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vehicle power

This forum is dedicated to helping people with questions about installing radio equipment in vehicles. This can include antenna installs, electrical wiring questions/problems, and mounting systems. Pictures of installs are welcome.

Note: Discussions regarding lighting, sirens, and other equipment now has its own forum in the 'off-topic' section below.

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vehicle power

Postby Irishfirepics » Wed Jun 11, 2014 11:29 am

Hi all

I have a motorola gm300 in my car running on 12v if i move it to a 24v truck would you know if it will work or will i blow the radio?

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Re: vehicle power

Postby RFguy » Wed Jun 11, 2014 3:06 pm

You'll let the smoke out of it.

You will need a 24 to 12-volt converter sized for the radio (25-watt radio, minimum of 10-amps at 12 volt. A 45 watt radio, minimum of 15-amps at 12-volts).

Don't try messing around with half tapping the batteries in an attempt to get your 12-volt radio to work. It will be on-going grief.

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Re: vehicle power

Postby Jim202 » Wed Jun 11, 2014 7:00 pm

Unless the truck batteries are new, it is not a good idea to center tap the two 12 volt batteries at the center point where you would expect to find 12 volts. This point is only 12 volts under a low current draw condition. As current input or output goes up, this 12 volt point can fly around and be either low or high in voltage. Bottom line it is only 12 volts if both batteries have a low internal resistance.

The problem compounds itself as the batteries age. The internal resistance goes up and the voltage filtering ability of the batteries goes down. As this happens, the ability of each battery to hold the mid point tap goes lower. As an example, if you go to start the truck engine, this mid point tap voltage could go real low or even high due to the high current draw from the starter motor. Once the engine starts, the charging alternator will try to bring the total electrical system 24 volt voltage back to normal. If the low side battery that has one terminal grounded, has lost it's low internal resistance, there is the possibility that you might even see close to the 24 volts at this mid point tap.

As has already been mentioned, using a voltage converter power module is the way to go. These units are made to take the full 24 volt input and convert it to the 12 volts the radio requires. It doesn't rely on any battery internal resistance to keep the voltage going to the radio at 12 volts.

I have used the 24 volt to 12 volt converter power modules on many large trucks and big front end loaders. Never had a radio problem once they were installed. Just make sure you get one large enough to carry the transmitter current demand.


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Re: vehicle power

Postby Bill_G » Wed Jun 11, 2014 8:01 pm


There are many fine 24-12 converters out there. Newmar is an excellent choice.

We won't discuss using a GM300 in the post-narrowband days.

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Re: vehicle power

Postby Irishfirepics » Thu Jun 12, 2014 12:39 am

thanks for the info guys, I'll pick up a voltage converter.

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