When you add a DC-DC charger from the truck as in several threads about that, does that put the house batts in parallel with the engine batt? (as with normal "alternator charging" via 7-pin)Or is it just charging the house batts, which are isolated from the engine batt being charged from the alternator?

There’s more than one way to configure it, but for the most part it’s a parallel scenario, and often* necessary to protect the alternator when using LiFePo4 batteries - Why?? Because of (if one dare believe Victron, et al...) LFP’s high current receptivity...Also, for newer vehicles, the dc to dc converter needs to be ‘Smart Alternator’ or Euro 5&6 compatible...*depending on components and loadHere’s an interesting primer on the subject:https://www.youtube.com/watch?v=dzU_pQFa19s&feature=youtu.be3 tons

Parallel would imply bi-directional flow and that is not really the case. The DC-DC just allows power in one direction. Then it is just a matter of how the DC-DC is connected to the truck and if that connection has any isolation.

Thanks, interesting.My question was more about the Renogy 20 amper with input from starter and output to house. After some more reading of its manual, it says: (after saying the input and output negs are all on the same chassis for grounding) for the positives:"the DC-DC output will connect to the 12V auxiliary or house battery that you are intending tocharge. These batteries may be a different chemistry from the starter battery. The DC-DC inputand output terminals are isolated, meaning that the output voltage can be kept stable withoutinterference from the input circuit. This ensures stable and correct charging of auxiliary batteries. It is best to place the DC-DC closer to the battery you will be charging primarily."I don't want my SiO2s in parallel with the FLA starter although I am told that is not a problem when floating or charging them. So right now in the TC I have disconnected the 7-pin charging to the camper batts.I would like to have "alternator charging" as a back up to the solar, since we can't carry a portable gen along in the TC set-up. I was thinking of using the 7-pin pos and neg in the camper that normally go on the house batt terminals as the inputs to the Renogy and output to the battery. Not too worried about the amps that would supply and the truck has a 40 amp fuse for the pin 4 charging wire so that would cover the higher draw from input than max output of 20 amps. I think.So would that work like it says and not put my SiO2s in nparallel with the starter batt?https://www.amazon.com/Renogy-Battery-Batteries-Multi-stage-Charging/dp/B07Q5VYPCF/ref=sr_1_3?crid=24ISMNS5CEXIC&dchild=1&keywords=renogy+20a+dc+to+dc+charger&qid=1613847979&sprefix=Renogy+20a%2Caps%2C399&sr=8-3EDIT--could I get the same thing by just using a solar controller with the 7-pin pos and neg into the array side?

Put the dc to DC charger in the existing "house" charging circuit on the "house" side of the isolation solenoid.While in use--there is no back feed path.Then when the truck is shut off, the dc to DC will be isolated from the chassis battery.

pianotuna wrote:Put the dc to DC charger in the existing "house" charging circuit on the "house" side of the isolation solenoid.While in use--there is no back feed path.Then when the truck is shut off, the dc to DC will be isolated from the chassis battery.This would be in the Chev truck using the 7-pin which is always live. No isolator as with a Ford. so when engine off, the input to the Renogy would be live and still charging the house? except no charging to the starter batt? OOPs ?OK same as usual with the 7-pin , have to unplug it when parked and no shore power. Except with the Renogy there, shore power won't float the starter batt too. I put a trickle charger on the starter batt now if parked for a longer time. Could still do that- so the Renogy and the converter would be in parallel on the house. that's ok.

DC-DC Charging ? | Good Sam Community

pianotuna
Nomad III
Feb 20, 2021
BFL13,

You convinced me to get one, too! Thanks for finding what, for me, is the right size for the right price.
BFL13
Explorer II
Feb 20, 2021
Slownsy wrote:
BFL13 according to Renogys manual you will have to install 6-4 Wire from truck to FW as distance 20-30’
Frank.

Yes, thanks. I will see how it goes first. It is more like 12 ft one way, but the 7-pin charging wires are more like #10 or 12, so amps will suffer. It is a Truck Camper and camper batts are near the front of the truck bed, so closer. I get 15 amps on the 7-pin now to the camper batt (at first, then it tapers)

My purpose is not to get more amps (but that would be nice), but to not have the two types of batteries "mixed", which might not even be an issue either. I just don't want to mix them and that's that.

