Every portable station eventually comes down to one question: what is going to power the radio? For decades the answer was a sealed lead-acid battery, and AGM (absorbed glass mat) versions remain popular. Lithium iron phosphate, written LiFePO4, has become a serious alternative for field and emergency use. Neither is universally better. This guide compares them on weight, cost, cycle life and voltage behaviour so you can match the chemistry to how you actually operate.
Start With Your Load
Before comparing batteries, know what you are powering. A typical 100 watt HF transceiver draws a couple of amps on receive and may pull around 20 amps at full-power transmit, depending on the model. Check your radio's manual for its real figures. A low-power QRP rig, or a radio running reduced power, draws far less.
Voice modes such as SSB transmit only part of the time, while digital modes can key the transmitter for long stretches. Your average current, not the peak, decides how many hours a battery lasts. Divide the battery's usable amp-hours by your average draw for a rough runtime.
Weight: Where LiFePO4 Wins Clearly
For the same rated capacity, a LiFePO4 battery typically weighs about half as much as an AGM, sometimes less. That difference is obvious when you carry gear to a hilltop or a park, or when you need to move a go-bag quickly. Exact weights vary by brand, so compare datasheets for the specific models you are considering.
Because you can use more of a lithium battery's capacity, the gap in usable energy per kilogram is wider than the label suggests.
Cost: Upfront Versus Over Time
AGM batteries cost less to buy. LiFePO4 batteries cost more upfront, often several times as much for the same rated capacity. Over years of regular use, the picture can reverse, because lithium lasts through many more cycles and gives you more usable capacity per charge.
If you operate a few times a year, or the battery mainly sits as emergency backup, the lower AGM price may make more sense. If you head out weekly, the cost per cycle usually favours lithium.
Cycle Life and Usable Capacity
A cycle is one full discharge and recharge. Lead-acid batteries last longest when you avoid deep discharges, and a common guideline is to use only about half of the rated capacity. At that depth an AGM might give a few hundred cycles, depending on quality and treatment.
LiFePO4 batteries are commonly rated for well over a thousand cycles, and many manufacturers quote 2,000 or more, while allowing most of the capacity to be used. Treat these as manufacturer ratings under set conditions, not guarantees. In practice, a 20 amp-hour AGM gives you around 10 amp-hours of healthy use, while a 20 amp-hour LiFePO4 can give you close to all 20.
Voltage Behaviour Under Load
This is the biggest practical difference. Most HF transceivers are designed around 13.8 volts, usually with some tolerance either side; check your manual for its range.
- AGM: a full battery rests at roughly 12.7 to 12.9 volts and falls steadily as it drains, with around 12.2 volts at rest being roughly half full. Voltage also sags when you transmit, so peaks on SSB can dip noticeably as the battery ages or runs down.
- LiFePO4: a nominal 12.8 volt battery holds roughly 13 to 13.3 volts through most of its discharge, which is approximate and varies by model. Voltage stays steady under load, so the radio behaves consistently. Then the battery management system (BMS) cuts power when the cells are nearly empty.
That sudden cutoff matters. An AGM warns you gradually, and a sagging voltage acts as a crude fuel gauge. A LiFePO4 battery can run normally and then simply switch off, mid-contact or mid-message. Because voltage barely changes, it is a poor gauge for lithium, so use a battery monitor or a model with a built-in state-of-charge display.
Cold, Heat and Storage
Cold reduces capacity in both chemistries. The important lithium rule is that LiFePO4 cells should not be charged below freezing (0°C or 32°F) unless the battery has low-temperature protection or a built-in heater. Many can be discharged in the cold, so powering a radio is usually fine, but charging from a solar panel on a frosty morning can damage the cells.
AGM batteries tolerate cold charging better, though they accept charge more slowly. For storage, keep AGM batteries charged, since leaving them discharged causes sulfation and permanent capacity loss. Lithium batteries self-discharge very slowly, and manufacturers generally suggest storing them partly charged rather than full or empty.
Charging: Match the Profile
Use a charger or solar charge controller that matches your battery. Many LiFePO4 batteries charge to somewhere around 14.2 to 14.6 volts, but follow the maker's specification. Some lead-acid chargers add a high-voltage equalisation or desulfation mode that is not suitable for lithium, and a long float voltage designed for lead-acid is not ideal either.
Do not mix chemistries in parallel, and do not connect a lithium battery directly to a vehicle charging system unless the manufacturer says it is acceptable. A DC-DC charger is often the safer route.
Which One Suits Which Job
- Hiking, summit and park operating: LiFePO4, mainly for weight.
- Fixed emergency backup on a trickle charger: AGM is a reasonable, economical choice, since it sits charged and rarely cycles.
- Frequent multi-day field operating with solar: LiFePO4 for cycle life and usable capacity.
- Very cold conditions: either can work, but plan charging carefully, especially for lithium.
Our guides to a 72-hour emergency power plan and a ham radio go-bag cover how batteries fit into a broader kit.
Setting Up Safely, Step by Step
- Work out your average current and how many hours you want to operate.
- Choose a battery with enough usable capacity, remembering that AGM gives you roughly half of its label.
- Check the continuous discharge rating of a lithium battery's BMS against your radio's transmit current.
- Fit a fuse or breaker on the positive lead, close to the battery.
- Use cable thick enough to keep voltage drop low, and keep runs short.
- Charge with the correct profile and top up fully before you leave.
- Plan your operating window with the live HF propagation page so you spend battery hours on bands that are open.
Common Mistakes
- Draining an AGM to empty regularly, which shortens its life quickly.
- Trusting resting voltage as a fuel gauge on a LiFePO4 battery.
- Charging lithium below freezing.
- Using the wrong charger profile.
- Skipping the fuse, because both chemistries can deliver very high currents in a short circuit.
- Undersized wire, which wastes voltage and can overheat.
- Flying with lithium batteries without checking airline rules. Limits are generally based on watt-hours, so confirm with the airline first.
Safety and Good Habits
LiFePO4 is regarded as one of the more thermally stable lithium chemistries, but it is not risk-free. Do not charge or use a battery that is swollen, cracked or damaged. AGM batteries are sealed but can vent gas if overcharged, so give them ventilation and never charge them in a closed box.
Also keep RF in mind: run your antenna at a safe distance from people, and remember that transmitting requires the appropriate licence and following your local rules, which differ between Canada and the USA. If you are still studying, the free exam practice questions are a good place to build confidence.
Wrap-Up
Choose LiFePO4 when weight, cycle life and steady voltage matter, and accept the higher price and the need for a proper charger and a monitor. Choose AGM when budget matters most, or when the battery mainly sits as a charged backup. Either way, size it from your real current draw, fuse it, charge it correctly and test the whole setup before you need it. After your next outing, record the contacts in your free online logbook and note how long the battery lasted so your next plan is even better.
How HamRadioList can help
Once your power is sorted, check the free live HF propagation page before you head out so your battery hours go toward contacts that are actually possible. Then log your field contacts in the free HamRadioList logbook, and if you are preparing for a licence, try the exam practice questions.
- Live HF propagation - see current band conditions before you call CQ.
- Free online ham radio logbook - log every contact, import and export ADIF, sync with QRZ.com and keep your awards organised.
- License exam practice - practice questions for the US and Canadian amateur exams.
- Live broadcasting guide - share your own station live on the air.
- Ham radio quick reference - bands, modes, phonetics and Q-codes on one page.
New here, or getting back on the air? Create your free HamRadioList account to log contacts, check band conditions and practise for your exam, or browse more guides from the blog. HamRadioList is built by amateur radio operators in Nova Scotia for operators everywhere.