Not only is this idea possible, it's practical too. By removing the load the batteries present to the alternator we typically see 5 KW or more per alternator in AC applications with no batteries. In DC applications the batteries serve two purposes. The first is to provide an external power source to start field excitation. The second is to pick up the slack when the alternators are at an idle.
The second reason is also why AC boxes can not be run at an idle in the vehicle. If you're willing to accept the same issue with DC, then you really have no reason at all to keep a truck load of batteries in the vehicle to run your DC equipment. All you need is a bank of high current filter capacitors and to pulse the field from the 12 volt system to start the alternator.
The capacitor bank should have a total capacitance of at least 100,000 uf and they must be rated to handle the working voltage. Because the current draw can be hundreds of amps, do not try to use a single capacitor to filter this output. Depending on the load, you may need between 2 and 4 equal value caps in parallel to handle the current.
The caps replace the battery however, since there is no outside power source, the field usually will not have enough residual magnetism to start producing output. To solve this problem you simply pulse the field through a diode and resistor with a momentary contact switch connected to the 12 volt system. Once the field is magnetized, no outside power is required.
There are pros and cons to running this system. The pros are, you can remove the charging load from the alternators and operate them at higher output in intermittent use since they cool down in receive. You have a wide range of useable output voltage that can be changed at anytime simply by adjusting the regulator. You are no longer stuck at the fixed voltage appropriate for charging your batteries, you're free to go from 12 to 20 volts in the blink of an eye.
The first con is Leece Neville voltage regulators suck. They usually use a transistor as the active control device. The conduction points of the transistor are not sharp enough to provide complete regulation from full load to no load. This makes SSB operation of this system unsafe without construction of a better regulator that uses an op amp comparator as the trigger to control the field pass transistor. The op amp vastly improves the reaction time of the regulator.
For AM use this is not a problem as long as you're close to the standard 4 to 1 carrier ratio. However, when you unkey, there will be a voltage spike as the load is removed. This doesn't cause a problem because the amp is unkeyed and the transistors are in cutoff at this point. If this bothers you, the problem can be rectified much the same way it's handled in AC applications. The alternators can be wired so the field only comes on when the amp is keyed and the field is shut off at or just before the amp is unkeyed. This can be done automatically through a relay. Using a capacitor that charges off the 12 volts system in receive. When the relay is activated, it dumps the stored capacitor energy into the field to start it.
There are many variations of how this system can be wired in. Some will work better then others. The basic outline I have given was used successfully throughout the 1990's. It is tried and tested, not just theory.
The second reason is also why AC boxes can not be run at an idle in the vehicle. If you're willing to accept the same issue with DC, then you really have no reason at all to keep a truck load of batteries in the vehicle to run your DC equipment. All you need is a bank of high current filter capacitors and to pulse the field from the 12 volt system to start the alternator.
The capacitor bank should have a total capacitance of at least 100,000 uf and they must be rated to handle the working voltage. Because the current draw can be hundreds of amps, do not try to use a single capacitor to filter this output. Depending on the load, you may need between 2 and 4 equal value caps in parallel to handle the current.
The caps replace the battery however, since there is no outside power source, the field usually will not have enough residual magnetism to start producing output. To solve this problem you simply pulse the field through a diode and resistor with a momentary contact switch connected to the 12 volt system. Once the field is magnetized, no outside power is required.
There are pros and cons to running this system. The pros are, you can remove the charging load from the alternators and operate them at higher output in intermittent use since they cool down in receive. You have a wide range of useable output voltage that can be changed at anytime simply by adjusting the regulator. You are no longer stuck at the fixed voltage appropriate for charging your batteries, you're free to go from 12 to 20 volts in the blink of an eye.
The first con is Leece Neville voltage regulators suck. They usually use a transistor as the active control device. The conduction points of the transistor are not sharp enough to provide complete regulation from full load to no load. This makes SSB operation of this system unsafe without construction of a better regulator that uses an op amp comparator as the trigger to control the field pass transistor. The op amp vastly improves the reaction time of the regulator.
For AM use this is not a problem as long as you're close to the standard 4 to 1 carrier ratio. However, when you unkey, there will be a voltage spike as the load is removed. This doesn't cause a problem because the amp is unkeyed and the transistors are in cutoff at this point. If this bothers you, the problem can be rectified much the same way it's handled in AC applications. The alternators can be wired so the field only comes on when the amp is keyed and the field is shut off at or just before the amp is unkeyed. This can be done automatically through a relay. Using a capacitor that charges off the 12 volts system in receive. When the relay is activated, it dumps the stored capacitor energy into the field to start it.
There are many variations of how this system can be wired in. Some will work better then others. The basic outline I have given was used successfully throughout the 1990's. It is tried and tested, not just theory.
