Tablet charging


Editor’s note: The use of smartphones and tablets on board voyaging boats has become commonplace, and with this widespread use has come the question of how best to recharge them. This piece from marine systems tester Rod Collins looks into the best way to do it.

Recently, when my iPad had a dead battery, I decided to use the opportunity to test how long it took to recharge it. To do the test, I connected my iPad Air to PentaMetric Battery Monitoring System software and tracked how much energy it used to charge from 0 to 100 percent state of charge (SOC). I noted the SOC using two methods: 1) the Pentametric computer software, and 2) the SOC as reported by the iPad’s screen. In the test below, these data are labeled as “monitoring software” and “iPad screen,” respectively.

Test 1: 400-watt inverter and Apple 120-volt adapter

For the inverter-powered test, Collins used a 400-watt inverter and an iPad power supply.

For this first test, I chose to run the iPad through a small 400-watt inverter off a 12-volt 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 to 100 percent charging via a 12-volt USB adapter. Suffice it to say, it is much more efficient than using the inverter.

The battery bank was LiFePO4, so the voltage remained more than 13.3 volts 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 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.

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 and track solar performance, in addition to tracking multiple channels for current, energy used/supplied, voltage, watt-hours, battery efficiency, etc.

For this article, I decided to use my Bogart Engineering PentaMetric Analyzer to track the amp-hours (Ah) and watt-hours (WH) of energy consumption. Throughout the test, the battery bank voltage remained at 13.3 to 13.4 volts. The iPad was charged via a 400-watt Cobra inverter to mimic the way most of my customers charge iPads.

Fig. 1: The PMComm display at 0 percent SOC at the start of the test at 11:22 a.m.

I was curious to find out how much energy it takes to charge an iPad Air 3G from a 12-volt bank via an MSW inverter.

Monitoring software: 0 percent SOC at 11:22 a.m. = -0.02 Ah

I left the screen capture at full screen so the computer time could be compared with the iPad time, mainly for me to sort everything.

iPad screen: 2 percent SOC at 11:22 a.m.

I snapped my first photo of the iPad at 11:22 a.m., and it had already gone from 0 to 2 percent SOC.

Monitoring software: 76 percent SOC at 1:58 p.m. = -3.91 Ah

In order to get to 76 percent SOC, the iPad and inverter inefficiencies consumed -3.91 Ah or -51 WH from the battery bank.

iPad screen: 76 percent SOC at 1:58 p.m.


Fig. 2: The iPad screen showing 76 percent SOC at 1:58 p.m., left. Fig. 3: The PMComm screen at 76 percent SOC at 1:58 p.m., right.

After nearly two hours and 40 minutes of charging at 1.5 amps, the iPad Air is nearing 80 percent SOC.

Monitoring software: 98 percent SOC at 3:08 p.m. = -5.5 Ah

The iPad has approached 98 percent SOC and consumed 5.5 Ah to get there.

iPad screen: 98 percent SOC at 3:08 p.m.

The iPad Air has a 32.5-WH battery. Some older iPads have a 42.5-WH battery and can consume more energy to charge than the current Air model does.

The iPad air: If we figure the average Li-ion battery voltage in the iPad Air is 3.6 volts, then: 32.5 WH/3.6V = 9.03 * 1000 = 9028-mAh battery (approximate size).

Older iPad models: If we figure the average Li-ion battery voltage in the iPad 2/3/4 is 3.6 volts, then: 42.5 WH/3.6V = 11.81 * 1000 = 11806-mAh battery (approximate size).

Keep in mind that most electronics makers are not using the full range of the internal Li-ion 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 percent 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 5.64 Ah to recharge the iPad Air.


Fig. 4: The PMComm screen at 100 percent SOC at 3:18 p.m., left. Fig. 5: The iPad screen showing 100 percent SOC at 3:18 p.m., right.

iPad screen: 100 percent SOC at 3:18 p.m. = -5.64 Ah

At 3:18 p.m., the iPad clicked over to 100 percent SOC and had consumed -5.64 Ah 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 percent SOC. It continued drawing 0.8 to 0.9 amps for about 40 minutes beyond when it displayed 100 percent. Li-ion batteries, in devices like this, stop charging when truly full. A 100 percent SOC 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-ion SOC can be very difficult.

Even if your screen says 100 percent, this does not necessarily mean it is full. When the charging icon disappears, then your iPad is fully charged.

iPad screen: 100 percent SOC at 3:18 p.m.

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 100-Ah battery bank, the iPad could use approximately 11 percent, give or take, of your usable amp-hour 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 percent depth of discharge, leaving them with 50 usable amp-hours.

If using the iPad while it is being charged, the current stays pretty steady at -1.5 amps 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 to 30 Ah per day if you’re not careful.

Test 2: Charging via 12-volt source

A Scosche USB insert.

For this second part of the charging observations, I chose to use a USB 12-volt adapter as opposed to an inverter.

This particular 12-volt USB adapter has an output rating of 5 volts and 2 amps, which is 0.4 amps less than the output of the Apple 120-volt adapter. This simply means that it will take a bit longer to charge.

This one is branded Rayovac and I grabbed it at Walmart. The standard Apple USB-to-lightning cable was used.

iPad screen: 2 percent SOC at 10:06 a.m.

My first screenshot is again at 2 percent SOC. It appears the screen won’t even turn on until it says 2 percent.

Monitoring software: 2 percent SOC at 10:06 a.m. = -0.01 Ah

This is the first recorded data point via the PentaMetric tracking software.

iPad screen: 94 percent SOC at 2:00 p.m.

I got sidetracked and finally came back when it was at 94 percent SOC.

Monitoring software: 94 percent SOC at 2:00 p.m. = -4.15 Ah

Fig. 6: The PMComm display for the second test using a 12-volt source (no inverter) at 100 percent SOC at 2:41 p.m.

Here is where the data begins to get interesting. It took just -4.15 Ah to go from 0 to 94 percent SOC with the 12-volt adapter. This is looking very good at this point.

iPad screen: 100 percent SOC at 2:41 p.m.

Sadly I missed the data print screen for 99 percent SOC, which occurred at 2:30 p.m. It finally ticked over to 100 percent sometime between 2:40 and 2:41 p.m. as I was doing a quick screen check every minute after 99 percent.

Monitoring software: 100 percent SOC at 2:41 p.m. = -4.36 Ah

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 to 100 percent SOC using straight DC-to-DC as opposed to DC-to-AC-to-DC, we saved 1.28 Ah of energy (5.64 Ah – 4.36 Ah = 1.28).

Put another way, when using the 400-watt inverter and Apple 120-volt adapter to charge this iPad, it uses 29 percent more energy for a complete charge cycle.

So what’s the bottom line? If you want to charge as efficiently as possible, stay with DC!

Rod Collins is an ABYC-Certified Marine Electrical Systems Specialist. He also produces the website, which has numerous tests of electrical and electronic gear.

By Ocean Navigator