Battery Runtime Calculator for Portable Ham Radio
The short answer
A 20 Ah LiFePO4 pack runs a 100 watt HF station for roughly four to five hours of normal single sideband operating, because the effective draw of about 4 amps combines a 22 amp transmit current with a 1 amp receive current over a typical transmit pattern. Lithium iron phosphate gives about 90 percent of its rated capacity; lead acid gives about 50 percent without shortening its life.
- LiFePO4 usable
- 90 percent
- Lead acid usable
- 50 percent
- 100 W HF transmit
- About 22 A
- Typical receive
- About 1 A
Battery runtime is the calculation that decides whether a portable activation ends when you want it to or when the radio starts resetting mid-transmission. The arithmetic is simple and two things make people get it wrong: assuming the whole rated capacity is usable, and forgetting that receive current runs for the entire session while transmit current only runs in bursts.
Calculator
Station runtime from a battery
Figures assume a healthy pack at room temperature. Cold weather reduces available capacity substantially for every chemistry.
Estimated runtime
Average current
Usable capacity
Receive contribution
Transmit contribution
Minimum fuse and wire
Comfortable for an afternoon activation with margin left over.
Why the rated capacity is not the usable capacity
A battery's label tells you how much charge it holds between full and completely flat. What matters is how much you can take out without damaging it, and the two numbers differ by a factor of nearly two between chemistries.
| Chemistry | Usable | Nominal V | Behaviour |
|---|---|---|---|
| LiFePO4 lithium iron phosphate | 90 percent | 12.8 V | Holds about 13 V nearly flat to empty, which matters to a radio that folds back below 11 V. About a third the weight of lead acid for the same capacity, and 2000 or more cycles. |
| Lithium ion, 3S or 4S pack | 85 percent | 11.1 V | Lighter still and cheaper per amp hour, with a steeper discharge curve and a chemistry less tolerant of abuse. A 3S pack sags below 11 V well before empty. |
| AGM sealed lead acid | 50 percent | 12 V | Only half its rated capacity is usable without shortening its life dramatically. Heavy, cheap, and entirely acceptable for a station that never moves. |
| Flooded lead acid | 50 percent | 12 V | Same 50 percent rule, plus outgassing that requires ventilation and a battery box. Not for indoors and not for a sealed vehicle cabin. |
The 50 percent rule for lead acid is the reason a 20 Ah LiFePO4 pack and a 35 Ah AGM deliver similar usable energy while the lithium pack weighs about a fifth as much. It is also why the lithium pack is cheaper over its life despite costing more on the shelf: a LiFePO4 cell handles 2000 or more cycles against a few hundred for a deep-cycle lead acid battery treated well and far fewer treated badly.
The second lithium advantage is the discharge curve. A LiFePO4 pack sits near 13 volts for most of its discharge and then falls off a cliff at the end. Lead acid sags steadily from the moment you start drawing, and a 100 watt radio that folds back its output below about 11.5 volts will be delivering noticeably less power long before the battery is empty. The lithium pack gives you full output right up to the point it stops.
Current draw by radio
| Equipment | Receive | Transmit peak |
|---|---|---|
| Handheld charging in its cradle | 0.1 A | 0.5 A |
| 25 W dual-band mobile | 0.8 A | 6 A |
| 50 W dual-band mobile | 1 A | 11 A |
| 85 W single-band mobile | 1 A | 16 A |
| 10 W portable HF (Xiegu X6100 class) | 0.6 A | 3 A |
| 20 W HF (Xiegu G90 class) | 0.9 A | 6 A |
| 100 W HF (FT-891 or IC-7300 class) | 1 A | 22 A |
| 600 W solid state HF amplifier | 1 A | 90 A |
| Automatic antenna tuner, tuning | 0.02 A | 1 A |
| Rotator controller | 0.3 A | 0.3 A |
| Station accessories, keyer, lamp, fan | 1 A | 1 A |
The receive column is the one people underestimate. A 100 watt HF radio drawing 1 amp on receive uses 1 amp hour every hour whether you transmit or not, which is 5 amp hours over a five hour activation before a single contact. On a 20 Ah pack that is a quarter of the usable energy spent listening. Radios with a large backlit display and a cooling fan running continuously are worse.
