BATTERY HOLD DOWNS IN A CASE

One thing you need to know is the rated minimum voltage
on the radio. It can make a big difference whether the
rig keeps operating when the voltage drops below 12 or 11
volts. Some ham gear designed for mobile use has problems
below 12.4 volts, while other gear is still happy at 9V
(though with reduced power output.) This makes a HUGE
difference in how much power you can get from your batteries
before the voltage drops below the minimum for the rig.

My suggestion is to minimize the power output and the
amount of time spent transmitting. 30% transmit time is
pretty high in the real world, except for a few specific
applications. The transmit time and power output (and
the current required therefore) are the biggest factor
in limiting the operating time.

(Personally, I only plan to use half the capacity of
a gel cell battery in my operating budget. Beyond that
the voltage may be too low, or it may give me some
spare capacity in a pinch if the batteries are new.)

Regarding mounting: I use 7aH gel cells in some of my
hidden transmitters built in ammo cans. Generally I
put a sheet of plywood along one side of the case and
attach some "L" brackets to it to hold the battery.
After slipping the battery and plywood into the case I
wad some foam down beside the battery to keep it from
wiggling. (You might put some heatshrink tubing over
the metal brackets to reduce the chance of cracking the
battery if the case is dropped.)

The exact mounting method, though, will depend on the
size of the battery and the shape of the case you are
using. Sometimes using several smaller batteries is
better than one large one because they give more
flexibility in arrangement. Tie wraps or metal clamps
should work, depending on how often you expect to have
to change the batteries.

A couple other suggestions from experience:

1) Don't plan to mount the antenna on the same case.
Better to have an external antenna that can be hung from
a tree or mounted on a vehicle. Not only is there a LOT
of RF floating around in a small space with the antenna
on the case, but gaining height will give you better
coverage with lower power, thereby giving longer
operating time from the same batteries.

2) PolySwitch (and others) make a line of Positive
Temperature Coefficient (PTC) thermistors that are used
as automatically-resetable fuses. These are your friends.
They will pass current up to a point, then overheat and
go to a high resistance state. Choose one that will
pass enough current to run your radio (using as low of
a power as is practical, of course) but will shut down
in case of a short circuit. After the short circuit has
been cleared, they will turn back on again and continue
operating as though nothing happened. This gives the
protection of a fuse, but without the problem of finding
a spare in the field, or opening up the case to do a
manual change/reset in case of an intermittent short.

3) Waterproof cases are not necessarily smokeproof, as
can be easily demonstrated by an internal short circuit
if the batteries aren't fused.

4) Provide a dedicated charger that can be carried in
the case where it won't get lost. Even better: include
a provision for DC charging in a car. Depending
on your needs, this may be independent of the ability
to actually operate the radio. Use a well-designed
charger that can be left on at all times so you don't
worry about overcharging, and make sure there is power
available wherever the radio is stored so you know that
the batteries won't be dead when you need it.