8 Gauge Wire

One possible solution is a dc to DC charger.

But likely an inverter is a better choice. It can be MSW and about 400 watts.

I second the use of a DC-DC charger ! You can use the "stock" wiring (including fuse) from the 7 pin trailer connector.

Depending on the calculator or table you use, distance is measured as one-way (as a crow flies) or two-way (round trip). If your battery and alternator are 25' apart, that would be 25' one-way or 50' two-way (25' POS + 25' NEG). The ever popular BlueSea voltage drop table is TWO-WAY based.

As for voltage drop, the 3% and 10% rules apply to most device types, but not necessarily to DC-to-DC converters which are designed boost voltage, aka counter voltage drop.

Sterling has a video where they compare straight alternator-to-battery wiring where the wire has to be "huge" to counter voltage drop, but then with a DC-to-DC converter, the wire can be much smaller and cheaper and still get the job done. For the alternator to DC-to-DC wire, see the manufacturers length to AWG recommendations rather than needlessly oversizing/overspending on AWG based on a 3% or 10% voltage drop rule. At least for Sterling, they support down to 11.0VDC, or roughly a 23% voltage drop off at 14.4VDC alternator. To counter voltage drop on the output side, the DC-to-DC will have to pull a few more amps, and Sterling specs the 60A output could require up to 80A of input from the alternator. I don't recommend pushing the 20%+ limit however.

Good info, all. The specs call for 4 to 6 awg depending on distance. My distance falls in the middle of the range listed for the run. I would like to run 4 awg from truck batt to truck bed, then step down to 6 for the last 6 or 7 feet of the run. Any concerns with that?

I am running a Victron DC to DC Charger in my AF 992.

From the engine bay to the truck bed, I have 4AWG wire with an Anderson connector where it plugs in to the camper.

From the DC to DC Charger to the truck bed pigtail, I used large wire too, but I honestly don't remember what size it is.

Here's a few videos of that project, which might give you some ideas:

Truck to RV High Amperage Battery Charging

Installing a Victron Orion TR Smart 12-12-30 DC to DC Charger

ticki2 wrote:

LaneW wrote:

ticki2 wrote:
If you are 25’ from battery to charger isn’t the run actually 50’ there and back ?

I don't know. Is that true??? Its not AC.

That has been my understanding . Perhaps one of the electrical guru’s will clarify

no, you use one wire to get there (positive) and a different wire to get back (ground). the rating of the wire is the amout of amps for so many feet of lenght so it would be a 25' length of wire.

LaneW wrote:

ticki2 wrote:
If you are 25’ from battery to charger isn’t the run actually 50’ there and back ?

I don't know. Is that true??? Its not AC.

That has been my understanding . Perhaps one of the electrical guru’s will clarify

LaneW wrote:
. . . What I am looking at installing is DCC30S 12V 30A Dual Input DC-DC On-Board Battery Charger with MPPT. I am looking at the specs but not finding the input current. I am still looking for that. But, and I’m sure I dont understand as well as you, I will be at a bit under 25 ft from truck batt to charger, which based on the specs I see online would be on the long end fog 6 awg, or safe with 4 awg. Right?

According to the Renogy website, max rated input power for the DCC30S is 400w. Input voltage range is 13.2 - 16v (alternator voltage minus any voltage drop). Supposedly this unit is capable of up to 97% efficiency--impressive. Unfortunately, they don't state what conditions are necessary to obtain 97%, so efficiency could drop off significantly for other conditions. Based on these specs I'd guess-estimate input current would max out around 31a (current varies dependent on input/output voltage).

25 ft of 4 gauge would probably net you approx. 3% of voltage drop at 30-31a. This is based on the one-way distance (using the voltage calculator link I provided earlier). 3% should be fine. I wouldn't want it any higher.

ticki2 wrote:
If you are 25’ from battery to charger isn’t the run actually 50’ there and back ?

I don't know. Is that true??? Its not AC.

Input current will vary depending on wire size and length feeding the charger, that is probably why they don’t list it. I run a 50 amp Redarc and it outputs 50amps with an input of about 53-54 amps, but I have a 1/0 DLO charge line.