Mobile voltage problem

[Marty]

Hi, I'm new here... just upgraded to my General in March after being off the air for 11 years (I had no idea what D-star is), and tried to install my old Standard C5900DA tri-bander in my truck, a '92 Ford Ranger. I pushed the power button... nothing. Next, I tried the Icom 706. Nothing.

So, I tested the power leads with a meter (10 gauge copper wire, straight from the battery) and the battery terminals, and got 12.5 volts with no attenuation. Yet both radios worked just fine when I tried them with the 13.8V power supply which runs on 115V.

I can think of only 2 ways to overcome this voltage problem. I can either fiddle with the DC power input regulators in the radios, something I definitely don't want to do, or else open up the alternator and mod the regulator somehow to bring it up to 13.8V.

Anyone else ever had this issue? Any info you might have will be appreciated.

73, Marty, KE6WNH

[M0HAK]

So, I tested the power leads with a meter (10 gauge copper wire, straight from the battery) and the battery terminals, and got 12.5 volts with no attenuation. Yet both radios worked just fine when I tried them with the 13.8V power supply which runs on 115V.

12.5V sounds about right if the engine isn't running. You should expect to see more like 14.2-14.5V with the engine running. If not, then you might not be charging the battery very well, or have some other current draw somewhere.

Andy

[va3mw]

You've eliminated the radios as part of the problem, but you didn't say if you had any fuses in line.

You should have fuses, and you might try replacing them. I had one in my mobile setup that was in the engine compartment. With no load, you could measure voltage. If you ohm'd out the fuses, they were good. If you drew any current, it opened up.

Clean the connections and make sure you don't have a polarity issue.

Mike VA3MW

[AE6ZW]

12.5 V with engine running sound like too low of voltage, battery to even charge. most of the car I have driven always had at least 13.3 V I believe most radio still work just fine even with 12.5 V however. may be DC power is not coming to radio some how, with 12.5 V, at least it should turn it on. may be negative connection is not good.

[dereckbc]

Is that 12.5 volts in RX or TX?

12.5 volts indicates a 100% fully charged battery at rest with no charger connected. Depending on size of conductors, length, and current demand while in TX mode the voltage will drop. It you see it dropping below say 1 volt below RX voltage level while in TX mode, you have a power wring problem. There should be no significant difference between 13.8 and 12. 5 volts except for a few watts lower transmit power.

Does the radio light up in the vehicle and receive OK? Then go dark or very dim when you transmit?

The problem can be bad connections or undersized wire between the radio and battery. It is simple Ohm's Law messing with you.

[KM3F]

I hope you fused lead the power at the battery. It's dangerious not to do so at that point.
Modern vehichles now run at 14 volts plus a little bit depending in the battery state of charge at the moment.
Most radio designs will take as high as 16 without damage.
You have a high resistance in your feed at some point that is dropping the voltage.
A voltmeter at as low as 1000 ohms per volt will not detect it. Most modern VOM are at least 10,000 ohms per volt and will not load the 'problem' enough to detect it.
Test it with a 12 volt bulb that draws a few amps and see what happens.
Good luck.

[dereckbc]

You have a high resistance in your feed at some point that is dropping the voltage.
A voltmeter at as low as 1000 ohms per volt will not detect it.

Well you can use any volt meter and it should work, and you do not really need any special test apparatus to find the problem. I agree the problem is excessive voltage drop, but you only need the radio and a volt meter to find the problem. Most times you usually do not need any equipment to find the problem, just observation by looking and feeling will find the problem.

Monitor the voltage on the back of the radio input power plug. Usually the voltage will be fine in receive mode, but when you key the microphone the voltage will crash when you demand higher current. Assuming the wire is adequate size and not too long, the problem will always be found in a connection terminations. Either poor crimps on compression connectors, loose connection hardware, or even forgetting to remove paint if using the vehicle chassis as the battery return.

Most time you can find it by having someone key the mic in FM or AM mode, then just use you hands to find the problem by feeling the heat being generated at the poor connection point. If it is a really high powered transmitter you might be able to see the problem from the wire insulation melted or distorted from heat at the poor connection point.

Last point is to make sure you are using adequate size wire. In low voltage systems like 12 volts you want to keep voltage drop to 3% or less (.35 volts), and that requires some significantly large wire to accomplish at 12 volts. A modern 100 watt transmitter draws 20 to 25 amps from the battery, so wire loop resistance needs to be .014 ohms or less which includes connection resistance. So some pretty extreme measures need to be taken to achieve that goal. For example lets say you need to use 10 feet of wire to make up both positive and negative to the battery post. You would have to use at least a #10 AWG wire which yields .012 Ohm's of resistance in wire alone not counting connection and fuse resistances. So realistically you are looking at using at least # 8 AWG minimum (# 6 AWG preferably) on a 100 watt radio with only 10 loop feet of wire.