An important aspect of amateur radio is power during emergencies. The grid is down so what do you do? If you have a fuel powered generator in your house, you're all set – as long as you don't need to move your radio elsewhere. If you need to be portable, then a battery supply is the way to go.
The three largest factors to consider in a battery are price, Amp hours, and battery type. Price is affected by your choices of the latter two.
Amp hours are used to determine how long a battery will operate under load. The term Amp Hours (Ah) means how much amperage a battery can continuously provide for a one hour period before the battery drops off. A 20Ah battery should continuously provide 20 Amps for one hour or 10 Amps for two hours, etc, That's the theoretical. In the real world, it's not that cut and dry.
A battery's listed Amp hours are usually specified for a specific current such as 5 Amps. Thus, that 20Ah battery can provide a continuous 5 Amps for four hours before the battery drops off. However, if you were drawing more current such as 10 Amps, more power would be dissipated internally as heat draining the battery a little quicker than you would expect (Peukert Effect). At 10 Amps, you would theoretically expect the battery to last two hours but it will likely be shorter.
Want more real world? You can't cycle a battery down to 0% capacity without damaging it. For most lead acid batteries, you can only draw them down to 50%. That 20Ah lead acid battery won't give you more than 10Ah unless you choose to risk damaging the battery. Lithium-ion batteries can be cycled deeper (often 80 or 90%) so they give you closer to the rated Amp hours.
The Amp hours you need in a battery depend upon your radio and usage. You need to make an estimate of the duty cycle of your radio. Will you be transmitting continuously or mostly in stand-by. Will you be using high power all the time? How long do you need to operate with the battery before charging it? These are the questions you need to ask yourself. Thinking about these factors will determine your minimum Amp hour requirement.
You need to know how many Amps your radio will draw on average over a time period such as one hour. Your radio's power usage will be different when transmitting and receiving. The current needed for each should be listed in your radio's technical documents. Let's say it's 5 Amps transmitting and 2 Amps receiving. Now you need to guess what percentage of the time you will spend transmitting versus receiving. That's the duty cycle.
A standard Duty Cycle might be 20% transmitting and 80% receiving. With the 5 Amps transmitting and 2 Amps receiving listed above, your average ampage would be (5 Amps x 0.2) + (2 Amps x 0.8) = 1 Amp + 1.6 Amps = 2.6 Amps. Each hour, you would expect your radio to draw 2.6 Amps. In an emergency, your duty cycle might be 50% / 50%. Then your average ampage would be (5 Amps x 0.5) + (2 Amps x 0.5) = 2.5 Amps + 1 Amp = 3.5 Amps.
Bioenno Power, a local lithium battery company in Santa Ana, provides several duty cycle and battery comparison charts to help amateur radio operators select the best Amp hours. They have a 20/80 Standard Duty Cycle Chart, a 50/50 Heavy Duty Cycle Field Day Chart as well as Radio-Battery Compatibility Charts for some Yaesu and Icom Radios, some Elecraft, Kenwood, Flex Radio, and Powerwerx Radios, along with Alinco and TYT Radios. They may not list every radio, but it's impressive that a battery manufacturer would go to this much effort to cater to amateur radio.
In general terms, I consider twelve Amp hours as a bare minimum because you never know what other power needs might crop up. Twenty to twenty-eight Amp hours would be a lot better and thirty Amp hours or more is not uncommon for many radios/usage. Unfortunately, the higher the Amp hours, the higher the price (and usually the weight and size of the battery).
There are two main considerations in battery type:
- High Rate vs. Deep Cycle
- Lead Acid vs. Lithium
High Rate vs. Deep Cycle
A car battery is a good example of a high rate battery. To start most standard sized car engines requires around 400 amps. For SUVs and trucks, you might need around 1000 amps. That's a large amount of current but only for a very short time. After that, the car battery has a very low load until the next time you need to start the car. These batteries deliver only a small part of their capacity in short, high bursts. This isn't how amateur radios usually operate so high output batteries are not the best choice.
Deep cycle batteries are designed to operate for extended continuous use. These batteries can be "drawn down" deeper than high rate batteries. These are what you are looking for in an amateur radio battery.
