What Size Battery For Portable Ham Radio Operations

It'due south e'er some variation of "How big of a battery do I need to run my (fill in the blank) radio?" Information technology comes up a lot, non just in my electronic mail but as well on the diverse forums and blogs I visit. The question is too open ended and comes with too many variables to give a definitive reply, just there are some bones battery concepts that will help yous sort through this confusing topic.

Before asking the question, provide some answers.

It certainly does non help that many of the answers floating effectually the net are based on guessing, hypothetical conditions, and overly generous manufacturer data. Before you can know how much bombardment you "demand", first notice out how much power all your stuff consumes and what yous plan on doing with it in the real earth. Off Filigree Ham reader James (whose question was the inspiration for this commodity) asked nigh going off filigree with his Yaesu FT-450 radio. The official Yaesu specifications state that this radio consumes maximum 22 amps/304 watts on transmit, and 0.55-one.5 amps/8-21 watts on receive depending on the audio level (these numbers are rounded).

Belch reality.

James wants to run his radio with a 35 amp hour AGM battery and charge it with a ii amp plug in charger. He plans on adding a solar console at a later time. So what can he realistically expect from this setup?

A 35 amp hour bombardment can provide 35 amps for one hr. This is known every bit the C-rate or 1C-rate. The 0.5C-charge per unit (or c/2) would exist 17.5 amps for two hours, and then on. Post-obit the math, the 35 amp-hour battery should push button James' 22 amp transmitter for a little over 90 minutes. In the receive simply mode, assuming an boilerplate of i amp, the bombardment will get for 35 hours. As we'll notice out presently, these estimates are manner overstated.

Only start let'south deconstruct this. Nether about conditions an amateur volition be transmitting only part of the fourth dimension, and even then, the radio will not continuously pull maximum amperage. A modulated signal will cause the electric current to swing up and down, hitting the peaks merely momentarily. In fact, the radio will depict continuous peak power just when transmitting in the AM mode, which is seldom used anyway.

Duty cycle and why it matters.

Duty wheel is expressed every bit a percentage and is the term used to describe the ratio of transmit to receive. If your duty cycle is fifty%, that means you lot split your radio fourth dimension equally between talking and listening. Keeping your duty bike low (ie, don't run your oral fissure on the air as well much) volition stretch battery life.

Battery functioning varies greatly depending on the temperature, age of the battery, and internal characteristics. Even identical batteries tin yield different results due to pocket-sized inconsistencies in the manufacturing process. There is likewise the rule of diminishing returns: From the moment a battery is placed into service, its abilities begin to dethrone. Whatever performance you get on Day One, information technology volition merely become downwards from there.

I mentioned earlier that manufacturer claims are overly generous. I'll bet a gold microphone that the average 35 amp hr battery, fifty-fifty a brand new one, volition not produce 35 amps for an hour. It might not striking 35 amps at all. Do not always wait a battery to perform to its published specifications, so the "35 amps for i hr" merits is BS without fifty-fifty doing whatever calculations or measurements. Batteries can exist discharged downwards to 60-40% of capacity, depending on what you lot have (lithium batteries can go lower), then the real-life run fourth dimension on your battery may not even exist one-half of what the specifications imply.

QRP for the win!

One fashion to get more runtime from a battery is to refuse the transmit ability. I heavily promote QRP operating here on Off Grid Ham not merely considering I personally like it, just because it has real, applied value in that it conserves a lot of energy for only a modest sacrifice in signal effectiveness.

Getting back to the question…how much tin can James get from a 35 amp hr battery when hooked up to his FT-450? Nosotros've gone over the math: Depending on his duty bicycle, he'll accept at near a few hours of operating time if he's running total power.

BATTERY AGM

GRAPHIC COURTESY OF AUSSIESOLAR.COM

There is one last wrench in this motor. The 22 amps/304 watts specified for the FT-450 is predicated on a 13.lxxx volt power supply. An AGM bombardment volition likely not even take a charge to 13.80 volts, and if it does, it won't stay there in one case the battery is placed on a load. As the voltage drops, the radio will continue to work but volition not put out full rated ability. Ultimately, the radio may operate erratically or shut downward completely. Some radios, such as the FT-817, easily tolerate voltage swings and volition piece of work between 8.00 upward to about sixteen.00 volts. Exercise some inquiry on your item model and find out how low the supply voltage tin become before you take problems. You could end upward in a situation where you have an otherwise skillful battery that isn't useable because the radio is too picky. Or, the radio works on receive but shuts downwardly when you try to transmit because the voltage drops too low.

Going in reverse: Charging the bombardment.

What should James await when he'southward ready to charge his 35 amp hour bombardment support? A commonly accepted formula for calculating charge fourth dimension is:

amp hours÷accuse current= charge time.

