Battery runtime calculator

How the calculation works

A radio draws a small current while receiving and a much larger one while transmitting. The average depends on how much of the time you transmit and how hard the transmitter works while you do:

T is the fraction of time you're transmitting. The mode factor allows for speech: an SSB transmitter only draws full current on voice peaks, so the average while talking is around 40% of the full-power current. CW draws full current while the key is down, about 60% of the time when sending. FT8, FM and other data modes draw full current for the whole transmission.

Usable capacity

You can't use all of a battery's rated capacity. LiFePO4 batteries deliver almost all of it at a nearly constant 13 V or so, which suits radios well, and they're about a third of the weight of lead-acid. Lead-acid and AGM batteries shouldn't be discharged below about half their capacity, or they wear out quickly, and their voltage falls as they discharge. Many radios reduce power or switch off below about 11.7 V, so a sagging lead-acid battery runs out sooner than its capacity suggests.

Measuring your radio

The figures in your radio's manual are usually for full power at maximum volume. For an accurate answer, measure with an inline DC power meter: note the current on receive with the volume at your normal level, and on transmit at the power you'll use. Turning down the backlight, the volume and the transmit power makes a big difference on a portable activation. Halving the power from 10 W to 5 W saves less than half the transmit current, because the radio's own consumption stays the same.

Always fit a fuse close to the battery, rated for your radio's maximum current. A LiFePO4 battery can deliver hundreds of amps into a short circuit.

Questions