Until now, I have been running wall adapters to my Raspberry Pi’s and an Ethernet switch, but it’s time to get things wired in a bit better. From determining our power needs and choosing our equipment to soldering it all together, today we are taking a look at our EmComm server’s electrical setup.
For portability, our server needs to be able to run off a battery as well as AC power when available.
Determining Power Requirements
Before we can determine things like which battery or power supply to buy, we have to determine the energy needs for our system.
The best way to do this is to list everything downrange of the battery, fill in their electrical specifications, and add everything up.
| Quantity | Item | Wattage | Voltage | Amperage |
|---|---|---|---|---|
|
x4 |
10w |
5v |
2a |
|
|
x1 |
10.5w |
24v |
450mA |
|
|
x2 |
5w |
5v |
1a |
|
|
x2 |
1.5w |
5v |
300mA |
|
|
x2 |
1.5w |
5v |
300mA |
|
|
x1 |
1w |
5v |
200mA |
|
|
x1 |
500mW |
5v |
150mA |
|
|
x1 |
250mW |
5v |
50mA |
|
|
x1 |
240mW |
12v |
20mA |
This should get us pretty close, but isn’t the full story. Every time we convert our power from one source to another (like our POE equipment or step up converters), there is a power loss associated with the conversion. When it comes to gauging our battery runtime, we will need to account for this power loss as well.
To calculate this, we will need to know the efficiency rating as well as the amount of power going through each component. For our POE HATs, this means knowing how much power is needed for each Raspberry Pi and its periphery equipment; for our switch, we will need to know the total power consumed by all the Pi’s and any other equipment drawing power off the switch, and so on.
This isn’t a difficult step when you have all the information at your disposal, but unfortunately, the power efficiency rating isn’t something that is always listed. To make things a little simpler, I used ChatGPT to estimate the power loss of some of these based off similar models when the actual efficiency wasn’t listed.
| Efficiency | Item | Max Watts | Req. Watts | Power Loss |
|---|---|---|---|---|
|
90% |
120w |
60w |
6w |
|
|
78% |
100w |
48w |
10.56w |
|
|
96% |
288w |
108w |
4.3w |
Now we know the power needs for our setup assuming we are powering all components at once. Actual power consumption may be less as we will be configuring our software to allow the hardware more efficiently (like not using all 4 SDRs or both LoRa transceivers at once, or at their maximum duty cycles). But to be on the safe side, we will actually add 20% to our required wattage when calculating battery runtime and the respectful ratings of our hardware and cables.
Rechargeable Battery Types
Choosing the right battery is an important consideration when building a portable server such as this. Not only do we have to balance runtime with size and weight, but the type of battery you go with can make a huge difference in performance and longevity.
Lead Acid
Traditional lead acid batteries are very affordable and widely available, however they were also immediately ruled out for my setup. Not only are they heavy and bulky, but lead acid batteries (sealed or flooded) have a shorter lifespan and are more sensitive to things like overcharging and cold temperatures. They have poor energy efficiency, losing more energy in the form of heat.
Un-sealed lead acid batteries also often require maintenance (such as adding distilled water) and are slower to charge. Lastly, they are not intended to be discharged below 50% as this can shorten their lifespan even more. Overall, the difference in price wasn’t worth all the drawbacks of using lead acid.
Lithium Ion (Li-Ion)
Lighter and more energy dense than lead acid, Li-Ion batteries were the first batteries I initially considered, though they were also less than ideal for my needs.
Compared to lead acid, lithium ion batteries have a longer lifespan both in terms of recharges and their low discharge rate.
Lithium Iron Phosphate (LiFePo4)
From my research, LiFePo4 batteries seemed like a clear winner. Not only can they be recharged thousands of times, but they also have a greater depth of discharge and are lighter weight than the other options we’ve discussed.
Rather than go over all the specs again to show why Lithium Iron Phosphate is the way to go, here’s a chart breaking down the main differences:
| Lead Acid | Li-Ion | LiFePo4 | |
|---|---|---|---|
|
Depth of Discharge |
50% |
20% |
~0% |
|
Operating Temps |
-4F – 140F |
-4F – 140F |
-4F – 140F |
|
Recharges |
200-300 |
3,000 |
4,000+ |
|
Weight |
19.5lbs |
11.7lbs |
6.3lbs |
|
Price |
<$50 |
$200-$350 |
$75-$100 |
Sizing The Battery
Our POE switch can accept an input voltage of 12v-48v, but runs more efficiently and is capable of delivering more power at 48v. So to give ourselves ample head space in case of spikes in usage and so we can more easily add more equipment in the future, we will use 48v as input power. But does that mean we need a 48v battery? Not necessarily.
While 48v batteries are typically more energy dense (meaning they can store more energy while weighing less) and more efficient overall, they are quite expensive. The 12v 30AH battery I ended up selecting was only $65, but a comparable 48v battery would be closer to $300. For under $50 you can find decent 12v-48v step up converters which is a lot easier on the budget. Additionally, since 12v is much more common, we will be able to swap the battery out or connect additional power sources more easily.
Given the amperage we calculated above we can gauge the battery runtime. As we will be using roughly 13A-15A total, we can estimate a runtime of roughly 2 hours with a 30Ah battery. Originally, I wanted to go with a 10Ah battery for weight savings, but this would only give us a 40 minute runtime before we’d have to connect to additional batteries or AC power.
