Field Operating - Power Sources
Power for portable radio operating can come from a variety of sources, in a wide variety of power levels and overall capacity. What solutions you find appropriate is largely dependent on what I’ll call your power budget, and your power budget is in turn determined by the equipment you plan to use, how you plan to use it, and how long your operating period will be. Your power budget consists of two independent values: your peak demand, and your capacity requirement.
Peak demand is just your maximum current demand at your operating voltage. If you’re on a mountain top, operating CW and running an MTR-3b at 12v, putting out about 5W of RF output, your peak demand might be as low as 700mA. On the other hand, if you’re operating for Field Day, and your station is operating FT-8 running 100W on a not very efficient transceiver, your peak demand might well exceed 25A at 13.8VDC. If you’re running a full contest capable station, with multiple transceivers and amplifiers all putting out 1.5KW, your peak power demand might well be on the order of 50A at 220VAC.
Capacity is a matter of average current demand, multiplied by total operating time. Average current demand is different from peak current demand, because ordinarily you transmit some of the time and are receiving the rest of the time. The ratio of transmit time and receive time is known at the duty cycle, and the formula for average current demand is
Anyway, I just want to point out that in some cases your power solution will sensibly be a rechargeable LiPo 9v battery weighing an ounce, and in other cases it might be a diesel genset. Which is to say, it’s horses for courses, so you need to decide what course you’re headed for, because there isn’t (and can’t be) a one size fits all solution.
Line power
In some places you might operate portable, you’ll have access to line power in whatever the local variety might be. In North America, that’ll be 120VAC 60Hz, but in other places it might be something else.
In those cases, all you need is a power supply that runs off the line and provides whatever flavor of power your radio needs - that’s most often 13.8VDC +- ~15%.
In that case, if you decide you’re going to run off the available line power, you probably want a compact, fairly lightweight switching power supply. Unless you’re running multiple radios or big power amps, your peak power demand will probably be less than 30A at 13.8VDC, which is handy because there’s a wide variety of power supplies from various manufacturers that will take 120VAC in and put 13.8VDC out and handle up to 30A.
My favorite is the PowerWerx SS30-DV, which fits the bill nicely and has Anderson PowerPoles on the front panel, which is convenient for portable work, and also binding posts on the rear panel just in case.
I’m also a fan of Samlex power supplies, which in my experience are really well built, compact, and modestly priced for what you get.
If line power is your plan, be sure to include extension cords that will let you operate where you want and still connect your power supply to the provided outlet. It would be wise to consider ways to prevent that extension cord from turning into a safety hazard as well. In some cases (in a picnic shelter, for instance) you can secure the cord to the rafters and route it overhead, and then have it drop down to the work surface and thus avoid the tripping hazard.
It’s worth considering, though, that it might be a good idea to have a backup plan for power, in case when you arrive on the scene line power is unavailable for some reason. For instance, I’ve been to a number of parks where there were line outlets at the picnic pavilion but the power to the pavilion was turned off for some reason.
Generator
An alternative to using local line power would be to bring a generator, and provide your own line power.
That’s certainly a viable solution, often used for club Field Day operations. My objection is that generators tend to be heavy, often bulky, generate both acoustic and RF noise, and need tending for things like fuel.
It’s worth noting that generators are most efficient when under significant load, not loafing along putting out just a few watts. Even when just idling, they’re burning fuel. In situations where it seems impossible to have enough battery capacity to meet your power budget, I’d be inclined to use batteries as the main power source - nice and quiet - and when the batteries get low, fire up the genset to charge them up at their maximum charge rate. Maximum fuel efficiency, AND maximum time with no noise!
Note that many places you might want to operate (parks, in particular) have limitations on when and where you can run a generator.
Batteries
Far and away, my preferred power source for field operating is batteries.
Batteries range in voltage from 1.5V up to well north of 48V, and in capacity from 400maH to over 400aH, so battery choice is largely about fitting the battery characteristics to your power budget and constraints like physical size, weight, recommended and maximum charge rates, and other incidental externalities like whether the battery must remain upright or can be inverted, vibration and shock resistance, resistance to dust and water, and so on.
There are various battery chemistries available, as well. For a long time, the rechargeable battery field consisted of either lead-acid or nickel cadmium (aka NiCad).