This is also why turning down transmit power helps less than expected. Halving a 100 watt radio to 50 watts cuts the transmit current from about 22 amps to about 13, but the receive draw is unchanged. On a station transmitting 30 percent of the time, that is roughly a 40 percent runtime improvement rather than a doubling.
Sizing a pack for what you actually do
| Session | Radio | Pack | Weight | Comment |
|---|---|---|---|---|
| A summit, walking in | 10 W portable | 6 Ah | About 2 lb | A full day of activation. Weight is the constraint, not capacity. |
| A park, from the car | 100 W HF | 20 Ah | About 5 lb | Four to five hours of voice. The standard answer. |
| A full day event | 100 W HF | 30 Ah | About 8 lb | A full day at normal duty with margin. Add a panel if it runs into evening. |
| Emergency backup at home | 50 W VHF plus accessories | 50 Ah or more | Heavy | Weight stops mattering. Lead acid becomes reasonable again on cost. |
| Continuous digital, unattended | Any | Add 50 percent | Full carrier duty cycle roughly doubles the transmit contribution. |
The packs worth owning: a DCDG 12 V 6 Ah lithium battery pack$34.99 for a summit kit where two pounds matters, a GOLDENMATE 12 V 20 Ah LiFePO4 battery$67.49 as the general-purpose portable answer at IP67 and about five pounds, and an ERYY 12 V 30 Ah LiFePO4 battery$75.99 with a built-in state-of-charge display when a full day at 100 watts is the target. All of them are compared on LiFePO4 batteries for ham radio.
Fusing, wiring and connectors
A 12 volt lithium pack can deliver hundreds of amps into a short. The wire will glow before anything else in the circuit reacts, and it will do so inside a bag or under a vehicle seat. Fuse at the battery, not only at the radio, and size the fuse to protect the wire rather than the load.
| Peak current | Fuse | Wire, run under 10 ft | Typical station |
|---|---|---|---|
| Up to 5 A | 7.5 A | 16 AWG | Handheld charger, 10 W portable radio |
| Up to 10 A | 15 A | 14 AWG | 25 W mobile, 20 W HF |
| Up to 20 A | 25 A | 12 AWG | 50 W dual band mobile |
| Up to 25 A | 30 A | 10 AWG | 100 W HF transceiver |
| Up to 100 A | 125 A | 4 AWG | 600 W solid state amplifier |
Standardise the connectors. A bag of 30 A quick-disconnect power connectors, 40-piece$9.99 genderless 30 amp housings turns every DC connection in the station into the same connection, which means any battery runs any radio and a borrowed pack works without an adapter. A Powerwerx PD-5F fused distribution block$50.85 gives five individually fused outputs from one feed, which is what stops one accessory fault taking the whole station down.
Cold weather, and the mistake that ruins packs
Every chemistry loses capacity in the cold. Lead acid at freezing delivers roughly 20 percent less than at room temperature, and lithium is similar on discharge. That is inconvenient rather than damaging.
What is damaging is charging any lithium chemistry below freezing. It plates metallic lithium onto the anode, the damage is permanent and cumulative, and it reduces the safety margin of the cell rather than just its capacity. A good battery management system blocks charging below about 0 degrees Celsius; many cheap ones do not. If you charge from a FlexSolar 100 W foldable solar panel$80.99 panel in winter, bring the pack indoors to warm before charging it.
The other rule: never leave a lithium pack fully discharged, especially in a cold vehicle. Store at around half charge if it is going to sit for months. The full treatment is on portable power and battery safety.
