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post #1 of Old 02-14-2014 Thread Starter
Maine Sail
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iPad Charging - 12V vs. Inverter

The other 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 test 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.

120V Inverter vs. DC/12V

Using an inverter to charge an iPad or any internally DC powered devices, such as computers or TV's etc., requires the conversion from DC to AC then back DC again. Once back at DC it gets further stepped down to 5V from nominal bank voltage or boosted to 19V for devices such as some TV's or some laptops.....

Using an inverter for this type of device is essentially: DC>AC>DC.

The losses in each of these conversions creates inefficiencies.

Using an inverter that is larger than necessary also results in added inefficiencies. A 400W inverter to charge an iPad is already inefficient but by using a 400W model to power a 10W brick we compound more inefficiencies onto this already inefficient means of charging..

I see many, many, many boaters using 1000W/1kW, 2000W/2kW and 2500W/2.5kW inverters to charge piddly little DC devices such as phones, tablets and computers. This is horribly inefficient. Some of these behemoth inverters use more current in standby mode then it takes to charge the iPad. D'oh.....:doh:

Boat owners often assume the inverter is 90% efficient as the literature claims. Don't always believe what your read... Into the right load, at the right inverter load, with sufficient battery voltage, that may be the case. In the realm of using an inverter to do DC>AC>DC those total inefficiencies can run 20-35% or more and not the 10% literature would lead you to believe you will see.....

When ever possible it is more efficient to stay DC>DC, even if stepping up to 19V for computers, or stepping down to 5V for tablets and phones.......

Most every computer or tablet on the market has a 12V adapter than can be purchased for it. For the iPad or other USB charged devices they run $6.00 - $30.00 depending on where you shop. Be sure to get a USB device capable of a least 2.0A for an iPad or it usually won't work. The cables for an iPad have a pin that identifies a charge source as either high output or standard output. A standard output USB port used with an iPad simply won't charge it.. Ideally you want a USB charger that has a 5V/2.1A or 5V/2.4A output rating.

This is but one example of a high output USB adapter that actually lasts:

TEST #1 - Charging Via 12V to 120V Inverter

The battery bank was a LiFePO4 lithium variant 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. Considering the 12V USB adapter outputs 5V I suspect the difference between 12.5V and 13.3V is not going to make much difference.

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). After 99% I opened it once per minute to check SOC. The "ON" duration of the screen was less than 2 seconds..

These tests may not be representative of how you use an iPad on your boat but we charge ours at night and while they are off or are in sleep mode.

Test Specifics:

*Battery Bank - 400Ah LiFePO4
*Inverter - 400W Cobra CPI 475 MSW
*iPad Power Supply - Apple 5V/2.4A 120V to USB Adapter
*Charging Cable - Apple Brand USB to Lightning
*iPad - In sleep mode and all apps closed
*Tracking Equipment - Bogart Engineering Pentametric Monitoring System With USB102 Interface.

400W Inverter & Apple 120V Adapter

I have been conducting a lot of battery testing this winter and needed my test station in my downstairs office area. It makes it very convenient to have the batteries at a good constant temperature and this area makes that possible compared to my barn/shop..

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..

The Testing Station

NOTE: I cropped the screen captures down from a full screen shot so the data could be more easily seen. I still have the original screen captures showing computer time, as compared to the iPad, but this made viewing Ah data tough..

From top To Bottom:

*Battery Voltage
*Ampere Hours Used
*Watt Hours Used

Data Point 0% SOC @ 11:22 AM = -0.02 Ah's

I snapped my first photo of the iPad at 11:22 AM and it had already gone from 0% to 2% SOC..

11:22 AM - 2% SOC

For this test I decided to use the 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 13.3V - 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.

You can see here that in order to get to 80% SOC it consumed -3.91 Ah's or -51 watt hours from the battery bank.

Data Point 79% SOC @ 1:58 PM = -3.91 Ah's

After nearly 2 hours and 40 minutes of charging at 1.5A the iPad Air is nearing 80% state of charge.

1:58 PM - 79% SOC

Here the iPad has approached 98% SOC and consumed 5.5 Ah's to get there.

