Building Portable Devices
BOARDS
Building Portable Devices
2016-08-19
By
David "DeMO" Martínez Oliveira

A portable device or a handheld have something special. It may not look that special nowadays when everybody is carrying a mobile phone all the day long. But, that feeling comes back when you try to build your device. Being able to walk away from the wall power socket is something great. So, let's build portable devices.
You can easily use one of those USB power packs to quickly make your design into a portable device, however, that solution is bulky and, let's face it, it is not cool.

In this short post we are going to explore the basics on how to add a battery to our devices. This is going to be a practical post targeting what you can easily get nowadays (2016). This is not intended to be a handbook on power subsystems that can be very tricky.

Fortunately, nowadays, building a portable device is pretty simple. We can acquire most of the circuitry we will need for building our device and just connect them together with a bunch of wires. Basically, what we need is:

  • A Battery
  • A Battery Charger circuit
  • An optional DC-DC converter

Let's quickly go trough this list...

DISCLAIMER
The author of this post is not responsible of any damage that may be caused out of the content of this post.

    Be careful to avoid hurting yourself and to avoid damaging your equipment

    Batteries may be dangerous and can even explode if not properly used. Read carefully the information provided with your battery before manipulating it

Batteries

The batteries are our power source. Batteries are devices that produce electric energy out of chemical energy. Depending on the chemicals used for this process many different battery technologies are available.

The most common batteries nowadays, specially for small portable devices are the so called LiPo batteries. LiPo stands for Lithium Polymer.

This batteries, as many others, are actually composed of at least two elements:

  • Battery Cells. These are the elements containing the chemicals that produces the electricty. For LiPo batteries each cell produces 3.7V nominal voltage. Using more cells will produce batteries outputting higher voltages.
  • Protection Circuitry. The chemical reaction in the cells, if not properly controlled, may produce fire or an explosion. The protection circuitry provides basic safety.

LiPo Battery. Can you spot the protection circuit cover with the yellow tape?

So, always use a battery with its protection circuitry... do not play with battery cells unless you really know what you are doing (and if you are reading this, it is very unlikely you can deal with that).

As we said, a LiPo battery has a nominal voltage of 3.7V, however, that is a nominal value. In the real world a LiPo battery voltage will vary from 4.2V (fully charged) to 3V (discharged). We need to take this into account when powering our design.

The other parameter you need to know about batteries is their capacity. Capacity is usually measured in mAh. This represents how many milliampers the battery can provide for 1 Hour.

Suppose that our device requires 500 mA to work (this is a typical average value for small computers). A 1000mAh battery will allow us to run our circuit for 2 hours (1000 mAh/500mA = 2h). Therefore a 4000mAh battery will keep the device running for 8 hours.

Unfortunately, the higher the capacity of the battery, the bigger its size and weight. Something to take into account.

Charging Circuit

A portable device shall use rechargeable batteries. You can build something with non-rechargeable batteries indeed... but at the end, that is not very practical (continuously disposing depleted batteries).

We are not going to describe the charging process of a battery, actually, we do not think we can do it in a proper way, but what you need to know is that, it is a non-trivial process that usually requires a special chip to control the process. Building the circuitry ourselves is not very difficult, however, unless you are a PCB/SMT master, your board will be much bulky that anything you can buy out there.

For my tests, I have used this USB LiPo Charger.

LiPo Charger. Note the two connections to power our device

This is a very small board with all the elements we want:

  • A JST connector for the battery... the white connector. Most batteries use this kind of connector
  • A mini-USB (it would be better to have a micro-USB but this is not really a problem), to re-charge the battery
  • Two pin holes to power a external circuit. This is going to be our circuit

With this small device, we can power our circuit and also recharge the battery using a standard USB port from a computer.

DC-DC Converter

The final element we may need for our portable device is a DC-DC converter. This circuit basically rises or lowers voltages. DC stands for Direct Current, as this devices converts one Direct Current (with some voltage) into another Direct Current (with a different voltage).

Depending on your device, you may need one of those or not.

There are different types of DC-DC converters. For the kind of applications we are interested on we will be usually interested on the so-called Step-Up converters. These ones are able to get as input a low voltage and output a higher one.

Why are we interested on this?. When there are many devices, specially all those able to connect to to USB peripherals that requires 5V (this is what USB requires). And we already know that LiPo batteries just produce 3.7V, which is not enough for many USB devices to work.

DC-DC Converter
In those cases we need one of those Step-Up converters. The converters will also stabilise the variation of the voltage in the battery as it get depleted. Remember a LiPo battery can vary from 4.2V to 3V. Many circuits nowadays can work without problems using a wide range of input voltages, but you will have to check if that is the case for your specific device.

For my tests I used this handy DC-DC Converter, specially for prototyping and experimenting as the output voltage is adjustable.

For this kind of devices you need to pay attention to the input and output ranges and also to the maximum output current. The one I chose can provide up to 1A, enough to power even a hungry Raspberry Pi with some USB peripherals. If the output current of the converter is not high enough, your device may not work.

Testing

Now is time to see these stuff in action. We have done a couple of tests, booting some small computers from a small LiPo battery with a capacity of 1400mAh, and using the support circuits described in the previous sections.

First we solder some cables into the LiPo charger circuit to be able to power up our computers. Then we tested and adjusted the DC-DC converter.

DC-DC Converter adjusted to 5V aprox.

With all the elements set up we first powered up an Arietta G25 directly from the battery. We used the 5V pins in the expansion header to boot the board up, and that worked fine.

Arietta G25 powered through the expansion header 5V rail

Even the companion wifi board worked fine. Here you can see it connected to a portable hot spot configured on my Android phone.

Arietta G25 Wifi connected to an AP using the companion Wifi board

The Arietta is a very nice device. As you can see from the pictures you can build a pretty small device with wifi capabilities, basically just using a battery and a charging circuit. Note, however, that I have not done extensive testing, so I do not know at which voltage the Arietta will stop working and if the use of an Step-Up converter will be beneficial.

The other test we did was using a Rasberry Pi 2. This requires 5V, so we used the DC-DC converter. In this case we powered the board using the normal microUSB connector for power, instead of feeding directly the power in the expansion header.

Raspberry Pi 2 powered by a 1400mAh LiPo Battery

Overall, this also worked fine. The board with a keyboard and connected to a external display booted fine.

Done and Done

So, this is it for my basic experiments on battery powered devices. Nothing really complex or fancy, but a simple way to improve your designs getting rid of that bulky external USB power pack and producing something a bit more neat.

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