--PT, what do you want one for? And why not get the 60 amper, or will that overtax your alternator ? (input amps draw being higher than output amps.) Also you said you will have two banks so do you need two?
theoldwizard1
Explorer II
Feb 20, 2021
BFL13 wrote:
Or is it just charging the house batts, which are isolated from the engine batt being charged from the alternator?

This !

Some have a manual "backfeed" button to charge the vehicle battery if it is lower than the house battery.
theoldwizard1
Explorer II
Feb 20, 2021
BFL13 wrote:
Yes, thanks. I will see how it goes first. It is more like 12 ft one way, but the 7-pin charging wires are more like #10 or 12, so amps will suffer. It is a Truck Camper and camper batts are near the front of the truck bed, so closer. I get 15 amps on the 7-pin now to the camper batt (at first, then it tapers)

The whole point of a DC-DC charger is NOT HAVE TO WORRY ABOUT VOLTAGE DROP ! The charger should be as close as practical to the house battery. The limiting factor will be the fuse that is powering the 7-pin charge wire.

BFL13 wrote:
My purpose is not to get more amps (but that would be nice), but to not have the two types of batteries "mixed", which might not even be an issue either. I just don't want to mix them and that's that.

This is even MORE important if you are running different battery chemistries.
BFL13
Explorer II
Feb 21, 2021
The Renogy will be within 2 ft (one way) of the house batts. Input 7-pin #4 is fused at 40 amps Stud 1 in the Chev truck under the hood. Output max is 20 amps. We'll see what happens (early March). :)

They are different chemistries but have the same sort of charging voltage specs. Don't care--I still don't want to mix them.
pianotuna
Nomad III
Feb 21, 2021
Hi BFL13,

I got the 20 amp because the alternator is only 130.

My charging paths are both equipped with automatic 50 amp circuit breakers.

I have bank switching so it is easy to switch from one bank to the other, so no need for two dc to DC chargers.
MEXICOWANDERER
Explorer
Feb 21, 2021
Gdetrailer wrote:
DC to DC converter should be a "one way" device but to ensure it is..

Insert a Schottky Diode in front of the DC to DC converter to ensure your vehicle battery and charging system are not going to damage your house batteries.

Read HERE

"Low Forward Voltage Drop

One of the chief characteristics Schottky diodes are known for are their low forward voltage drops.

One note of interest although my 500 ampere dual schottky rectifier needs an isolated heat sink, it passes 166 amperes at vf .304 showing .214 quiescent. I used it on the BORG to insure absolute isolation protection of reversed polarity connection.

Their low forward voltage drops means that they consume less voltage than a typical diode.

A typical silicon diode consumes about 0.7V across its junction.

A typical schottky diode may consume only 0.3-0.4V across its junctions. Thus, it saves about 300mV of power. "

5 pack of 40A Schottky diodes HERE for $9

Those actually are a "dual diode" package, contains two diodes that you can parallel if you like..
BFL13
Explorer II
Feb 21, 2021
My 1991 C's alternator is fairly lame. They got new ones in the 1992s.

With alternator charging in the C as is, I get 30 some amps at first and then it settles around 20 amps. So no point in using a 20 amp DC-DC in mine to get more amps. Also house and engine batts are all FLA no mixing worries.

When there is solar, it has a bit more voltage than from the alternator so that cuts back amps from the alternator.

With the Renogy, I suppose I can dial the solar voltage to match more closely so they add their amps better than they do now. Haven't checked to see if the Renogy has adjustable output voltage--I think so. Then I could dial them both to 14.6 battery spec and they will add their amps better.
BFL13
Explorer II
Feb 21, 2021
So it wants a wire to the ignition circuit to turn the unit ON, but the unit will be back in the camper. It says any 12v source will do though, so I will connect that wire to the house.

I can leave the 7-pin plugged in with the always live Chev input and put a switch on the "ignition" wire from the house to turn the unit on and off.

"The DC-DC will not power on or operate until the D+ ignition cable is connected to the ignition
circuit where it will detect a 12V source to operate ON. The purpose is to toggle the DC-DC
switch on when the vehicle is running with the alternator to prevent the DC-DC from operating
incorrectly with just the starter battery leaving you with a drained starter battery."
noteven
Explorer III
Feb 21, 2021
I’m thinking of using a 3 pole electric trolling motor connector to make the connection between truck and camper. One pole for the ignition on wire that turns the charger on and the other two for size large cable to carry the charge current. I will disconnect the “charge wire” in the camper 7 way plug.