The NVIDIA Jetson TK1 is the development kit for the Tegra TK1 embedded processor. Naturally, there are a lot of embedded projects with the requirement of being mobile, and running on battery power. Running the Jetson on a battery is pretty simple. Looky here:
There are a large variety of battery technologies, each one having advantages and drawbacks. At their core, batteries store energy in chemical form and convert the energy into electrical form. Currently the Lithium Polymer battery (LiPo) is among the smallest, lightest and most energy dense packages available. For smaller mobile devices, such as mobile phones, LiPo batteries are usually the first choice. For larger applications, another form of Lithium battery, called Lithium Ion is usually preferred. Unfortunately, Lithium Ion batteries are a little more fragile and require a protection circuit. For this article, we’ll be talking about using LiPo batteries to power the Jetson TK1 because we hate fuss.
The Jetson TK1 FAQ states:
Jetson TK1 is presently characterized to accept an input voltage of 12V ±10%. The board may not reliably turn on below an input voltage of 9.5V, and a voltage above 13.2V may damage SATA HDD using 12V. Above 16V, the main board may be damaged. Input voltage ranges from 9.5-10.8V and from 13.2-16V have not been characterized. It may be possible to run the system on batteries around the 10-16V region (without HDDs requiring 12V), but NVIDIA has not tested this configuration.
From this, we can calculate that we’d really like to have a battery that supplies 12 volts. You can read more about powering the Jetson with a battery on the Jetson Wiki.
Here’s a fun fact about people who sell batteries: They are invertebrate liars. Only some of that is their fault. Batteries are typically sold at a specific voltage, say in the case of a LiPo battery 11.1 Volts. However, the batteries deliver 11.1 volts for a short amount of time as they go from a state of full charge to low charge during use.
LiPo batteries are made up of one or more cells. Each cell typically supplies between 3.0 to 4.2 volts depending on the charge. Multiple cells are combined to get higher voltages. A LiPo 1S battery is 3.7V, a 2S battery is 7.4V, a 3S battery is 11.1V, all the way up to 6S which is 22.2V.
In a fully charged state one cell of a LiPo battery is 4.2V, in a discharged state around 3.3V. As an example, a 3S LiPo battery fully charged is delivering 12.6V, and when discharged is providing 9.9V. You’ll note here that those numbers happen to fit the operating range of the Jetson TK1, and are thus a good match for our use case.
Note about safety: You should never go above or below the recommended limits, as damage may occur to both the battery and the surrounding environment. If the battery is overcharged, bad things tend to happen which usually start off with a loud noise and then a fire, then men in a big red truck show up. Batteries that have been physically damaged or charge abused also tend to demonstrate that behavior. A LiPo battery that appears to be “puffy” is actually outgassing, and should be disposed. A puncture to the battery tends to be really bad, as a lot of energy is released all at once.
As you saw in the video, we selected Venom 20C 3S 2100mAh 11.1 LiPO Battery (available from Amazon), a LiPo 3S battery made by Venom. The battery has a capacity of 2100mAh. The battery will last several hours under normal loads, but both smaller and larger capacity LiPo batteries are available.
You will probably need a battery charger too. LiPo batteries require a balancing charger, which means that each cell in the battery is individually monitored and charged to the correct voltage. In the video, the demonstration used a SKYRC iMAX B6AC V2 (available at Amazon). It’s a popular unit, and you’ll be able to find a lot of YouTube videos on how to use it. The SKYRC can charge several different types of batteries in addition to LiPo. To set the charge, I used 2.1A at the normal charging rate. Typically at normal charging rates, it’s just the capacity of the battery expressed in amps (2100mAh converts to 2.1A).
Please use a LiPo Charging Bag. The men in the big red truck have more important things to do than to watch your house burn down.
Once you have the battery fully charged (it comes from the manufacturer charged at about 3.8V per cell, which is also a good voltage for long term storage), you can hook it up to the Jetson. You will need a connector. For the demonstration a Power Cord with 2.1mmX5.5mm Barrel Plug to XT-60 Male plug was used. The Venom battery has a “universal” plug, which allows one of four different adapters to be used for different connector types. The XT-60 connector was attached to the battery universal plug, then the barrel plug power cord was connected to that. Then the barrel plug was plugged into the Jetson. Fire her up, and Bob’s your uncle.
Roboticist Myzhar (http://myzharbot.robot-home.it/blog/) wrote in with words of wisdom:
A little note: it is really easy to damage a LiPo battery. The charge level of a single cell should not go under 3.0V, otherwise the cell will be damaged and the battery life reduced.
Furthermore the discharge rate is not linear and a cell takes very little time to drop from 3.3V to 3.0V.
To be sure to not damage the battery you can use a simple circuit like this
Integy C23212 Lipo Voltage Checker/Warning Buzzer
The circuit must be connected to the “balance” connector and will advise you with a noisy beep that your battery is near to dangerous charge level.
I use this one: Venom Low Voltage Monitor for 2S to 8S LiPO Batteries. I was shortsighted and was going to include this information when doing an upcoming project, but Myzhar correctly points out that this information is important enough to be broadcast when talking about using the batteries in general.
As you saw, it’s pretty simple to get the Jetson to work on battery power. If you’ve seen some of the previous JetsonHacks videos, you’ll know that we configure the Jetson to run flat out, with no consideration given to power consumption. Obviously that’s not the way to go if you’re running on a battery! We’ll have to address that in the future.