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Author Topic: BATTERY HOLD DOWNS IN A CASE  (Read 1864 times)

Posts: 17476

« Reply #15 on: August 15, 2005, 01:43:15 PM »

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.

Posts: 295

« Reply #16 on: August 18, 2005, 05:31:49 PM »

If there is the possiblity of AC power available you might want to combine a power supply and adevice like this one:

To keep the batteries charged for use & power the radio when AC is available.
Also i would resis cutting into the side of the case for the radio. Instead consider making a plate that would fit in the box under the cover with the radio face sticking out, along with some ventilation to let out the hot air that will be produced by this radio & other equipment. A pelicam case may be over kill actually. You might want to look at a large plastic tool box. I have seen some nice setups built into these.

Posts: 8

« Reply #17 on: August 23, 2005, 09:38:33 AM »

Well I would like to thank all of you who have been helping me with your ideas and advice. All is going well with this project because of all of you.

 I am going to install a ventilation hole in the pelican case and I need to know if it would be a good idea to use an aluminum computer fan cover with a steel mesh screen to help keep dirt and water from easily entering.

 Any other advice with this part of the project? I am going to use an 3" wide vent grill.

Thanks again.

Posts: 3289

« Reply #18 on: August 26, 2005, 03:34:25 PM »

I recently made a portable battery box and had the same ventilation issues.

-You can find vent covers at boating stores like West Marine.   They have them in white/black plastic and probably chrome.   The covers have slats that are canted downwards to keep spray and rain from falling into the vent hole.  I used a black 3" vent cover.  It had countersunk screw holes at the corners.
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