How do I determine if DC-DC charger is necessary?

[thewarden]

Originally Posted by dznf0g

I was just going to post that bulletin. I know I have played with the tow haul mode while driving and seen no rise in voltage. I'm going to have to work it again when I leave for AZ in 3 weeks. I had to turn on lights to get voltage rise. Now that I installed the victron DC/DC, its irrelevant, but I'm curious.

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I have played around with my tow-haul mode as well, but I have never seen the voltage gauge deviate, even a bit, no matter what the condition. It always reads well over 14v. However, I don't put much stock in these gauges.

Mine is a 2018 GMC 3500 Denali. I took it in when it was new out of concern that the gauge never moved, the GM dealer checked it out and said all was normal. Go figure.

[dznf0g]

OK, I finally found that verbiage in my owner manual. I never had a problem getting a full charge when I had FLA batteries and my trailer solar settings were for FLA. Once I installed AGM and a correspondingly higher solar settings, I started arriving at the next night site with less than a full charge.

[dznf0g]

Originally Posted by thewarden

I have played around with my tow-haul mode as well, but I have never seen the voltage gauge deviate, even a bit, no matter what the condition. It always reads well over 14v. However, I don't put much stock in these gauges.

Mine is a 2018 GMC 3500 Denali. I took it in when it was new out of concern that the gauge never moved, the GM dealer checked it out and said all was normal. Go figure.

What battery chemistry are you running....before and after the dc/dc install.

[uncle_bob]

Originally Posted by dznf0g

It's done through the ecm and bcm by sensing system amperage and voltage. Where noted below, switching components will command certain outputs. Before all this discussion, I was using headlamps in order to get a charge to the AS batteries from tv and solar.

"Charging System Operation
The purpose of the charging system is to maintain the battery charge and vehicle loads. There are 6 modes of operation and they include:

Battery Sulfation Mode

Charge Mode

Fuel Economy Mode

Headlamp Mode

Start Up Mode

Voltage Reduction Mode

The engine control module (ECM) controls the generator through the generator turn ON signal circuit. The ECM monitors the generator performance though the generator field duty cycle signal circuit. The signal is a pulse width modulation (PWM) signal of 128 Hz with a duty cycle of 0-100 percent. Normal duty cycle is between 5-95 percent. Between 0-5 percent and 95-100 percent are for diagnostic purposes. The following table shows the commanded duty cycle and output voltage of the generator:

Commanded Duty Cycle Generator Output Voltage

10% 11 V
20% 11.56 V
30% 12.12 V
40% 12.68 V
50% 13.25 V
60% 13.81 V
70% 14.37 V
80% 14.94 V
90% 15.5 V

The generator provides a feedback signal of the generator voltage output through the generator field duty cycle signal circuit to the ECM. This information is sent to the body control module (BCM). The signal is PWM signal of 128 Hz with a duty cycle of 0-100percent. Normal duty cycle is between 5-99 percent. Between 0-5 percent and 100 percent are for diagnostic purposes.

Battery Sulfation Mode
The BCM will enter this mode when the interpreted generator output voltage is less than 13.2 V for 45 minutes. When this condition exists the BCM will enter Charge Mode for 2-3 minutes. The BCM will then determine which mode to enter depending on voltage requirements.

Charge Mode
The BCM will enter Charge Mode when ever one of the following conditions are met.

The wipers are ON for more than 3 seconds.

GMLAN (Climate Control Voltage Boost Mode Request) is true, as sensed by the HVAC control head. High speed cooling fan, rear defogger and HVAC high speed blower operation can cause the BCM to enter the Charge*Mode.The*estimated battery temperature is less than 0°C (32°F).

Battery State of Charge is less than 80 percent.

Vehicle speed is greater than 145 km/h (90 mph)

Current sensor fault exists.

System voltage was determined to be below 12.56 V

When any one of these conditions is met, the system will set targeted generator output voltage to a charging voltage between 13.9-15.5 V, depending on the battery state of charge and estimated battery temperature.

Fuel Economy Mode
The BCM will enter Fuel Economy Mode when the estimated battery temperature is at least 0°C (32°F) but less than or equal to 80°C (176°F), the calculated battery current is less than 15 amperes and greater than -8 amperes and the battery state-of-charge is greater than or equal to 80 percent. Its targeted generator output voltage is the open circuit voltage of the battery and can be between 12.5-13.1 V. The BCM will exit this mode and enter Charge Mode when any of the conditions described above are present.

Headlamp Mode
The BCM will enter Headlamp Mode when ever the headlamps are ON (high or low beams). Voltage will be regulated between 13.9-14.5 V.

Start Up Mode
When the engine is started the BCM sets a targeted generator output voltage of 14.5 V for 30 seconds.