Two hours of connectivity in an emergency isn’t ideal, but I felt that it was better to keep the total package lighter and more compact with additional battery boxes for scalability. This prevents a single point of failure and allows it to be more portable. Remember, if it’s inconvenient to take with you, it’ll end up left behind.
Connecting External Power
With everything running off the battery, we need a way to keep it charged.
Our battery will power our equipment through a fused Anderson Powerpole distribution block and our battery will in turn connect to outside power through an Anderson Powerpole bulkhead in the side of the Pelican case. This power can be supplied via external Batteries or AC power. Here’s how I intend to handle both:
In addition to the server’s native battery, I want to build some field batteries similar to this that I can plug directly into the server to extend runtime. Compared to using a single larger battery in the server case, this methodology keeps the individual components (the server and external batteries) can be lighter weight and more compact for ease of portability.
It also offers good scalability and flexibility as I’ll be able to run practically any 12v source with the right power adapters and bring the right number of batteries for the mission at hand.
I plan on doing a separate post for the battery box build, but basically each field box will be equipped with their own capacity monitor, Anderson Powerpoles, USB-A, USB-C and 12v cigarette lighter outlets. This will allow them to power a variety of equipment, not just the portable server.
Connecting to AC power is also pretty straight forward with my configuration. The battery is supplied with an external Anderson Powerpole bulkhead, allowing it to interface with an AC wall adapter that will charge the battery with pass-through capabilities. The charger I selected is specifically made for LiFePo4 batteries and supplies enough power to both charge the battery at a decent rate and power the server simultaneously.
Wiring It All Together
Warning
First of all, keep in mind that I am not an electrician, so be sure to verify any guidance given here. Also be sure to use proper PPE and common sense when working with electricity. Ensure all components are powered down and disconnected from any power source before starting.
While this isn’t the most complex electrical setup, I made myself a simple wiring diagram to remind myself where each part went and what it was there for. This not only helps when it comes to wiring everything together, but also when comparing and rethinking different parts during the planning phase.
This is the diagram I came up with.
Battery Monitor
A good place to get started is the battery monitor as it will attach directly between the terminals. I decided to go with this one from Drok which displays the voltage, remaining charge, and temperature of the battery. It supports a wide range of battery types, but its important to insure the monitor you choose is optimized for the battery you go with.
Fuse Block & Fuses
From there, we can run a splitter between the battery, fuse box, and our bulkhead. This will allow us to power our server equipment via AC power when it is plugged in, or its onboard DC power when not. As I’ve said before, fuses are cheap, so this block may as well help protect our equipment as well as distribute the power.
As with our other electrical components, double check that the fuse block you select is properly rated for the amount of power your server will be running with some headroom. Likewise, use appropriate fuses that are rated higher than your normal power requirements that will still trip before damage occurs to your equipment.
Step Up Converter
Since we are working with a 12v battery and the POE switch is optimized for 48v, we need to add a step up converter to bring the power coming from the battery up to the right voltage. This is pretty self explanatory, but ensure that it is properly rated for your power needs.
Power Over Ethernet
Power over Ethernet (PoE) allows you to both provide power and data to compatible devices with a single cable. Even for our small set up, this means we can use 5 cables instead of 10, and of course cable management is even more crucial in larger clusters.
To provide power and connectivity to our Raspberry Pi’s, we will need 3 things: a PoE switch, cables, and PoE HATs on the Pi’s themselves.
PoE Switch
Even more crucial than our battery is our PoE switch. I wanted to balance size and weight with capabilities so chose this 5 port managed switch. I was planning on an L2 managed switch, but it was 8 ports, and I only needed 5. Unfortunately, this meant losing a bit of added control over the network, but it was something I could live with. Additionally, only 4 of the ports supply PoE with the 5th one being Ethernet only. This means the router will either need to use a PoE injector or be wired in separately. But as with everything, there are no free rides.
PoE HATs
To allow the Raspberry Pi’s to accept power over Ethernet, you need some additional hardware. I went with the Waveshare PoE HAT (f) as it checked all the boxes. Max 25w PoE, stackable GPIO headers, and even an integrated heat sink and cooling fan for around $30.
Ethernet Cables
Since this server will be used for RF communications, I was concerned with potential interference and reliability issues. To alleviate these I chose to go with 1ft shielded Cat 6a network cables. They are capable of 10Gbps and provide reduced electromagnetic interference compared to some other cable options.
Cables & Connectors
Speaking of cables, we will obviously have to use a couple different gauges of wire and various connectors when assembling our system. Of course the gauge wire will be determined by the current going through those components and any cables that may generate EMI will have appropriate ferrite chokes.
For connectors, we will primarily be using Anderson PowerPoles. Theseare great for modularity should we need to replace any parts in the future and provide a secure connection.
Recap
Powering our server effectively and efficiently is essential to getting the most out of it in an emergency. From selecting the right battery to calculating the power needs for your equipment, this is an essential part of the planning process for our portable server project.
If you haven’t already, be sure to check out our other articles on the topic and take a look at our YouTube where we post even more content. Don’t forget to subscribe to be updated when we release future content and join the discussion below.