The newest entrants in the field are various chemistries that collectively get rolled up as lithium - Lithium Ion (aka LiPo), and LiFePO4, which offer substantially better performance in terms of energy density, energy to weight ratio, number of lifetime charge cycles, maximum charge rate and maximum discharge rate, self discharge rate, and so on.
Of the two lithium based battery technologies, LiPo offers slightly better capacity by volume, and slightly better capacity by weight, at the expense of being susceptible to thermal runaway, a cell voltage which is not quite as well suited to use in a nominal 12.8v system, a lower cycle lifetime, and a greater voltage swing as the battery discharges.
Putting it another way, with a cell voltage of 3.2V, 4 cells of LiFePO4 put you up at 12.8 volts, whereas with a cell voltage of 3.7V, 3 cells in series of LiPo puts you down near 11 volts and 4 cells puts you up near 15V.
The LIPO disadvantages of less felicitous voltage choices and lower cycle lifetimes are not as significant for QRP radio operating, where radios are often able to operate happily well below 12VDC and the lower current demand will drive the battery through fewer charge cycles over time. Once you move up into radios that will give lower performance if the operating voltage drops below 12VDC, LiFePO4 batteries become the better choice.
I’m sure there are reasons why you might choose some variant of lead acid battery (e.g. SLA or Sealed Lead Acid, AGM, Gel Cell, or even flooded cell) but at this point I can’t imagine what they would be. There was a time when lead acid batteries had a price advantage, with inexpensive SLA batteries being quite a lot less expensive than alternatives like LiPo or LiFePO4, although the LiFePO4 batteries could withstand so many more charge cycles before loss of capacity that over the long run they were cheaper. But the market has moved very swiftly and for the price of an SLA deep discharge battery, you can now buy vastly more capacity in LiFePO4.
It’s possible to beat this very dead horse for a long time with comparisons, but the bottom line is this: LiPo and LiFePO4 are lighter, more compact, have higher power density, high discharge and charge rates, and much longer lifetime as well as being cheaper out of the gate.
Battery Specifications
So here are my thoughts on various battery specifications and how you might evaluate them based on your use case for a battery.
Dimensions
Length, width, height - it’s not just a matter of a smaller battery is handier, but also a matter of the shape meeting your needs. I have a 12aH battery that’s a long rectangular prism and that’s a useful shape primarily because it fits neatly into a certain spot in the bag I carry my Yaesu FT-891 in.
For a bigger, higher capacity battery, the question might be whether it fits easily into the footwell or trunk of your car.
Weight
I will admit to being a freak about gear weight. I’m reasonably strong and quite fit but I still dislike gear that is heavy, and batteries have a definite tendency to be heavy if they are also high capacity.
Weight (or more appropriately viewed, power density) is where LiFePO4 batteries are a dramatic win over lead acid batteries. I have a Group U1 sealed lead acid deep discharge battery that weighs ~25 lbs and has a capacity of 35aH. Recall that with a lead acid battery, discharging it below 50% capacity dramatically reduces the lifetime of the battery, so that’s really a 17.5aH useable capacity.
In contrast, I have a recently purchased LiFePO4 battery that weighs 19 lbs and has a usable capacity of 100aH - nearly six times the capacity of the SLA battery at 80% of the weight.
In any case, you need to consider how far you’ll want to carry a battery when you’re choosing. If you need to carry it 15 miles, you’re going to want a battery that’s a lot lighter than something you might consider if you’re only ever going to lift it into the trunk of your car, and then at the end of the day lift it back out.
Maximum Discharge Current
Big batteries generally have very high maximum discharge current specs, often in the hundreds of amps. Very small batteries generally have maximum discharge current specs that might be as low as one amp.
So it’s important to make sure that you consider not just capacity, but maximum current, to make sure the battery fits the power budget demands you expect when using it. Sometimes you need to get a bigger, heavier, higher capacity battery than you really want simply because your power budget demands higher max current.
Voltage
Obviously, you want a battery that has a voltage that matches the operating voltage for your equipment. For most ham gear, that’s going to be 13.8VDC +- 15%, but it’s worth noting that there are a fair number of QRP radios that demand operating voltages lower than 12VDC, and you put the finals at risk if you run them with higher voltages.