Voltage Reduction Mode
The BCM will enter Voltage Reduction Mode when the calculated ambient air temperature is above 0°C (32°F). The calculated battery current is less than 1 ampere and greater than -7 amperes and the generator field duty cycle is less than 99 percent. Its targeted generator output voltage is 12.9 V. The BCM will exit this mode once the criteria are met for Charge Mode"

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Hi

..... and absolutely none of that does anything to impact the voltage drop on the wires to the 7 pin. Indeed it does virtually nothing with a 5 to 10A load anywhere on the vehicle.

Bob

[thewarden]

Originally Posted by dznf0g

What battery chemistry are you running....before and after the dc/dc install.

Stock wet cell in the truck and lithium in the trailer.

[dznf0g]

Originally Posted by uncle_bob

Hi

..... and absolutely none of that does anything to impact the voltage drop on the wires to the 7 pin. Indeed it does virtually nothing with a 5 to 10A load anywhere on the vehicle.

Bob

I don't understand the meaning of your statement????

[uncle_bob]

Originally Posted by dznf0g

I don't understand the meaning of your statement????

Hi

If we are talking about charging a battery in the trailer (which is how this started) the limitation is the voltage drop in the wiring. The claim was made that a smart alternator made charging easy. Since it does not impact the voltage drop in the wires, that's not the case.

Bob

[Justinfinity]

DC-DC charger or not, you won't get a decent charge through the 7-way connector. So, if you are planning to use the 7-way, you are probably wasting your money on a DC-DC charger.

If you really want to do this right, you have to run a separate "umbilical" from the tow vehicle to the trailer. And you have to have a robust alternator.

I have posted on this topic several times (including today), describing exactly what I did to achieve reliable 50-60A charge from TV to AS.

[dznf0g]

Originally Posted by uncle_bob

Hi

If we are talking about charging a battery in the trailer (which is how this started) the limitation is the voltage drop in the wiring. The claim was made that a smart alternator made charging easy. Since it does not impact the voltage drop in the wires, that's not the case.

Bob

OK, I'm not sure where the "easy" comment came from, nor what that means.
For me the thread is about whether a DC/DC is necessary. For him (OP), probably not. With 200Ah of lithium (and no rooftop solar), he probably would never see a huge enough current flow to overheat the alternator with long term use. This is because of your mentioned relatively low 10A (?) flow through a restrictive wire. BUT, his Lion batteries have quite low internal resistance (especially when substantially run down). Since circuit resistance is additive, he might see more current through the 7 pin than if he had LA batts. I still doubt that he would see substantially charged LiOns at the end of a day's drive if he were down to 20 or 30% SoC when starting out...or even 50%.
HOWEVER, if one were to simply install a large wire, say 4 or 6ga, AND no DC/DC, all bets are off in my book.

My wandering on this subject, is what happens when we add solar charging with a smart alternator with two totally different charge routines, opposing current flow, especially when the TV has some "hard commanded" overrides in addition to its own SoC routine and it's own temp compensation where the TV battery and AS battery are different chemistries and at very different temperatures. (Underhood vs. inside AS).

[Free Pony]

This is all such great technical info everyone thank you!

I recently had rivets go bad on a pulley that held my serpentine belt and the belt seized and burned up, alternator was ruined, and the belt flew into my starter - poof! $3000 later, and my BB LIon batteries were dead dead dead!

BB recommended VJs Automotive in my area. I took my rig to VJs in Arcadia, CA, terrified the batteries were ruined. VJs was amazing! Incredible customer service, welcoming and kind and highly communicative and REASONABLY PRICED! I called BB, gave the phone to VJ and the two companies worked through my issues collaboratively.

I don't know whether the company that originally installed my BBs installed a DC-DC charger or not, I just bought the batteries from Dragonfly Energy and took them to Ernies RV and said "here put these under the couch!" VJ did not find a DC-DC charger when he looked under the hood. I had to decide if I wanted one. The way he explained it to me was that it protects your alternator by regulating the flow of electricity to maximize battery charging, and minimize wear on both batteries and alternator. I have a B Class and I almost never hook up to shore power (I do have a Zamp suitcase but don't use it when I'm camping it in the city) so I require excellent battery charging when I drive. For me, the DC-DC charger made sense.

[Steamy1]

Originally Posted by Justinfinity

DC-DC charger or not, you won't get a decent charge through the 7-way connector. So, if you are planning to use the 7-way, you are probably wasting your money on a DC-DC charger.

If you really want to do this right, you have to run a separate "umbilical" from the tow vehicle to the trailer. And you have to have a robust alternator.

I have posted on this topic several times (including today), describing exactly what I did to achieve reliable 50-60A charge from TV to AS.

