🔋 Ham Radio Battery Calculator

Choosing a 12V Battery Chemistry for Ham Radio

LiFePO4 (Lithium Iron Phosphate): the default choice for POTA, SOTA, and Field Day today. Nominal 12.8V (4 cells), flat discharge curve that holds voltage under heavy pulsed loads far better than lead-acid of the same capacity, roughly 2,000-5,000 cycle life, and about 2-3× the energy density of AGM. A built-in BMS protects against over-discharge, so most of the rated capacity is genuinely usable. The tradeoff: higher upfront cost, and charge/discharge performance drops sharply below freezing — many packs' BMS will refuse to charge below 0°C.

AGM / Sealed Lead-Acid (SLA): cheap, widely available, and tolerant of abuse, but heavy for its capacity and only safely usable to about 50% depth of discharge before cycle life drops off badly. Voltage sags noticeably under the pulsed current of SSB or CW, which is why a small 7-9Ah "brick" SLA battery is a common cause of a radio resetting or an amplifier folding back mid-transmission — undersized AGM is one of the most frequent field-day power complaints.

Gel Cell: similar chemistry and characteristics to AGM, slightly better deep-cycle tolerance in some designs, slightly lower maximum discharge rate. Treat it the same way as AGM for sizing purposes.

Deep-Cycle Flooded Lead-Acid: the traditional choice for base-station and Field Day club setups (golf-cart and marine batteries), inexpensive per Ah at large capacities, but heavy, requires the battery to stay upright and ventilated, and needs periodic maintenance (topping up electrolyte). Good for a fixed club station; impractical to carry for a POTA hike.

Sizing a Battery for 100W SSB/CW Operating

Peak current matters as much as total amp-hours. A 100W HF radio can pull 20-25A the instant it keys up — a battery that can't deliver that current without its voltage sagging will trip the radio's low-voltage protection or reset it, even if there are technically "enough" amp-hours left. As a rough continuous-discharge guideline, LiFePO4 packs are generally happy supplying about 1-1.5× their Ah rating in amps, which is why a 15-20Ah LiFePO4 pack comfortably runs a 100W rig on SSB/CW. Lead-acid chemistries (AGM, Gel, flooded) should be sized much larger relative to the peak current — roughly 2-3× the amp-hours of an equivalent LiFePO4 pack — because their safe continuous discharge rate is far lower and their internal resistance causes more voltage sag under load.

FM and digital modes (FT8, RTTY, PSK31) run at or near 100% duty cycle for the length of every transmission, so they draw much closer to peak current on average than SSB or CW. Budget accordingly if a Field Day or emergency plan leans on FM repeaters or digital modes rather than SSB/CW.

Field Day / POTA / Emergency Power Planning Notes

• Always fuse the battery-to-radio connection close to the battery, sized to the radio's maximum rated current, regardless of chemistry.
• For a full Field Day station running most of a weekend, club stations commonly plan for 200-300Ah of total battery bank capacity (often several batteries in parallel, sometimes recharged by solar or a generator overnight).
• For a single POTA/SOTA activation of a few hours on SSB or CW at 100W or less, a 9-20Ah LiFePO4 pack is typical and well-proven in the community; QRP rigs (5-10W) can often run a full day on a much smaller pack.
• Cold weather cuts usable capacity on every chemistry, but especially LiFePO4 — carry margin, and keep lithium packs insulated or warm if operating in freezing conditions.
• This calculator estimates typical operating runtime, not a safety margin — for emergency communications planning, size the battery for your worst-case (highest duty cycle, coldest temperature) scenario, not the average one.

Sources: Yaesu, Icom, Xiegu, and Elecraft published specifications and independent current-draw measurements (oh8stn.org, ve3ips); ARRL and PA9X guidance on PEP/duty cycle; Bioenno Power field-battery sizing guidance.