Going out into nature and setting up a station sounds like fun . . . but how do you actually do that? The first thing is figuring out a power plan; if you can’t power your gear reliably then nothing else matters.
Antennas, radios, and even computers can suffer from minor issues without taking you off the air, but if your power goes down . . . you’re done!
Define your needs
When looking around at power systems it is easy to get overwhelmed. The market is packed with high-performance, high-price, super lightweight gear. But the latest and greatest is not always the best fit for your needs.
Sometimes you just need something simple, reliable, and easy to maintain. When building these packs I intended to use them for Parks on the Air activations, so I first did a realistic assessment of what my needs were:
- Most of these activations take place close to a vehicle, so I didn’t need something ultralight. I could use lead-acid batteries and save money.
- I needed to be able to recharge the batteries in the field . . . in Alaska . . . in winter. Solar isn’t a realistic option in the winter, which means the system must be chargeable from a running vehicle.
- The packs have to be waterproof in case they get rained on.
- I need at least two power pole connections for gear to tie into.
- The system has to have a fuse in case something shorts out.
I already had a few lead acid batteries that were waiting for a project, so I decided to build the packs around those. The next step was to find a container and figure out the wiring. For the container I used two plastic ammunition boxes I picked up at Cabela’s. The wiring diagrams are shown below. Click on each for a downloadable PDF.
It took some work to assemble the packs, but they came out great and have worked perfectly on multiple POTA activations.
I have seen packs like this built using metal ammunition cans before, but I would actually recommend against that. While it is true that the metal cans are stronger, they are a bit overkill for this pplication. Any impact that is able to crack the plastic ammunition cans is hard enough that you should probably take the pack apart and inspect it.
Also, the plastic cans are immune from corrosion, and are non-conductive. Both of those are good benefits for a system that you are planning to take into the field.
To finish out the packs I ordered some panel mount gauges and power pole connectors from Powerwerx. They make good products, and some components are better to buy pre-made than to cobble together from junk drawer pieces.
The only part I really had trouble with was wiring the batteries in parallel. I used 10 gauge wiring in the packs to keep resistance to an absolute minimum, but there isn’t any chance of getting two ten gauge wires into a single spade terminal for connection to the battery.
What I ended up doing was removing the insulating collar from the spade connectors and opening up the part where the wire goes. That part of the connector is just folded over and rolled, so with a pair of needle nose pliers you can open it up more to get a larger wire in. You can’t crimp the wires that way though, so I soldered all of the connections together where they connect to the battery terminals. It works.
To charge the batteries I just use an automotive trickle charger. The car charger has the right charging cycle for this battery chemistry, and with the batteries wired in parallel they all charge as though they were a single, larger cell.
I have also tested field charging using solar panels, and it works great. My camping solar panels have raw DC output, which I feed into a charge controller. With the charge controller set for lead-acid batteries the system just works.
I can also charge from my truck, using power pole connectors wired into a battery isolator. While you might get away with connecting the pack to the vehicle’s electrical system using cables, you really shouldn’t try it! If the pack is deeply discharged you could get a pretty severe surge of power from the car battery into the pack, likely damaging both.
If you absolutely must charge a pack like this from your vehicle, without going through something like an isolator, then there is another way to do it safely.
- Start the vehicle.
- Disconnect the battery from the vehicle. It will continue to run because the alternator is still being driven by the engine.
- Connect the positive and negative leads of the pack to the corresponding battery terminals on the vehicle.
- Monitor everything closely while it charges, watching out for heat buildup.
The idea with the above method is that you can’t get a surge of current from the vehicle’s starting battery since it is disconnected. The vehicle’s alternator and charging circuitry should do a good enough job of charging the battery pack to get you by. But keep in mind that the charging circuitry in the vehicle is made for a much bigger battery, and so you need to keep an eye on the pack to make sure it isn’t getting overcharged.
The point: use an isolator for charging from a vehicle.