How much battery should you carry for a POTA activation, a SOTA summit, or an afternoon of portable FT8? The answer depends on your radio’s actual receive and transmit current, how often you transmit, and how long you plan to operate—not simply the RF wattage printed on the front of the radio.
The simple amp-hour calculation
A battery’s capacity in amp-hours (Ah) tells you how much current it can supply over time under specified conditions. Start with the average current draw:
Average amps = (receive amps × receive fraction) + (transmit amps × transmit fraction).
Required Ah = average amps × operating hours.
Then add a practical reserve for unexpected operating time, cable losses, cold weather, accessories, and battery aging. For a conservative field estimate, planning around 80% of a battery’s rated capacity is a useful starting point; it is not a universal limit on LiFePO4 batteries.
Example: Yaesu specifies about 2 A on receive and 23 A on transmit for the FT-891 at its rated 100 W output. At 80% listening and 20% transmitting, the planning average is (2 × 0.8) + (23 × 0.2) = 6.2 A. Two hours uses about 12.4 Ah before reserve. A 15 Ah battery may be adequate in favorable conditions, but a 20 Ah pack gives a more comfortable margin. Actual SSB current can be lower than this conservative full-transmit-current calculation because speech power varies.
QRP, 100-watt voice, and FT8 need different plans
For a lightweight station such as an Icom IC-705, the manufacturer lists approximately 0.5 A on receive with maximum audio and less than 3 A on 10 W transmit using a 13.8 V supply. At an illustrative 80% receive / 20% transmit split, that is up to roughly 1 A on average. A three-hour activation works out to approximately 3 Ah before reserve, so a 6 Ah external pack gives useful headroom. The IC-705 can also operate at up to 5 W using its own BP-272 battery; you do not always need an external pack.
For an FT-891 operating at 100 W SSB, consider a 20 Ah class battery for a roughly two-hour activation at the conservative 80/20 duty cycle above. Smaller batteries may support shorter sessions or lower power; check both runtime and the battery’s ability to deliver the radio’s transmit current.
FT8 changes the calculation. If a station transmits during half of its operating time, the FT-891 example becomes (2 × 0.5) + (23 × 0.5) = 12.5 A average at the radio’s maximum specified transmit draw. Two hours would require about 25 Ah before reserve; a 20 Ah pack is not a safe two-hour assumption at those settings. Many operators run FT8 at substantially less than 100 W, which can reduce power consumption. Measure the current at your chosen RF output rather than assuming the transmit draw scales exactly with the wattage setting.
| Illustrative setup | Time and duty cycle | Calculated use | Planning capacity |
|---|---|---|---|
| IC-705, external 10 W | 3 hr, 20% TX | ~3 Ah | 6 Ah |
| FT-891, 100 W SSB | 2 hr, 20% TX | ~12.4 Ah | 20 Ah |
| FT-891, 100 W high-duty-cycle digital | 2 hr, 50% TX | ~25 Ah | 30–40 Ah, or lower RF power |
These examples use published current figures and simplified operating cycles. Your antenna tuner, laptop, hotspot, fans, radio settings, and measured current can change the totals.
Discharge current matters as much as capacity
Do not choose a battery by Ah alone. A 100 W radio may need more than 20 A during transmit, so the battery’s built-in management system (BMS), wiring, connector, and fuse must support that load. Bioenno says its BLF-1215A and BLF-1220A batteries use 30 A protection boards and can support the roughly 23 A demanded by a 100 W amateur transceiver under typical intermittent voice duty cycles. Check the specifications of the exact pack you own, especially for extended high-duty-cycle digital operation.
LiFePO4 is popular for portable radio because it is lighter than comparable lead-acid batteries and delivers a relatively stable voltage through much of its discharge. A four-cell LiFePO4 pack is nominally 12.8 V, but its terminal voltage changes as it charges and discharges. Watch the voltage at the radio under transmit load: long or undersized cables can introduce enough voltage drop to trigger reduced power or a shutdown.
Five field-power habits worth adopting
- Measure your actual current: a DC wattmeter or ammeter is more useful than relying on other operators’ numbers.
- Fuse near the battery: use adequately rated wire, compatible connectors, and proper polarity.
- Lower RF power when practical: going from 100 W to 50 W reduces transmitted power by 3 dB, but current savings depend on the radio and do not automatically double battery life.
- Budget for accessories: laptops, digital interfaces, hotspots, and screens may use more energy than expected on a long FT8 activation.
- Charge safely: use the manufacturer’s LiFePO4-compatible charger and observe the battery’s temperature restrictions. Do not charge below freezing unless the battery is specifically designed to permit it.
What I would take into the field
For short QRP activations, a modest 6 Ah pack is a sensible starting point after checking the radio’s draw. With a 100 W FT-891, a 20 Ah LiFePO4 battery offers useful reserve for a typical voice activation; for extended FT8 operation, reduce transmit power, carry more capacity, or plan to recharge. The goal is not the biggest battery—it is a dependable station that lasts through the final QSO.
Roger Quintana, NJ2RQ
Keep exploring: POTA Setup: From Start to First QSO, FT8 Without the Confusion, and FTX-1 Field vs. FT-891 for Portable Operating.
Manufacturer references: Yaesu FT-891 specifications; Icom IC-705 specifications; Bioenno battery-discharge FAQ.