Extending a session without carrying more battery
- Turn the power down when it is not needed. On a good path, 10 watts works as well as 100 and draws a fifth of the current. The QRP comparison puts numbers on when this is true.
- Switch off the display backlight and the internal fan where possible. Small savings that run for the whole session.
- Charge the laptop separately. Running a logging laptop off the station battery through an inverter is the least efficient thing you can do with the pack.
- Add a panel rather than a bigger battery. A FlexSolar 100 W foldable solar panel$80.99 folding panel with a charge controller matched to your chemistry keeps a 20 Ah pack topped up between operating bursts, which is lighter than doubling the pack.
- Operate CW or a low-duty digital mode. The duty cycle difference is real and it is on the transmit side, which is the larger term.
Common questions
Questions people ask about this
How long will a 20 Ah battery run a 100 watt HF radio?
On single sideband voice with a normal transmit pattern, roughly four to five hours from a LiFePO4 pack. The arithmetic: a 100 watt radio draws about 22 amps on transmit and 1 amp on receive, and a typical voice conversation transmits about 40 percent of the time with the transmitter at about 40 percent of peak on average, giving an effective draw near 4 amps. Eighteen usable amp hours divided by 4 is about four and a half hours.
How much of a battery capacity can I actually use?
About 90 percent of a LiFePO4 pack and about 50 percent of a lead acid one. Lead acid loses cycle life dramatically when discharged below half, which is why a 100 Ah AGM and a 50 Ah lithium pack deliver similar usable energy despite the label. That is also why lithium has taken over portable amateur operation despite costing more per nominal amp hour.
Does transmit power change battery life much?
Less than people expect, because receive current dominates for most of the session. Dropping a 100 watt radio to 50 watts halves the transmit current but leaves the 1 amp receive draw untouched, so a station that transmits 30 percent of the time sees runtime improve by roughly 40 percent rather than doubling. A 10 watt portable radio is a completely different case because both figures fall.
What size battery for a portable field station?
A 6 Ah pack runs a 10 watt radio for a full day of park activation and weighs about two pounds. A 20 Ah LiFePO4 pack runs a 100 watt radio for a long afternoon and weighs about five pounds. A 30 Ah pack covers a full day at 100 watts with margin. Beyond that you are carrying weight you will not use in one session, and a solar panel is the better answer.
Can I charge a LiFePO4 battery with a car charger?
Only with a charger that has a LiFePO4 mode. A lead acid charger running an equalisation or desulphation cycle applies voltages that damage lithium iron phosphate cells, and the built-in battery management system is a last line of defence rather than a substitute for the correct profile. Never charge any lithium chemistry below freezing; it plates the cells and the damage is permanent.
Do I need a fuse on a battery?
Yes, at the battery rather than only at the radio, and sized to the wire rather than to the load. A 12 volt lithium pack can deliver hundreds of amps into a short circuit, which will glow a length of undersized wire before anything else reacts. This is the most commonly skipped safety step in portable amateur radio and the one with the most immediate consequences.
Keep going
Related on this site
- Power supply sizing The mains equivalent of this calculation.
- The $1,032 portable field kit Radio, antenna, battery and a way to carry it all.
- LiFePO4 batteries compared What each capacity weighs and what it runs.
- LiFePO4 versus lead acid The 50 percent rule and what it really costs.
- Portable power and battery safety Fusing, charging and cold weather.
- Working parks on the air What the battery is actually for.
Keeping your own cut lengths, SWR sweeps and exposure record? The Station Build & Antenna Planner is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.
Disclaimer Researched guidance for planning purposes, not professional engineering, electrical or legal advice. Antenna work, RF exposure, mast and tower work, grounding and battery handling all carry real risk that depends on your specific site and installation. Verify anything safety-critical against current FCC rules, the National Electrical Code and the manufacturer's own documentation, and hire a professional where the job calls for one. We research equipment from published specifications, regulatory documents and verified owner reviews rather than claiming hands-on testing we have not done.