Couple of questions about DC-Dc charging

otrfun wrote:
Most of the Renogy dc to dc charger cable recommendations result in a 25-50% alternator load penalty. For instance, based on their recommendations, their 40a dc to dc charger at full output would result in a 50-60a load on the alternator. In effect, you end up "wasting" 10-20a of alternator output.

Nothing is “wasted”. The DC-DC charger boosts the voltage from what’s available (which will ALWAYS be lower at the end of those 23 ft cables than what you measured at the alternator) to what is needed by the charger based on which charging stage it’s in. It does that by drawing more amps. There are always conversion losses, but it is NOT wasting 10-20 amps of alternator output.

The cable run from our truck battery to our 40a dc to dc charger mounted in our truck camper was approx. 23 ft. Renogy recommends 4 gauge for this particular run. We used 2 gauge instead. With a 40a output from the dc to dc charger, our alternator only incurs a 43.5a load (8-9% load penalty). Alternator output voltage was 14.1v at the time we took this current reading.

While the higher gauge wires certainly don’t hurt, assuming the 4 gauge wires were capable of carrying the maximum amperage for the length of your circuit, up-sizing them to 2 gauge isn’t what’s responsible for what you are calling the “8-9% load penalty”. The alternator voltage being as high as it was at the time (14.1) was responsible for that. If the DC charger was in bulk or boost mode, it only needed to raise the voltage to 14.6, so it did that by drawing more amps. Referencing the DC charger’s output voltage to the alternator output voltage is irrelevant anyway. The DC charger should be connected to your starting battery, not the alternator. What matters to the DC charger is what the voltage is at it’s input terminals. What matters to the alternator is the voltage it’s sensing of the starting battery. The DC charger is just another electrical load on the starting battery. If the voltage at the DC charger’s input terminals drops to 13 volts or even lower, but it needs to be in boost mode for the camper battery (which is going to happen at some point), it’s going to draw as much current as it needs to raise the voltage to 14.6.

We chose to mount/use a battery isolator next to the Renogy dc to dc charger in our truck camper vs. running a 20-25 ft. sensing wire from the dc to dc charger in our TC to the truck's engine bay. Another added benefit of mounting the battery isolator in the TC (vs. the truck) is zero parasitic current when the TC is disconnected/off-loaded from the truck.

There is no parasitic current from the camper battery to the DC charger when the TC is disconnected from the truck. There IS a less than 0.4 amp idle current draw of the charger FROM the truck starter battery when the TC is connected to the truck. That’s why you either run a long D+ sensing wire all the way to the truck (which would be foolish, imho) or you run the +12v input wire to the charger through an ignition switched constant-duty solenoid or some other such device mounted in the truck, and connect the D+ sense wire directly to the DC charger’s + input terminal.