Amp Draw of iPad / iPhone Tested

BlueKhufu

macrumors regular
Original poster
Nov 27, 2010
181
19
To answer some questions for myself I cannibalized a usb extension cable, shortened it to about 6 inches and left the red +5V line open so I could use an ammeter to check current levels using different cables and devices. The results are basically "duh" and many of you wont be surprised by any, although I did clear up a few misconceptions I had.

I have several 10' charge/sync cables from dealextreme that have worked very well for my iPhone, but when using them with my new ipad the math just didnt add up and the charge was slow.. anyhow, here are the results:

IPAD RESULTS
10' cord - maxed out at 1.4 amps. I cut it down to verify length was the only limiting factor. It was.. it maxed at 2.0a when I cut it to about 3 feet.

Stock cord - 2a max. I did NOT cut it down to see if I could get that up to the 2.1a rated capacity of the charger.

Also interesting (yet expected) as the ipad nears a full charge the draw decreased (I'm going to do a time lapse and I'll graph it to get exact numbers). When the screen was turned off, the draw would drop about .2a (not when the charge was low, it maintained the 2a draw).

IPHONE 4S
I know a lot of people have said the iPhone will charge faster on the ipad charger due to the available extra current, however my iphone4s only drew a max of 1a on both the ipad3, iphone, and aftermarket charger (same ipad stock cable for all three), so there is clearly a limit set in the phone. On the bright side, it confirms Apples statement that it is perfectly ok to use either charger (even though 2a would still be safe for the lipo battery as many have mentioned). Unless there is new math I dont know about, there is no way the ipad charger charges the iphone4s faster... the phone is the limiting factor. When the phone reached 100% the draw dropped to around .2a and was variable dependent on brightness and what kind of load the A5 was handling, ect.. basically everything you'd expect to ding the battery.. Also, when you turn the phone off, current drops to zero for a second or two, then jumps back up to whatever the charge rate was when it was on, so it's clearly "smart" circuity whether the phone is on or off.

Like I said, none of this is particularly shocking.. It should be no surprise that a piece of technology like the iDevices would have very specific charge curves and hard limits to protect itself. The one thing that I didnt expect was the iphone hard cutoff at 1a.. I too thought my ipad charger worked quicker, but I cant ignore the numbers and chalk it up to wishful perception..

SOOOOO....

If your new ipad is charging slow, make sure you are using the stock charger and the stock cable... My 10' cables are awesome for in bed and at work as the length lets me keep everything at 100% without feeling tied to a short leash, but they just barely keep up with the ipad when it's in moderate use.

Hope this helps at least one person. :)
 
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Mrg02d

macrumors 65816
Jan 27, 2012
1,097
1
Longer wires drop more voltage than shorter wires. I'm surprised that the charger isn't compensating for the voltage drop, though. (raise the voltage output to offset the drop across the wire ).I plugged my charger into a power meter and see that it uses 12 watts while the iPad is plugged in wether it's just charging or I am playing on it.

Matt
 

BlueKhufu

macrumors regular
Original poster
Nov 27, 2010
181
19
Longer wires drop more voltage than shorter wires. I'm surprised that the charger isn't compensating for the voltage drop, though. (raise the voltage output to offset the drop across the wire ).I plugged my charger into a power meter and see that it uses 12 watts while the iPad is plugged in wether it's just charging or I am playing on it.

Matt
In fairness it may be. I only checked the current, as I figured any drops in voltage would be corrected as well.
 

TB07-NJ

macrumors 68020
Jul 7, 2008
2,306
130
FORMERLY NJ now FL
In fairness it may be. I only checked the current, as I figured any drops in voltage would be corrected as well.
+1 Nice job and proves beyond a doubt (with your iPhone test) that even if you could find a higher rated charger it would make no difference as to faster charging time as the device only takes what it's circuity is programmed to take.
 

Mrg02d

macrumors 65816
Jan 27, 2012
1,097
1
In fairness it may be. I only checked the current, as I figured any drops in voltage would be corrected as well.
My guess would be that the charger is already running near its limits and has already raised the output voltage to its max. These things are so tiny, some sacrifices must have been made...

Matt
 

sectime

macrumors 6502a
Jul 29, 2007
529
0
+1 Nice job and proves beyond a doubt (with your iPhone test) that even if you could find a higher rated charger it would make no difference as to faster charging time as the device only takes what it's circuity is programmed to take.
Adult threads with numbers seem to not get a lot of posts:cool:
 

Macman45

macrumors G5
Jul 29, 2011
13,196
133
Somewhere Back In The Long Ago
Stock cables second cycle...10% warning, plugged into stock charger and cable at 11:15 AM UK time, charged to 100% at 4:30PM UK time. Charged as I always do with my phone as well with Airplane mode on. 4 hours and 15 mins to full charge...I can live with that.
 

wharzhee

macrumors 6502
Apr 27, 2010
279
0
texas
you shd get a charge extension cable before the adapter, not after.
5V has a lot of loss, thats y ur wall plug is at a much higher voltage.
 

BlueKhufu

macrumors regular
Original poster
Nov 27, 2010
181
19
you shd get a charge extension cable before the adapter, not after.
5V has a lot of loss, thats y ur wall plug is at a much higher voltage.
Yep! Definitely good advice (and I'm ordering one from DX tonight). I will defend my 10 foot usb cable when it comes to the iPhone though.. most of the DX cables I've bought in the past wont even sync properly at 6 foot lengths.. the "snake" at ten feet charges the phone and syncs perfectly.. I have 10 of them.. at different job sites, my car, one in my pc, two in wall chargers in my room, and a few spares etc. :)
 

VoltVision

macrumors newbie
Oct 1, 2012
1
0
iPad3 charging current quickly drops below 2amps....

BlueKhufu,
In your original post you had mentioned....
"Also interesting (yet expected) as the ipad nears a full charge the draw decreased (I'm going to do a time lapse and I'll graph it to get exact numbers)."

We are very interested in seeing some "Current over time" charging curves. I am designing a new 6-port "high current" USB charger. The customer wants it to do a continuous 2.1amps on each of the 6-ports of course, but when I did my preliminary testing I found that when the iPad3 battery is "very low" then it charges @ 2amps....but within 30mins or less the current started decreasing and it spent most of its time charging at currents far less than 2amps. I am wondering why it didnt stay @ 2amps longer? Is the iPad3 throttling itself back or was it the TI TPS2511 causing this? I will share some charging graphs with you in the next few days. It would be great to compare notes/graphs.

Thx!
--
Respectfully,
Steve French

President, Volt Vision
www.voltvision.com