cables that have thicker wires for the two wires for power.  Of cources, thicker wires are only required for the power, and not for the data pair.

Apple sort of solved this problem with the 10 watt charger that has the exteneded wall portion, so that it i can deliver 120 volts to the USB transformer. This is ok, but for me it is not practical for travel.  Optimally, I would like a 6 foot cable with a 28 AWG data pair and 20 or 22 AWG power pair which would be compact and light enough for travel.

In mean time, Griffen has gone overkill with a new cable that is 10 ft and has 16 AWG for power pair.  Pro: it charges at full 10 watts at 10 feet.  Con:  it is heavy and thick ( not very flexible ) so the connector wants to come out of the iPad a bit.  It is $29.95 and I purchased at the Apple Store in Boca Raton, but cannot say if all of them carry it.

Finally, some notes about iPad actual charging.  It will charge off PC with good cable and screen off.  Measuring via my amp meter I show the iPad using 0.4ma on sleep mode and about 330ma when charged and awake but idle.  With 3G active, it ran between 400 and 600ma, so it is no surprise that the 500ma most PCs and USB hubs provide will not charge it when awake.  Additionally, when syncing, the USB port is allowed ( required? ) to drop the power rate to the unit so as not to overdraw the ports when transmitting data.  So trying to sync and charge on a normal cable and USB power is not going to happen.  Some Macs and new motherboards by Gigabyte will provide more power on the USB power but the USB power is shared by multiple ports in most configurations.

Using the dual USB plugs( 500ma + 500ma  ) common with portable hard drives did not seem to make any difference in charging the iPad because the iPad seems to negotiate for power so that it doesn't melt your USB ports or chargers if they are too small.  In the iPhones, it appeared they did this by looking for certain voltages on the data pair, so it may be possible to create a custom cable that would show the iPad 1000ma so that you could charge and sync at same time, though a bit slower than the included charger.

Like most modern smart batteries it will charge fastest for the first half, then begin to slow down ( and draw less power ).  This is to preserve the life of the battery and allows for a more efficient charge as well.  Most Apple batteries are rated at 400 charge cycles.  In most cases a charge cycle counts whether you charge 80 percent or 10 percent so it does not make sense to plug it in every 3 percent.