The hardest part may be figuring out how much current your rig draws
on transmit, especially if you are using SSB.
Its easy with FM, as the transmit current is constant for each power setting
whenever the mic is keyed. So let's start with that as an example.
Say your 2m FM rig draws 8 amps on transmit, and 1 amp on receive. Then
you need to consider how much of each hour you plan to be doing each of
those. In a normal QSO you might figure 50% duty cycle. Listening to a
net you might transmit less than 10% of the time and listen the rest. Let's
use 20% transmit as an example.
That means that, in each hour, you would draw 20% * 8 amps or 1.6 amp-hours
on transmit and 80% * 1 amp or 0.8 amp-hours on receive, for a total consumption
of 2.6 amp-hours of battery capacity for each hour of operation.
That is:
Transmit power required per hour = % transmit time * transmit current
Receive power required per hour = % receive time * receive current
Total required power per hour = transmit power/hour + receive power/hour
Now, here's the problem with calculating the transmit current: it varies with the
mode, output power, and other factors. On CW, the rig draws full current when
it is keyed (and this can be measured at different power levels) but the output
is interrupted by the spaces between the dits and dahs. So I generally figure
about 50% duty cycle during the time I'm transmitting (though it might be more
like 40%). In my case, the rig draws receive current when the key is up, so I
can just cut my transmit cycle percentage in half and get reasonably close. If you
are running VOX or manual switching, then the rig may draw more current when
the key is up than in receive mode, but not as much as in full transmit.
SSB is even more difficult, because the current varies with the voice waveform.
When you press the mic button, the rig switches to transmit mode, which, with
a typical 100W radio might draw 4 or 5 amps. Then, the louder your talk, the
higher the current goes from there up to a maximum of, say, 20 amps. As a
rough guess, the average current is probably around 1/3 to 1/2 of the maximum
value, so with a radio rated for 20 amps on transmit you might figure only 10 amps
in the equation (more if you are using speech compression).
And if you reduce the power of an SSB rig, the current doesn't drop linearly as
one might expect. Even if you turn the power down to zero, there is still that
minimum 4 - 5 amps of current. So if you turn a 100W radio down to 40W output
or so, it might still draw 12 amps rather than 8. You really have to measure that
with your own rig, and such numbers are not often included in the specifications.
The result of that formula is how many amp-hours of capacity you need to operate
the station for an hour. Multiply that by the number of hours you want to work and
it gives you the required battery capacity. Or divide the battery capacity by that
number and it gives you the number of hours of operation the battery will support.
Well, it isn't quite that simple, either. Batteries have a maximum current rating
that you don't want to exceed, so you can't draw 120A for 1 minute out of a
2 amp-hour battery and expect it to survive. And often (depending on the rig) the
battery voltage will drop below the minimum required by the radio before it is
depleted to the specified limit. With lead gel batteries I allow a 50% capacity margin
(especially with used batteries). With lithium types you probably run them much
closer to full depletion, as long as you don't totally drain them.
So the math is pretty simple, but there are a number of design choices (like how
much of the time you are transmitting) that will vary depending on your specific
application and operating style, and some that you really need to measure with
your rig if possible in the configuration / mode / power level you plan to use.