Battery Life Calculator for Field Operations

Battery life estimation is one of the most critical planning tasks in portable amateur radio operation. Whether preparing for a POTA activation in a remote park, deploying an ARES emergency communications station during a power outage, or running a Field Day alternate-energy station, knowing precisely how long your battery will last determines whether you complete the mission or go QRT prematurely. This calculator accounts for the four factors that most significantly affect field battery runtime: battery chemistry, depth of discharge, transmit duty cycle, and reserve margin.

Battery chemistry determines how much of the rated capacity is actually usable without damaging the cells. A conventional flooded lead-acid battery should not be discharged below 50% state of charge (SOC) — deeper cycles drastically reduce cycle life. AGM (absorbed glass mat) lead-acid can tolerate 60% DoD. LiFePO4 (lithium iron phosphate) chemistry can be safely discharged to 80–90% DoD with thousands of full cycles and excellent recovery, making it the dominant choice for ham radio portable operations since roughly 2018. Alkaline primary cells can be fully discharged but are single-use.

Transmit duty cycle is the fraction of total operating time spent transmitting. This varies dramatically by mode and operating style. FT8 uses fixed 15-second TX/RX slots; in a normal FT8 QSO you transmit one period and receive the next, for an average 50% duty cycle. CW contest operating involves listening as well as calling CQ, typically around 30% TX. SSB casual ragchewing involves talking roughly 15% of total operating time. A POTA activator hunting QSOs on SSB may only transmit 10% of the time between contacts. The mode selection is the single largest variable in battery runtime estimates.

Reserve margin is the capacity intentionally left in the battery at the end of operations. Discharging a lead-acid battery to 0% SOC is permanently damaging; even lithium BMS units will cut off at a low-voltage threshold, leaving you without power. Operators typically keep 10–20% reserve for pack protection, plus the ability to keep the radio running while packing up. Emergency communicators often maintain a 30–40% reserve to ensure the station can be quickly reactivated if the incident extends beyond the planned window.

The biggest single improvement most POTA operators can make to their portable power setup is switching from sealed lead-acid (SLA) to LiFePO4. A 10 Ah SLA weighs about 3 kg and delivers only 5 Ah usable at 50% DoD. A 10 Ah LiFePO4 weighs 1.4 kg and delivers 8.5 Ah usable. The lithium pack provides 70% more usable energy at less than half the weight — a critical difference when hiking to a trailhead activation site. Brands such as Bioenno, Dakota Lithium, and Battleborn offer packs designed for ham radio with integrated BMS protection.

For FT8 operation, the 50% transmit duty cycle is often surprising to operators accustomed to SSB. At 100 W, your transceiver is running at full carrier power for 7.5 seconds out of every 15-second slot. Many FT8 operators reduce power to 25–50 W to extend battery runtime significantly: at 25 W the TX current drops from ~20 A to ~5 A, and because WSJT-X reports signal-to-noise ratios down to −20 dB, you will still complete QSOs with stations who have good antennas. Use the minimum power that successfully completes QSOs rather than running maximum power throughout.

Always measure actual TX and RX current draw from your specific radio rather than relying solely on published specifications. Use a clamp ammeter or a DC power meter (like the popular Powerwerx meters) on your coax power lead during a real transmission. Manufacturers publish maximum TX current, but many radios draw less at ALC-limited output levels. The RX current figure is also highly variable — full display brightness, backlighting, fan speed, and powered accessories all add up. Accurate measured current gives runtime predictions within ±5% versus ±25% from nameplate specs alone.