400W Inverter & Apple 120V Adapter
This morning I woke up to a dead iPad battery. I had to do some tax work for the business so I decided I might as well use the opportunity to connect my iPad Air to the Pentametric Battery Monitoring System and track how much energy it used to charge from 0% to 100% SOC.
For this first test I chose to run the iPad through a small 400W inverter off a 12V battery bank. I chose this method because it is the most popular way I see customers charging iPads. It is NOT the most efficient way!!
Later on in this article you will see that I also tested 0-100% charging via a 12V USB adapter. Suffice it to say it is much more efficient than using the inverter.
The battery bank was LiFePO4 so the voltage remained well over 13.3V for the entire duration. This could potentially make the inverter operate a tad more efficiently, but we are talking peanuts..
For this test the iPad was asleep except for the few seconds I opened it to snap a shot of the state of charge (SOC). This may not be representative of how you use one on a boat but we charge ours at night and while they are asleep.
Battery Bank - 400Ah LiFePO4
Inverter - 400W Cobra MSW
iPad Power Supply - Apple 2.4A 120V to USB Adapter
Charging Cable - Apple Brand USB to Lightning
iPad - Asleep and all apps closed
Tracking Equipment - Bogart Engineering Pentametric Monitoring System With USB102 Interface
The Testing Station
I have been conducting a lot of battery testing this winter and needed my test station in my downstairs office. It makes it very convenient to have the batteries in a good constant temperature..
This system can be used to conduct 20 hour capacity tests on batteries, cycle testing of batteries, track solar performance and can track multiple channels for current, energy used/supplied, voltage, watt hours, battery efficiency etc. etc. etc. etc...
Today I was just curious to find out exactly much energy it takes to charge an iPad Air 3G from a 12V bank via a small MSW inverter..
Data Point 0% SOC @ 11:22 AM = -0.02 Ah's
NOTE: I left the screen capture at full screen so the computer time (lower right) could be compared with iPad time, mainly for me to sort everything. If you want to see the Ah data on the screen shot simply click the image to make it larger.
From top To Bottom:
Ampere Hours Used
Watt Hours Used
11:22 AM - 2% SOC
I snapped my first photo of the iPad at 11:22 AM and it had already gone from 0% to 2% SOC..
Data Point 76% SOC @ 1:58 PM = -3.91 Ah's
For this article I decided to use my Bogart Engineering Pentametric Analyzer to track the ampere hours and watt hours of energy consumption. Throughout the entire test the battery bank voltage remained at 13.3 - 13.4V.
The iPad was charged via a 400W Cobra inverter to mimic the way I see most of my customers charging iPads, via an inverter.
In order to get to 76% SOC the iPad and inverter inefficiencies consumed -3.91 Ah's or -51 watt hours from the battery bank.
1:58 PM - 76% SOC
After nearly 2 hours and 40 minutes of charging at 1.5A the iPad Air is nearing 80% state of charge.
Data Point 98% SOC @ 3:08 PM = -5.5 Ah's
Here the iPad has approached 98% SOC and consumed 5.5 Ah's to get there.
3:08 PM - 98% SOC
MUSINGS: The iPad Air has a 32.5 Watt Hour battery. Some older iPads had a 42.5 Watt Hour battery and can consume more energy to charge than the current Air model does.
AIR: If we figure the average Li battery voltage in the iPad Air is 3.6V then: 32.5Wh/3.6V = 9.03 *1000 = 9028 mAh battery (approx size).
OLDER MODELS: If we figure the average Li battery voltage in the iPad 2,3,4 is 3.6V then: 42.5Wh/3.6V = 11.81 *1000 = 11806 mAh battery (approx size).
Keep in mind that most electronics makers are not using the full range of the internal Li batteries in order to get the most life out of them. Consumers hate short battery life in products where batteries are not easily replaced.
They are likely using somewhere between 60% and 80% of the actual mAh capacity but this is kept under wraps... If they used the full rated capacity it would take a lot more than 5.64Ah's to recharge the iPad Air...
100% SOC = -5.64 Ah's
At 3:18 PM the iPad clicked over to 100% SOC and had consumed -5.64 Ah's to get there. At this point I stopped the test.
DISCLAIMER: It should be noted that my iPad Air does not discontinue charging when it says 100% SOC. It continued drawing 0.8-0.9A for approx 40 minutes beyond where it said "100%". Li batteries, in devices like this, stop charging when truly full. A 100% screen display on the iPad does not necessarily mean it is totally full or done consuming charging energy. After noticing this I discovered that this is a fairly well known issue. It makes sense because tracking Li SOC can be very difficult...
Just because your screen says 100% this does not mean it is full. When the charging icon disappears, then your iPad is fully charged....
3:18 PM - 100% SOC
While the iPad is a very efficient product it consumes more energy to charge it than many boat owners would assume, especially when run through an inverter.
On a small boat with a 100Ah battery bank the iPad could use approx 11% +/- of your usable Ah capacity, when charged via a small inverter. This is of course if we assume the owner is practicing good battery management and only drawing the bank to 50% depth of discharge leaving him or her with 50 usable Ah's..
Total Charging Time = 3:58
Average DC Current = -1.4 to -1.5A
Total Ah's Consummed* = -5.64 Ah (*to when the 100% icon appeared)
If using the iPad while charging it the current stays pretty steady at -1.5A when run through this inverter..
We often have three iPads on-board, plus phones and a laptop computer. The computer is an energy pig by comparison. When you add all these up you can easily break 20-30 Ah's per day if not careful.
TEST #2 - Charging Via 12 Volt Source
For this second part of the charging observations I chose to use a USB 12V adapter as opposed to an inverter.
This particular 12V USB adapter is has an output rating of 5V and 2.0A which is 0.4A less than the output of the Apple 120V adapter. All this means is that it will take a bit longer to charge.
This one is branded Rayovac and I grabbed it at Wal*Mart.. The standard Apple USB to lightning cable was used.
10:06 AM 2% SOC
My first screen shot is again at 2% SOC. It appears the screen won't even turn on until it says 2%.....