Capacity
Your total battery capacity divided by your power budget tells you your expected operating time without a recharge (or supplemental power).
Maximum/Recommended Charge Current
As with discharge current, small batteries generally have smaller charge current. As a rule of thumb, with LiFePO4 batteries you can generally expect that the maximum charge rate will work out to roughly 20% of the capacity per hour.
If you only ever plan to put your batteries on a charger at the end of the day, and then come back in the morning to a fully charged battery, this probably isn’t important.
If you are doing a POTA rove and your plan is to recharge your battery as you drive from one park to the next, you’re in a different situation and you care about maximum charge current quite a lot.
Consider the case where operating for half an hour to activate a park before moving on, your gear uses 2aH - not unusual for a 100W rig running full power. In that case, you might think a 4aH battery would be plenty for your needs. But remember you need to recharge that battery during the drive to the next park. If that drive is half an hour, you need to recharge the battery at a rate of 4A to get it topped back up, and most batteries in the 4aH capacity range will have a maximum charge rate of 1-2 amps.
In that case, you’ll need a battery with a higher charge rate, which in most cases is going to also mean a higher capacity battery as well.
Or, as an alternative, you’ll need two batteries to hook to the radio, and one big battery to use as a power source to charge them. Then while you are using one battery to operate, the other battery can be charging off the big battery, and as you drive from park to park you can charge that big battery at a much higher rate (perhaps 10-15A) to top off the big battery.
Internal Resistance
Internal resistance determines the voltage drop from the battery as current increases, so lower internal resistance is always better.
Smaller batteries will generally have higher internal resistance. Higher capacity batteries will generally have lower internal resistance.
Internal resistance may increase as a battery ages.
Cycle Lifetime
Some batteries (and I am here thinking of SLA batteries) have a fairly low number of charge/discharge cycles before the battery becomes unusable.
Even though LiFePO4 batteries generally have a very high cycle lifetime (thousands of cycles), more expensive batteries will often have incredibly high cycle lifetimes.
Sometimes you can save money over the long run by buying a more expensive battery that has a much higher cycle lifetime. Sometimes you can save money by buying a less expensive battery with a lower cycle lifetime and simply replacing it.
BMS
Unlike lead acid batteries, LiFePO4 batteries should have a Battery Management System (aka BMS) that handles things like overcurrent protection, short circuit detection, low voltage cutoff, charge management and cell balancing, and so on.
Not all BMSs are RF quiet. Unsurprisingly, cheap BMSs in inexpensive batteries are much more likely to generate RF noise than the high quality BMSs you find in more expensive batteries, and this is something to keep in mind as you shop on Amazon for the least expensive battery you can find.
On top of the RFI issue, newer BMSs will track the state of charge of the battery, and you can use a bluetooth app on your phone or tablet to communicate with the BMS and find out the remaining capacity of the battery. As you can imagine, this is damn handy, because it neatly avoids having to wrap the battery in a bunch of electronics that track current in and out and thus can tell you the remaining capacity. The app will also tell you useful stuff like how balanced the battery is, the state of health of the cells, and so on. In addition batteries with these newer BMSs often have a low temp charging cutoff, which prevents charging the battery if the cell temperature is below 32F - trying to charge a frozen battery will cause permanent damage, so that’s handy protection.
I’m a big fan of these newer BMSs and greatly prefer to get this feature especially on big batteries. Generally this seems to increase cost by about $30, which seems like a good deal to me.
Physical robustness
Small LiPo and LiFePO4 batteries are often just wrapped in shrink wrap, or encased in some lightweight stiffener (e.g. cardboard) and then wrapped in shrink wrap. As you can imagine, that leaves the battery relatively unprotected against the thousand mortal shocks that field equipment is routinely exposed to.
In contrast larger batteries are available in a wide variety of cases, ranging from the shrink wrap arrangement used for smaller batteries all the way up to robust cases that are watertight and dust tight. Unfortunately, those cases improve protection at the expense of size and weight, so you need to strike a balance that matches your needs.
It’s worth considering, though, that if your gear is going to get rained on, a battery that is at least weather resistant seems like a good plan.