I don't understand this statement; all I know is that after dry camping for four (4) days, according to my Victron 712 monitor, I had 67% left on our LION batteries (105AHx2) , drove about 2.5 hours, and the monitor was reporting 100%, then camped again for several days without hookups.

The only charging energy to the batteries was the dc-dc charger through the 7-way wiring 12awg positive and ground wires from the TV. Am I missing something?

[dznf0g]

Originally Posted by Steamy1

I don't understand this statement; all I know is that after dry camping for four (4) days, according to my Victron 712 monitor, I had 67% left on our LION batteries (105AHx2) , drove about 2.5 hours, and the monitor was reporting 100%, then camped again for several days without hookups.

The only charging energy to the batteries was the dc-dc charger through the 7-way wiring 12awg positive and ground wires from the TV. Am I missing something?

Nope, you're not missing anything, but you have a DC/DC. (You don't say what size). At 67%, you were only down 69Ah. So, you were charging 27(ish) amps per hour of travel. That's if you have everything calibrated to true capacity. I do worry a bit that apparently you have a 30 amp DC/DC on a 12ga wire and didn't pop the charge line fuse. Is it a 30amp circuit? It seems most 30 amp fused charge lines I've heard about are 10ga.

[Steamy1]

Hi Rich,
Thanks for the reply.

My mistake; double checked the install and the white (ground) and black (positive) 7-pin wires are 10awg. I installed a 30amp fuse on the black wire on the input side of the dc/dc charger. Then 10awg on the output side to the respective buss bar.

[dznf0g]

Originally Posted by Steamy1

Hi Rich,
Thanks for the reply.

My mistake; double checked the install and the white (ground) and black (positive) 7-pin wires are 10awg. I installed a 30amp fuse on the black wire on the input side of the dc/dc charger. Then 10awg on the output side to the respective buss bar.

Welcome. You're fine, and appear to be "right sized" for your lifestyle.

[uncle_bob]

Hi

If your shunt has been running up and down for a while without ever hitting max voltage, it's likely to be off a bit. How far off depends a lot on how careful you have been about all of the various calibration numbers for the shunt. You might be off 3%, it could easily be 30%.

When you *do* get up to a full voltage (which also is a setting .... do you have it at 13.6 or 14.2?) the shunt resets to 100%. Net effect is that you really can't be 100% sure how much charging got done in that process simply by looking at the two percentages.

Bob

[kmrice]

Probably a dumb question, but I’ve been reading about DC/DCs for months now and still haven’t figured out if they go in the truck or the trailer. If in the truck, will an Airstream dealer who is experienced with Lithium upgrades (thinking JC or Colonial) do work on a truck or do you have to get a truck mechanic to install it?

[Steamy1]

Originally Posted by kmrice

Probably a dumb question, but I’ve been reading about DC/DCs for months now and still haven’t figured out if they go in the truck or the trailer. If in the truck, will an Airstream dealer who is experienced with Lithium upgrades (thinking JC or Colonial) do work on a truck or do you have to get a truck mechanic to install it?

Since I only went with a 20amp dc-dc charger and connected to the feed wires from the pig tail, mine is installed in the trailer next to the inverter.

If you want to go all out, you can install a REDARC in the engine compartment and run cables and connectors to the rear of the vehicle to connect to cables from the trailer.

I was happy to at least have some charging capacity while traveling and am satisfied with the 20 amps.

[dznf0g]

Originally Posted by kmrice

Probably a dumb question, but I’ve been reading about DC/DCs for months now and still haven’t figured out if they go in the truck or the trailer. If in the truck, will an Airstream dealer who is experienced with Lithium upgrades (thinking JC or Colonial) do work on a truck or do you have to get a truck mechanic to install it?

I put mine in the AS, near the electrical center with all the rest of the equipment. Some folks install in truck, but I don't understand why. Mine is an 18 amp victron.

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[thewarden]

Originally Posted by kmrice

Probably a dumb question, but I’ve been reading about DC/DCs for months now and still haven’t figured out if they go in the truck or the trailer. If in the truck, will an Airstream dealer who is experienced with Lithium upgrades (thinking JC or Colonial) do work on a truck or do you have to get a truck mechanic to install it?

Because my AS batteries are still in the original battery box on the tongue, I mounted my DC-DC in the truck box. I have a hard cover, so water should not be an issue. I did it this way because I did not want to run heavy wiring from the truck into the trailer and then back out to the battery box.

If I had the batteries inside the trailer, then I would mount it in the trailer.

[kmrice]

Thanks, as always, for the helpful replies. I’m just looking to get some charging on lithium batteries while towing. I’m thinking the DC/DC would fit in the compartment where my solar converter is stored on my 2022 GT 23FBT (under the front of the right bed).