Data Point 98% SOC @ 3:08 PM = -5.5 Ah's

3:08 PM - 98% SOC

At somewhere between 3:17 & 3:18 PM the iPad clicked over to 100% SOC and had consumed -5.64 Ah's to get there.

100% SOC = -5.64 Ah's

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 as has been done here..

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 if we assume the owner is practicing good battery management and only drawing the bank to a max of 50% depth of discharge, leaving him or her with 50 usable Ah's..

On a large battery bank an iPad is but a small dent but as you get multiple iPads on board the energy used can add up..

I have one customer with two kids. They often have four iPads on-board.. They also run a 17" older computer & cell phones, cameras, a GoPro, small video games etc. etc. etc.. These items can add up to nearly 40 plus Ah's per day for just phones, tablets and additional little trinkets people often assume draw next to nothing..

Inverter Results:

*Total Charging Time = 3:58

*Average DC Current = -1.4 to -1.5A

*Total Ah's Consumed = -5.64 Ah

If using the iPad while charging it the current stayed pretty steady at -1.5A when run through this inverter..

Between my wife, our daughter and myself 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.

3:18 PM - 100% SOC

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 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 the iPad. Bot regulate to 5V max but one as a 2A output and the other has 2.4A of current capability.

This particular USB adapter is branded Rayovac. I grabbed it at Wal*Mart for about $9.00.. The standard Apple USB to lightning cable was used.

NOTE: This USB insert-charger is a venerable POS. I can literally burn one out in a weekend. USB insert-chargers are one of the fastest failing devices I see on boats. Go figure that most I see are el-cheapo crap purchased in the checkout aisle or at a convenience store.. Many of these USB insert-chargers are not designed to meet Apple's high-current charging standard so will charge very, very slowly and often default to 1A or less.

If you want a high quality long life USB insert, that actually works, consider the SCOSCHE USBC242M. This model, and it's 10W predecessor, have proven to be the most reliable USB inserts I have come across. The SCOSCHE USBC242M can charge two iPad Air's simultaneously at virtually the same speed as your 120V Apple wall charger. Amazon Prime usually has them for $11.99....


My first iPad screen shot is again at 2% SOC. It appears the screen won't even turn on until it says 2%.....

10:06 AM 2% SOC

First data point via the Pentametric tracking software.

Data Point 2% SOC @ 10:06 AM = -0.01 Ah's

I got side tracked and finally came back when it was at 94% SOC...

2:00 PM - 94% SOC

Here is where the data begins to get interesting. It took just -4.15 Ah's from the battery bank to go from 0% SOC - 94% SOC using this 12V USB adapter. It is looking very good for a DC>DC win at this point....

Data Point 94% SOC @ 2:00 PM = -4.15 Ah's

Sadly I missed the data print screen for 99% SOC, which occurred at 2:30 PM. Isnapped a photo of the iPad but the screen shot either I forgot or I hit the wrong key.

The iPad finally ticked over to 100% sometime between 2:40 & 2:41 PM as I was doing a quick screen check every minute after 99%..

2:41 PM - 100% SOC

As in many other observations one can easily see how much more efficient it is to remain DC throughout the entire charging process.

To go from 0% SOC to 100% SOC using straight DC>DC, as opposed to DC>AC>DC we saved 1.28 Ah's of energy (5.64 Ah - 4.36Ah = 1.28)!!!

Put another way, when using the 400W inverter and Apple 120V adapter to charge this iPad it uses 29% more energy for a complete charge cycle than it does to stay DC>DC.

12V Results:

*Total Charging Time = 4:41

*Average DC Current = -0.8A - 1.1A

*Total Ah's Consumed = -4.36Ah

So what's the bottom line?

If you want to charge as efficiently as possible, stay with DC!

Data Point 100% SOC @ 2:41 PM = -4.36 Ah's

DISCLAIMER: Every combination of inverter or 12V adapter may yield slightly different results due to differing efficiencies. The results here are for the products used and are intended as an observational guide only..

-Maine Sail / CS-36T

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Last edited by Maine Sail; 09-26-2015 at 10:17 AM.
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