When you’re traveling with mobile devices, electricity is one of the most important factors to take into account. No matter how advanced is the technology you carry, if you can’t power it up, it becomes useless.
4xAA USB battery case – version 2
I’ve been making some research over the past few years looking for good solutions to power up my devices. Selecting a mobile power source is really a question of how much current and voltage your electric device consumes, and how much weight you’re willing to carry.
When I travel, I usually carry with me a PDA, a camera operated by 4xAA batteries, a hard-drive based OTG backup device and a 4xAA charger. During my previous journey, I hacked a 4xAA battery case that I recieved along with my ELOOK LK-A6 OTG device, so it could also charge my PDA (back then it was AXIM X5). It required soldering a 3.5mm DC plug directly into the battery contacts. It worked great, but things changed, and I needed some adjustments.
For my next journey, this setup has not changed much. I’m still carrying a 4xAA based camera, the ELOOK LK-A6 (unfortunately nothing better has come out yet), and a new PDA, the HTC TYTN II.
This time I decided to make the battery case more USB-oriented. I drilled a hole in the case, soldered a USB port that I excavated out of an old USB hub. I also soldered a USB extension cord directly to the battery contacts.
The great thing about using AA batteries is that four of them can supply enough current to power up a 2.5″ hard-drive, or for my case – the ELOOK LK-A6.
Four rechargeable batteries give you 1.2V * 4 = 4.8V which is almost 5V – the required voltage for USB! Now since my batteries turn out to be slightly over-charged to 1.3V, that gives me the required 5V. Sufficient for every USB-based device.
The battery case that arrived with the ELOOK LK-A6 didn’t include any electric circuit or any kind of voltage stabilization. This is a direct connect to the batteries (connected in series). It worked without a problem powering many of my USB devices.
Modding the AA battery case
After soldering two USB ports into the AA battery case I can practically power any device that works on USB. Notice that the USB extension cable can’t supply enough current for a hard-drive. I used a low-quality thin cable that is suitable for USB 1.0 devices. If you’re planning to build one for yourself, get a proper USB 2.0 cable, or simply solder the USB female port directly.
You should be aware to the limitation of 4 AA batteries. Although they can power-up a 2.5″ hard-drive, as the batteries drain, the current drops down, which means they will not be able to supply enough current for high consumers, such as hard-drives, after some time. However, you will still be able to use them to charge other things, such as a PDA or using a camera.
How to solder – testing that it works
Cut the extension cable to the length suitable for you, then strip the USB extension cable, you’ll see two wires (might be red and white). Get yourself a cheap USB lamp to make the tests. Connect the lamp to the USB extension cable. Make sure there are four batteries inside the case, put the red wire on the Plus side, the white wire on the Minus side. Do you see any light coming from the lamp? If you don’t, switch the wires. When you find the correct polarity, solder the wires and close the case.
Soldering an actual USB port is a little more tricky. There are four tiny metal threads inside the port. The plus and minus are the left and right ones. It doesn’t matter which is which. Solder two wires to each thread. Be careful not to make contact with the two threads in the middle. You can take them out so they will not interfere using small pliers. Connect your USB lamp to the USB port, repeat the same test as before with the wires you soldered. When everything is working, solder the other side of the wires to the batteries’ contacts. You’re almost done! Now you have to carefully mount the USB port to the battery case so it will not move. This is realy important since you are likely to push and pull this plug many times while using it. Close the case and you’re done.
Surely, there are better ways to do it. This is the simplest way that I can explain.
Carry a few sets of batteries with you
Since I have a camera based on 4 AA batteries, I carry with me three sets of AA batteries. First one is in the camera, second one is in the battery case, third one is for backup.
So you need to power up a hard drive and your batteries are drained out? No problem, use the other set!
How to overcome AA batteries disadvantages
In my previous journey I learned that the last sentence doesn’t always apply in reality. Maintaining three sets of AA batteries isn’t easy. You have to remember which one is drained so you can charge it as soon as you get back to your hotel or hostel. Moreover, cold weather can easily effect NiMH AA batteries. You might find yourself with drained batteries although you charged them a few hours ago.
After I returned from my last journey, I searched for solutions that can prevent that from happening in the future. I found two type of things that can help: Li-ion USB batteries and LSD AA batteries.
LSD AA Batteries
A little miracle called NiMH LSD – Low Self Discharge. These batteries have smaller capacity compared to the regular NiMH batteries (2000mAH vs 2700mAH), but they can keep their charge for a long time (many months). More importantly they can work in cold temperatures. There’s nothing worse than climbing on a mountain, feel the temperature drop, and discover that all your batteries are dead.
Li-ion USB batteries
The other solution is using a rechargeable Li-ion battery stored in a special case that has a USB output. These are the same batteries found in mini-cameras, laptops, PDAs and other devices. They are usually refered as propriety batteries becaus they can be shaped to fit a device, unlike AA batteries which have a standard shape.
There are two problems using a USB Li-ion battery case.
The first problem is the voltage: Li-ion batteries have 3.7V to 4.2V voltage (when fully charged). It’s not enough to power USB devices. Many of these Li-ion battry cases can’t even supply enough current to power up a hard-drive. The case should have a well-planned voltage booster from 3.7V to 5V. This doesn’t go without a cost. You may lose upto 30% of the original capacity while boosting the voltage to 5V. For example, I have such a device that holds 2000mAH. The electric output, as stated on the case is only 1445mAH.
The other problem is the one-time-use till next charge. Unlike the AA battery case, you can’t replace a Li-ion battery in this kind of case. These batteries are not common like the AA batteries so even if you wanted to, you can’t buy them in a store.
The Li-ion batteries are unable to supply high current for a long period of time. As they lose their charge, the current goes down to a point where they can’t power up a hard-drive even after a couple of minutes.
Their real advantage is their relatively small size compared to AA batteries.
Using a AA battery case together with Li-ion case
Due to the rather small size of the Li-ion battery USB case, I find it very convenient to carry it along with the 4xAA battery case. The amazing thing is that even when one of the cases can’t supply enough current, together they can. Using a split USB cable, you can connect a 2.5″ hard-drive to both cases, while the batteries are substantially drained, to give it enough current. A split cable adds up the current, not the voltage. Thus, connected together I can continue using the power when one of the battery cases is drained, even though it wasn’t usable if I only had one of them.
I will write a few more words about Li-ion USB cases in a future article.
Note: I take no responsibilty for anything that might happen to your devices or to you by following the steps described in this article. You should deal with soldering and electricity only when you know what you’re doing.
Posted by: Amir.W
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