[Mobile-Portable] 500 Watt Mobil Amp

[K7fe@aol.com]

To run a 500 watt amp in your vehicle you might consider the following:
The extra batt should be in a "marine case' to prevent acid from damaging 
your vehicle interior.  I would also suggest a "sealed batt so that you never 
have to add water and of course, you will have fewer potential leaks of acid 
since it is sealed.  A "GEL" cell, like are use in electric wheel chairs are 
great, they will not contaminate your vehicle interior, but cost more.  The cable 
from the batteries should be connected with a solenoid that only provides 12 V 
to the amplifier only when the auto engine is running.  The original auto batt 
and the additional batt should be connected in parallel with cables 
consisting of #4 or larger wire depending on distance apart.  There is a benefit to 
install one of the batts very close to your amp, if you can, if not use a very 
large cable to your amp from the batts.  Car audio stores have very nice large 
conductor cables as do welding supply houses.  Both types are quite flexible 
and easy to run in your vehicle. 

Isolators are a diode network that has a 0.7 to 1.4 V volt drop and on a 12 
volt system, that is a high percentage voltage drop and I would not recommend 
their use.

Make sure that the amp is mounted in the passenger cabin so it does not 
become too hot.  If the amp does not have a fan for cooling, install a 12v fan that 
blows over the heat sink.  You could run the fan only when the amp is on, or 
use a thermostat and have the fan remain on when the heat sink is above a 
certain temperature.  Your 500 W linear amp is about 50% efficient, so you will 
need about 1000 watts of energy.  Power = EI , so 1000 / 12 =83 amps plus your 
xcvr draw of about 20 amps and normal vehicle current demands.  Make sure that 
your alternator is capable of the current that you expect to draw or it will 
blow out.  Many recent vintage autos have alternators that are capable of 100 
to 130 amps, which will be O.K. Older cars which were not so power hungry may 
only have a 40 or 60 amp alternator.  Ask your auto dealer, they usually have a 
chart that list the alternator capacity for each model and year.  Some auto 
electric rebuilding shop sell rebuilt alternators that are of a higher amperage 
than your standard factory supplied unit and are affordable.  There is also 
an after market "dual alternator" kit that some car audio stores sell where you 
can have 260 + amps capacity that you could run with two batts, if you need a 
full legal limit RF amp. 

I currently run a 1000 W output amp with two batts and a single 160 amp 
alternator.  My 1500 watt solid state output amp is under construction, and I will 
need to install "dual alternators" to avoid killing my present alternator. My 
antenna is a home brew "bug catcher."  

Other news: The auto industry has announced that they will soon switch from a 
12 volt battery to a 36 volt battery with a 42 volt charging system.  Low 
impedance matching problems of future RF power transistors in linear amplifiers 
will be almost eliminated and power cables will shrink in size.  Hurray!
73,
Terry, K7FE


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