DC to DC charge controller necessary

[Yoshi]

Hi,

I'm getting ready to swap out my lead acid batteries to lithium (2 in parallel). I'm reading conflicting information that the alternator in my 2021 GC limited (w tow package) will be damaged if a DC/DC controller is not installed. I eventially plan to install one, but right now I want to begin using the lithium batteries in my RV thank you

 

[Isaac-1]

It depends on what size LiFePo4 batteries you use, and how you use them, ie how deeply they get discharged, etc. Personally I would not risk it.

 

[Old_Crow]

Hi,

I'm getting ready to swap out my lead acid batteries to lithium (2 in parallel). I'm reading conflicting information that the alternator in my 2021 GC limited (w tow package) will be damaged if a DC/DC controller is not installed. I eventially plan to install one, but right now I want to begin using the lithium batteries in my RV thank you

Easiest way to do this would be to disconnect the hot wire from the vehicle charging system to the trailer at the point where the harness is wired into the vehicle until you could get the DC/DC charger. The car would no longer charge the batteries while driving, but maybe that's not a big deal to you.

 

[UTTransplant]

If the batteries are low, they will draw a lot from the alternator. It can burn up smaller vehicle alternators. My MH a is a 160 amp alternator, so I don’t worry, but you really should install one soon. Until then, either charge up the batteries before you leave with shore power or a generator, or disconnect the alternator from the charging system.

 

[Yoshi]

, or disconnect the alternator from the charging system.

Would this be removing the 12V on the 7 pin as @Old_Crow suggested? Just to confirm, this would have no impact on the emergency brakes on the trailer. They need 12V, but as long as the battery switch (in the trailer) is still on, they should still be powered? Thank you

 

[CharlesinGA]

Trailer break away switches are powered directly off the trailer battery. There can be no fuses or switches between the battery and the break away switch. I think this is a Federal requirement.

Installing Lithium batteries is a SYSTEM and not just a simple battery swap. The batteries should have rather expensive T class fuses on the outputs and your power converter may or may not be capable of properly charging lithium batteries. Many new RV's have WFCO (junk) converters that supposedly have "auto detect" and will change modes and charge lithium just fine, but from reading the forums, they generally won't switch over properly. Other brands of converters require you change a jumper setting or slide a switch, or replace the converter entirely. When you go to install an inverter, the wiring then gets really crazy, as you will need heavier gauge of everything and better connections.

Most people who install lithium also install a battery monitor system such as the Victron units.

As far as the power from the truck goes, you could install a small (20a max) DC/DC charger in the trailer's 12v feed from the truck. This will limit the amp draw from the truck, and also boost the charge voltage to the trailer batteries.

Modern vehicles do not use voltage regulators, but rather depend on the computer to regulate the voltage. After a start they boost it to retop the starting battery, then drop the voltage to operate the electronics/electrical system. Trucks, usually are programmed to detect the trailer and if they do, they will NOT drop the voltage but maintain a higher level for trailer charging.

If you chose to use a larger DC/DC charger and run separate heavy cables from the truck's battery, you will need to fuse that right at the truck's battery, and may want to install an automatic trailer power relay, though not necessary. Anderson connectors at the bumper and then I would disconnect the trailer 7 way power wire AT THE TRAILER'S JUNCTION BOX. Don't mess with the truck wiring, as you may need to pull something else, and will cause other issues. Alternatively, you can pull the fuse in the truck for the charge line to the 7 way.

Charles

 

[Lou Schneider]

I have two Lion Safari 100 amp-hour lithium batteries in my Safari motorhome and don't see the need for a DC-DC converter. When the engine is running each battery draws about 25 amps when charging, or 50 amps for the pair regardless of the state of charge. This is roughly the same as what a lead acid battery draws during it's bulk charging phase, the difference is the lithium's charge rate doesn't taper off as the battery gains charge. The voltage coming out of the stock Chevy alternator fills them to at least 90%.

If you have more than 200 a/h of lithium batteries, or a trailer with a 10 or 8 gauge charging wire, then yes a DC to DC charger is useful to regulate the charging amperage. But not so much in a motorhome with one or two batteries and thick cables that can handle 50 amps continuous current.

