There are a lot of misconceptions about batteries and charging. So many that I already wrote an article about it a year ago on Anything but iPod. One year later and here I am, talking about tablets instead of MP3 players. The same concepts are true for this new family of gadgets though, so I thought I’d resurrect my old guide and update it for the new year, the new site, and these new devices.

Charging: rumours and misconceptions

99% of all the rumors and misconceptions surrounding batteries for any electronic device have to do with charging them. A lot of “rules” on how to charge batteries are still around from the “old days” where electronics used completely different types of batteries, and has nothing to do with the reality today. Any modern gadget with a rechargeable battery will have a lithium based battery in it. Why this doesn’t say much, it’s very important when it comes to how the battery behaves. Most people have heard that you need to charge the battery for 12+ hours when you get it, leave it alone while charging, discharging and fully charging as often as possible etc. This is complete and utter nonsense when it comes to lithium based batteries. We’re talking a completely different battery technology, one which has a whole other set of “rules”. Here are some:

Lithium batteries don’t care when you charge them or for how long. All night, fine. 5 minutes at a time, fine. 10 times a day, go ahead.

There’s no priming needed, which means the first time you charge the battery is no different from the 100th time you charge the battery. You don’t have to charge it for an insane amount of hours, as the battery will shut down charging when it’s finished fully charging anyways.

Fully discharging the battery isn’t good for the battery. In most cases nothing bad will happen, but there’s a chance- especially if it’s stored for extended periods of time with no charge- that the battery won’t charge back up afterwards. The only reason to discharge it is in some cases where it will reset the battery’s ability to estimate the amount of charge left.

A lithium battery charges 70% of the capacity in 1/3 the total charge time, if charging from a fully discharged state. The last 2/3 of charging time is for topping off the battery. This has been very evident with my iPad when I’ve had to charge it a bit in the evening after using it all day (which is pretty rare, which says a lot about the battery life of that thing); it will pop up from 10% to around 50 in no time.

I'm normally a pretty frugal person, but I still compulsively buy any R/C aircraft that's less than $30. In the last few years, this has become a problem.

The total charging time however might depend on the charge you use. The iPad is a very good example of this, as it charges through a USB cable- meaning 5 volts. USB ports only give about 500-700mA though, which is basically the amount of power transferred at the specified voltage. If you connect an iPad to a computer, it says “not charging”, which isn’t strictly true as it simply charges very slowly. The iPad has a very large battery, and that is why the iPad charger is 2A, which is 2000mA. Multiply these, and 5v*2A = 10W. The iPhone charger in comparison is 1A, so 5W, which is half that of the iPad. Basically that means that charging an iPad through USB will take up to 4 times as long as doing it with the included charger, and twice as long with the iPhone charger. This also applies to other devices- for example the Archos 5 IT which I had last year, which was infamous for charging extremely slowly through USB but didn’t come with an AC adapter included (you could buy one separately)

This does not however mean that your iPhone will blow up if you connect it to the iPad charger, as it only draws as much as it’s built to when it comes to amperage.  A wall socket works the same way, with a constant voltage and a maximum amperage (anything from 15A and up) and a lot of different appliances using different amperage but the same voltage. Basically, if the voltage is the same and you can physically connect a charger to a device, it will work.

There are also other things to consider, but that are less important to the average user. If you want to geek out with battery information, take a look at the info over at Battery University. I especially recommend people take a look at the “do and don’t do” comparison chart for various battery types. They also have some info specific to lithium batteries that is interesting.

Maximizing battery life

Battery life, meaning how long the tablet will last on a charge, varies greatly between different devices. It can range from just a couple of hours on the crappiest of crappy tablets (and those that run Windows…..) to 10-12 hours on the iPad and some upcoming tablets and weeks of use on e-ink devices that only use power when the page is turned. The actual number of hours you get out of a device depends not only on the capacity of the battery, but also how effectively the device uses the power. That is the main cause of the difference in battery life between something like an Android tablet an a Windows tablet, as the latter can’t run properly on hardware as slow (relatively speaking) as the energy efficient components used in the former. You might be surprised to know that the iPad battery is only 25Wh (Wh = Voltage*battery capacity in Ah), which is slightly less than the 29Wh battery in the Dell Duo, a convertible 10″ Windows tablet. While I can attest to the iPad going strong for 12 hours and longer, testing has shown the Dell Duo to give up anywhere from 2.5-3.5 hours. My 13″ ultraportable laptop can match the iPad’s battery life, but requires a 84Wh to do so. It’s also important to look at what battery results people get in real life and in different usage scenarios, as manufacturer ratings are often optimistic (though for the ipad they are in fact pessimistic for some reason).

That brings us to what you can do to prolong the battery life of your tablet. The rule of thumb is that the more hardware is in use, the more the battery will drain. That goes for CPU/GPU activity as well as various modules (GPS, WiFi, 3G etc). A tablet with no Internet connection, no Bluetooth active, no GPS in use, screen brightness at the lowest and only running very simple apps would definitely get a few more hours on the battery than the tablet next to it that’s playing Need for Speed while streaming music from the Internet and running a GPS tracker in the background. Personally though, I don’t worry about it with the iPad, but with other devices I at least turn down the brightness if I know I’ll be using the device for many hours.

The other definition of battery life is regarding how many times you can recharge the battery before it gives up. First off, let’s get some more rumours and misconceptions out of the way. Laptop batteries often serve as a worst case scenario as they often lose their capacity a lot faster than anything else. This is due to several factors, but heat is often a major factor as anything that’s a full blown PC generates more heat than a low power portable Android tablet. Still, 500 recharge cycles (which is a number often thrown in for good measures with lithium batteries) is only about 1.5 years with daily charges, while others (like Apple) put the number closer to 1000. It’s not like the battery will go from working fully to being completely dead though, so you’ll gradually lose capacity over time. Tablets generally don’t have user-replaceable batteries (though some do), but even Apple doesn’t charge more than $100 for an iPad battery replacement and that isn’t half bad in my opinion if you can recharge it 1000 times (daily for 3 years) before it dies completely. By then, you’ll probably want a new one anyways as your device might not be updated any more and be left behind in other ways way before the battery gives up.

To make sure that you get as many charge cycles as possible though, there are some simple guidelines you can follow. Avoid extreme temperatures both hot and cold and just charge the player when you need to. As a rule I try to charge my iPad once a day, but I’m not beating myself up over doing it twice if it’s a day of very heavy use. With other tablets it might be more of a problem because of much shorter battery life, but a tablet is there to be used- so use it!

Bottom line

The point of this article is to get some pieces of misinformation set straight and put some things in perspective. With lithium batteries, the best thing you can do for your battery is to completely ignore how and when you charge the battery and just make sure you have enough power when you need it. Lithium batteries are made to serve the user, not the other way around, and the “tips and tricks” that are left from the old days of other types of batteries actually hurt lithium batteries.