Did I read correctly that you have converted your whole truck to 12volt?
If so, you should be able to use the radio power bus as a charging source. Make sure you use a fuse sized to protect the wire. A relay would be the easiest way to protect the truck electrical from your extra battery, connect the coil of the relay to the ignition wire from the vehicle system (the wire that goes hot when you turn on the ignition). This way when you turn on the ignition switch, it connects the battery to the radio power bus and charges it, and when your turn off the ignition it disconnects the battery from the radio bus.
You will get a spark in the relay if there is a significant difference in the state of charge of the two batteries. You really can't get around this without making hour system more complicated. For example adding current limiting resistor and a second rely to bypass it after a time delay...
As it happens, it would be easier if you still have a 24volt charging system in the front and 24volts at the radio bus. You can use a cheap step down voltage regulator from e-place to drop the 28.8volts from the running engine charging system to 14.4 voltage required for a single 12 volt battery. These cheap voltage regulators (found one for $7, look for "step down CC CV") sometimes feature current limiting which will make it act like a simple two stage charger:
- Bulk (current limited until the voltage matches the set point)
- Float (current tapers off as the voltage regulator maintains the voltage value)
This also prevents a fully discharged battery from appearing as a very high current load to the parallel batteries over the radio bus, and the current limiting keeps from burning up the radio bus wire, thus reducing the cause of arcing. You would still want to relay to the ignition wire so that it only charges your battery when the engine is running. The cheap step downs will only put out between 3-5amps, so to get enough to charge your battery you'd need several in parallel. I hear a good rule of thumb is the load plus 25% of the battery amp hours to get the C/20 rate.
There are also step-up and step-up/step-down CC/CV regulators on e-place, so you can mix an match to various effect. For example, you can use a 13Volt front battery through a radio wire bus that drops another volt (to 12volts) and boost that (step-up) to 14.4V to charge a battery. It's all in how much you want to do it yourself and how much money you have
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