James said he was using a 2 amp charger. And so the calculation would look like this: 35 amp-hours÷2 amp charge= 17.fifty hours. This general purpose formula is not 100% accurate for all battery types, merely it is a good basic rule that will get you close enough.

BATTERY AGM

GRAPHIC COURTESY OF COASTAL CLIMATE Command

Seventeen-plus hours for a make full-up makes for a very long wait! In one case over again, let'south bring it into the existent world. That accuse time assumes a fully dead 0% charge. And of grade, no one lets that happen! Suppose the bombardment is 50% full when placed on the two amp charger. Then nosotros're looking at virtually half of that 17.50 hours, or 8.75 hours. That'due south still a long time. What advice would you requite James? If you suggested bumping upward the charge current, congratulations…you've been paying attending.

The master concern when increasing charge current is that if it'due south pushed too high, information technology will cook the battery. The sweetness spot would be the maximum charge electric current that can be applied without stressing the battery. For AGM batteries, the magic number is 20% of the amp 60 minutes rating.

20 percent of 35 is 7. At vii amps of current, how long will it take to get James' battery charged upwards?

35 amp-hours ÷ 7 amp charge= 5 hours.

Don't forget that five hours is for a 0% charged, totally expressionless battery. If we start with a more realistic 50% accuse, and so our recovery fourth dimension drops to ii.50 hours. That's keen at all.

Walking and chewing glue at the same time.

It'south not plenty to have sufficient current to charge your bombardment in a reasonable amount of time. In an platonic off grid setup, you'll likewise want power to run your equipment simultaneously. This is a careful balancing act that requires filling the battery faster than yous drain it while nevertheless having actress juice to run your gear. On top of that, you must contend with the fact that any yous are using as a fuel source (solar, air current, etc.) is non consistently bachelor.

For this illustration, nosotros'll apply solar panels every bit a chief power source. Considering we took the time to exercise some enquiry (and didn't merely fly it) nosotros confirmed that nosotros'll need about 1.00 amp or then just to operate the FT-450 in an idle receive mode, plus (ideally) a 7.00 amp charge current. That'due south a baseline of eight amps before we've even started transmitting. A 130 watt solar console will produce a footling over 7.00 amps, merely that's at total sunlight. Y'all're not going to have full sunlight all the time.

So what should you do?

At this point there is a option to exist made: ane) Go with the 130 watt panel knowing it is merely barely enough. When the sun isn't strong, the battery will fill in the hole. The solar extends bombardment fourth dimension upwardly to a bespeak, but you're probably discharging the battery faster than you tin can fill it and eventually the tank will run dry out. If the dominicus is potent, the solar will do most of the work but you'll even so take to heed your duty wheel to avoid getting alee of your available power. 2) Add more solar chapters to requite yourself some "headroom" when the lord's day is weak. If you do information technology right, you can operate all twenty-four hours and however accept a 100% charged battery at sundown.

In either case, we've already adamant that the 35 amp hr bombardment lonely will not push button the FT-450 on full power transmit for very long, and a ii amp charger will technically piece of work only will have an unacceptably long accuse time. When it gets nighttime and you lose your solar, you'll either need to go off the air, reduce transmit wattage, switch to another electric source such every bit a gas generator, or have plenty boosted batteries (with respective solar capacity for charging) to pull you through the dark.

What nosotros learned today.

Before doing annihilation, know how much power you will really need. Practise not guess. Use existent numbers.
C-rate idue south how much electric current a battery can provide for one hour. It is synonymous with amp-hours.
Duty cycle is the ratio of transmitting to receiving, expressed equally a percentage. Duty cycle is a huge factor in how long your battery will concluding.
Manufacturer battery specifications cannot be completely trusted and are usually overly generous.
Amp-hours÷charge current= charge fourth dimension.
For AGM batteries, the maximum safe charging electric current is 20% of the amp-hour rate.
An constructive off grid operation will be able to charge a battery faster than it is tuckered.
Determining how much bombardment you "need" depends on your operating goals and power demands of your equipment.

Resources:
Bombardment University is an outstanding go-to source for everything there is to know nigh batteries. You lot'll never take time to read everything that'due south there. I Honeythis website! It deserves a bookmark in every radio amateur'due south computer.

Here is an easy on-line calculator that volition apace compute charge times.

These older Off Grid Ham articles provide boosted data about specific types of batteries:

Flooded/wet cells

Lithium batteries

Absorbed Gas Mat (AGM) batteries.

Correction 02/01/2021: Description of C-charge per unit edited to clear up incorrect terminology, Cheers to OGH reader Dan for pointing it out.