On the plus side, not having a DC to DC converter means the house batteries can still boost a dead starting battery and I've had to use this a couple of times since installing the lithiums. A DC to DC converter will block the reverse current flow to the starter.

 

[UTTransplant]

Lou, I think the OP has a Grand Cherokee SUV, not a MH.

 

[DonTom]

Lou, I think the OP has a Grand Cherokee SUV, not a MH.

I'm reading conflicting information that the alternator in my 2021 GC limited (w tow package) will be damaged if a DC/DC controller is not installed.

I had success by using one of these, on my Jeep SUV 4.8L as well as my 7.4 L Y2k RV.

I know the instructions say up to 6L but it will still work on a 7.4L as long as the small pack is fully charged and the large RV battery is not totally dead. I have never had it not work when needed. I carry them in my ICE motorcycles as well as in my RVs and other vehicles. I own several.

I do NOW have the DC2DCC in my Y2k RV, but I have used the RV a lot without it, I just made sure my lith battery voltage was as high or higher than the RV charging voltage. IOW, I made sure the lith (300 AH) was always well charged up so it cannot draw any current from the alternator. Can charge it up with the genny (that has a lith converter) when needed.

So if it is safe or not to your alternator to use without the DC2DCC, it depends on several things. It worked out well for me, but I decided to add the DC2DCC anyway.

In my 2022 RV, I have an added 2nd DC system that does NOT charge at all by the alternator and have not had the need for it to do such yet and I doubt if I ever will.

I have the same 300 AH lith in each of my RVs, but in my Y2k RV it is also my main house battery.

In my 2022 RV, the house battery stuff stays stock and I added the 2nd system so I can boondock and still run the MW oven, hair dryer, ham radio, etc. and have no effect on the main house L-A batteries.

-Don- Reno, NV

 

[TonyL]

Just wondering if instead of a DC-DC converter, could you not just fit a higher output alternator?

 

[CharlesinGA]

Just wondering if instead of a DC-DC converter, could you not just fit a higher output alternator?

Lithium batteries, due to their very low internal resistance, will take all of the charge they can get. If they are low on charge, this means very high amp draws, which will damage wiring unless the wiring is upsized enough to handle it (we are getting into welding cable sized wiring) AND a large enough alternator is provided.

Some will argue that the charge wire in the tow vehicles wiring harness acts as a resistor, due to its relatively small size, and that no DC-DC charger is needed, but you are then getting less than ideal voltage to the trailer, and at the risk of the wiring harness in your tow vehicle being damaged.

The other issue is that new vehicles use computer controlled alternators. No more regulators on them, just the computer. It raises voltage after a start to re-top the battery, and then drops the voltage to about 13.0 to 13.2 volts, which is ideal for maintaining a lead acid battery and for operating the equipment in a car or truck. However Lithium needs 14 volts or better to get fully charged. Many truck computers will sense the connection to the trailer and not drop the voltage down, but maintain a higher voltage for charging the battery on the trailer (which may only be a small sealed lead acid break away battery on a flat bed trailer or the like). Fact is, tow vehicle trailer charge lines are not designed to charge massive RV battery banks, but rather to keep a break away battery topped off.

The battery to battery charger does two things, one, it limits the charge amperage, which in turn limits the draw on the tow vehicle system to something it can handle. Second, it is a buck/boost transformer, and can either lower or raise the output voltage as necessary. Most DC-DC chargers have a way of setting them for either lead acid or lithium so they know what output voltage to generate. Thus, if charging Lithium, it will put out a high enough voltage (14+) to actually top off the lithium to 100% or nearly so if charged for long enough, and if it is charging lead acid it charges at a lower voltage (mid to low 13's) so as to not cook the battery, but again, get it fully charged in a reasonable time.

The other nice thing about DC-DC chargers is that they almost all have a solar input and a MPPT controller, so, unless you are running a huge solar panel array, that can serve as the solar input also without the need for another box.

Note that if the DC-DC charger is say, 20 amp, that is its OUTPUT max. It will draw quite a bit higher on the input side to be able to generate the power and voltage for the output.

Charles

 

[Yoshi]

tks to everyone for your replies and insights.