Couple of questions about DC-Dc charging

I still don't think you know if you need a DC-DC charger. In most cases you don't need one. Figure out what is actually happening with the alternator charging the batteries before investing in something you probably don't need that may make the situation worse.

My understanding is you want the signal wire to the charger hot with ignition on or manually switched in an ignition on circuit if you want to be able to shut off the charger in stop and go traffic or extended idling or other such low alternator output situations.

I plan to size my wire for the load for the wire length in both directions and fuse the + connection at the engine battery.

My understanding is the charger should be as close to the house batteries as practical.

One you tube channel that shows the installation of a Renogy is Ray at “Love Your RV” - he measures the output and goes about the description in a low key manner.

Mexicowanderer’s’z system sounds interesting but I am too much of a lectrical dummy to understand :)

Thanks everyone for your responses. So to summarize: My object here with the DC-DC charger is to properly charge my batteries but limit the amperage as to not burn out my alternator. It sounds like my 130 amp alternator may be at the low end of what is needed to really handle 40+ amps of charging for possible hours on end of normal driving. I'm not adverse to upgrading the alternator for a higher amp, or really, a heavier build that can handle the constant current needed to charge the (4) 100amp at possible 50% state of charge.

As to the solenoid that is currently separating the chassis and starter batteries. It should be disconnected and I will extend the wire that energizes that solenoid as the D+ wire for the DC-DC charger.

Does that sound about right? Any other suggestions of real world experience would be greatly appreciated.

There is an easy way to do this but it's going to make the people who have never done it howl in anguish.

The feeder circuit from the alternator to the house batteries must be restricted. Restricted means resistance. The circuit must be made correctly. Professionally designed.
A charge division must be incorporated and the only one I have found to be reliable is a White Rogers 200-ampere capacity solenoid that actuated automatically. A smart solenoid that prefers chassis battery first then house battery. This system needs no switches. Both the Blue Seas and Sure Power solenoids are good brands. The bi directional allows automatic charging in the opposite direction as well. Preferable.

Amperes limiting is done via a fifteen foot length of FUSIBLE LINK wire. 10 AWG and ten to twelve feet in length. Fusible Link wire is tin coated but it's secret is in the insulation. Cross-linked polyethylene which can tolerate extremely high temperature.

USA auto manufacturers have utilized restrictive alternator output wire size for decades and of course they have utilized fusible link wire for gross amperage violations for the same amount of time. Decades.

For many years, GM used a 13 gauge alternator charge wire on their 10DN, 10SI, 12, 15, and 17SI alternators. Ford and MoPar did the same.

The issuse you face with a Ford 3, 4, or 6G, is the alternator has twin internal rans driven by the rotor. And underhood temperatures can read 240 degrees which is punitive for an alternator cooling inlet temp.

Fifteen feet of restricted gauge wire will limit ultimate amperage passage to around 60-70 amps. The preferential charge protocol of the "smart solenoid" inherently limits total amperage delivery.
The fusible wire must not be bundled or wrapped.

For over a decade General Motors avoided using a separate block resistor on their ignition system by using a STEEL CORE coil ignition wire. There was an override for starting booster voltage originating at the R terminal on the starter motor solenoid. Ford used this design as well.

Just make sure your wire is cross link type and is fifteen feet in length.

It would be light years more reliable than any electronic circuit.

My Renogy DCDC which I haven’t installed yet says input amps are 1.3 x output amps so 60 amp draw for a 40 amp output charger.

The charger is an isolator as well so you don’t need a solenoid or another isolator between the starting battery and the house batteries.

The DCDC charger maintains a proper charging voltage profile for the batteries you have it set for. Modern alternators taper their voltage quickly so never really put a “bulk charge” into deep cycle batteries.

You need a signal wire to the DCDC to turn it on.

If you shorten the alternator life high output alternators are available and not that expensive.

If you are discharging your batterie completely on a daily basis you may have shorter alternator life vs light recharging cycles.

About 25% max on a continuous basis...

That is probably too much current for a 135A alternator to sustain, but does it actually provide this much current without a huge load? What is the current when just charging the batteries?

Most modern alternators are ECU controlled (although I don't specifically know about the F53/Triton) and are self regulating - the ECU monitors the alternator temperature and will decrease the output voltage (and thus the load) if the alternator gets too hot. However, this won't work with a DC-DC converter as it will draw more current if the input voltage decreases.

A general rule of thumb is that you can safely run an alternator continuously at 50- 60% of its peak rating, but this is just a guideline. Run the batteries down to 50% SOC or so and then fire up the engine and see what your actual current draw is, and if the alternator throttles back. If you in fact do have an an issue, instead of a DC-DC converter that will limit your charging current, a higher output alternator may be a better investment.

What wire size feeds the AGMs?

The wire feeding the house batteries, 200 amphrs, appears to be 2 gauge. I would assume it would be capable of delivering as much current as the alternator can deliver, safely. The second bank comes off the house batteries using 4 gauge and can be separated, and used independently from the house batteries. This second bank is also directly connected to my inverter. My intent was to get the Renogy 40 amp charger which I believe the wires can easily support but can the alternator?

How much current are you getting from the alternator with the existing set up?

Last I checked it was putting out 119 amps at 14.4 volts. I was lowering my jacks to put on a load. It may put out more.

How much current are you getting from the alternator with the existing set up? You really need to know this before changing things up.

What wire size feeds the AGMs?