Lead Acid vs. Lithium
Wet Cell (Flooded) lead acid batteries should be avoided in most situations. Their content is caustic and a spill can injure people and damage equipment. In addition, the vapors from these batteries are toxic. If you are operating in a tent or structure of some kind, these batteries should remain outdoors. This makes them a poor choice for amateur radio
Sealed lead acid batteries are a better choice. Unless damaged, they don't leak or give off toxic vapors making them acceptable for amateur radio use. Their main advantage is they are cheap and plentiful. Their disadvantes are primarily three. They are much heavier than other batteries. They are not environmentally friendly. Lastly, they can usually be drained no more than fifty percent of capacity. Draining these batteries beyond that often causes damage.
Older lithium batteries had a reputation for catching fire if over charged. Modern lithiums are much safer. Most have a Battery Management System (BMS) built into the battery. A BMS usually prevents under or over voltage of the battery and prevents short circuits from damaging the cells.
One main advantage of lithium batteries is their light weight compared to comparable lead acid batteries. In addition, they can be cycled more deeply (80-90%) than a lead acid battery (50%) and have more lifecycles. Over the lifetime of the battery, you can get a lot more power out of it.
The main disadvantage of Lithium batteries is their higher cost. An 18Ah sealed lead acid battery might cost around $40 or $50 while an 18Ah lithium battery might go for $130. In addition, they usually require special AC chargers and solar regulators rated for lithium batteries. You shouldn't use lead acid chargers on lithium batteries.
The two most popular sub-chemistries of lithium-ion batteries are Lithium Nickel Manganese Cobalt Oxide (NMC) and Lithium Iron Phosphate (LiFePO4 or LFP). LiFePO4 uses Lithium-phosphate as a cathode material while NMC uses Lithium, Manganese, and Cobalt as cathode materials. NMC has a higher energy density, meaning there will be more energy per the same amount of battery. LiFePO4 has a more stable chemistry, so the temperature threshold for thermal runaway is higher than that of NMC making LiFePO4 batteries safer.
The lifecycles of NMC and LiFePO4 batteries are also different. Typical NMC batteries can be fully cycled 500 – 1000 times before reaching 80% of original capacity. LiFePO4 batteries can be fully cycled 2000 or even 3000 times before reaching that 80% capacity mark. Thus, the total cost of ownership of a LiFePO4 battery is lower than an NMC.
Total Cost of Ownership
After determining your duty cycle plus other accessory usage along with your preferred hours of battery use between charges, you decide you need a battery with 20 usable Amp hours. You also expect to cycle that battery on average about 2 times per week. Now, let's take a look at the overall cost of a lead acid versus LiFePO4 battery.
Example 1 — Lead Acid Battery
A lead acid battery can only be cycled down to about 50% capacity so to get 20 usable Amp hours, you need to purchase a 40 Ah lead acid battery at an estimated price of $75.
With a lifecycle of about 300 cycles and 20 usable Amps per cycle, at the end of the battery's lifecycle, you've gotten about 6,000 Amps (20 Amps x 300 cycles) out of the battery. Taking 6,000 Amps and dividing by the price of the battery ($75) gives you 80 Amps per dollar. Since you are cycling the battery twice per week, the battery will last you 2.8 years.
Example 2 — LiFePO4 Battery
A LiFePO4 battery can be cycled down to 80% capacity. So to get 20 usable Amp hours out of it between charges, you need a 25 Amp hour battery. Let's estimate that cost at around $175.
With a lifecycle of about 2000 cycles, you will get around 40,000 Amps out of the battery. 40,000 Amps divided by $175 equals 229 Amps per dollar. Cycling it twice per week means the battery will last 19.2 years! Alright, due to other factors, you are unlikely to get a battery to last 19 years but you certainly will get a much longer life out of it than a lead acid battery.
Total Cost of Ownership Summary
You would have to purchase almost seven lead acid batteries (at $75 each) for a total of $525 to match the lifespan of one LiFePO4 battery (at $175). While the upfront cost of the LiFePO4 battery is higher, the long haul cost is much cheaper than a lead acid battery. This example was simplistic. It didn't take into account how a battery might degrade after 19 years nor the added cost of a LiFePO4 rated solar controller if you want to use solar panels, etc. The point is that the long term cost of a LiFePO4 battery is actually much cheaper than a lead acid battery, so don't let the sticker shock fool you when looking at lithiums.