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Old 01-13-2022, 02:34 PM   #1
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DC to DC charger question

I will be installing a dc to dc charger shortly, my question is should I disconnect the 7 pin charge line?
If it matters this is the charger
https://www.renogy.com/12v-20a-dc-to...ttery-charger/
Thanks
Tom
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Old 01-13-2022, 02:51 PM   #2
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I'm assuming you have a separate DC plus to use as a charging plus wire, with higher amps, instead of the seven pin plus wire. If you have AGM or flooded batteries, no dc-dc is required, but if you have lithiums, it is better to have a dc-dc charger. The dc-dc charger will have a regulated output based on the type of batteries you have. So yes, disconnect the seven pin hot. No need for two different strategies at the same time. Also, the seven pin hot can be used to signal the dc-dc charger to turn on, provided that that hot is controlled by the tow vehicle key. I think the Renogy uses a separate switched hot to turn on, whereas the Victron Orion uses voltage sensing and needs no separate hot, or switch.
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Old 01-13-2022, 04:24 PM   #3
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Guess I need some education here. I have no idea what this is or what its function is.

Walt
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Old 01-13-2022, 05:52 PM   #4
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Originally Posted by Raspy View Post
I'm assuming you have a separate DC plus to use as a charging plus wire, with higher amps, instead of the seven pin plus wire. If you have AGM or flooded batteries, no dc-dc is required, but if you have lithiums, it is better to have a dc-dc charger. The dc-dc charger will have a regulated output based on the type of batteries you have. So yes, disconnect the seven pin hot. No need for two different strategies at the same time. Also, the seven pin hot can be used to signal the dc-dc charger to turn on, provided that that hot is controlled by the tow vehicle key. I think the Renogy uses a separate switched hot to turn on, whereas the Victron Orion uses voltage sensing and needs no separate hot, or switch.
Thank You
Also, I knew the Victron is a better charger but Renogy had a good price
https://www.ebay.com/itm/20355408178...tem2f64c4cbf5:
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Old 01-13-2022, 07:12 PM   #5
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Guess I need some education here. I have no idea what this is or what its function is.

Walt

The DC-DC charger allows one to charge the trailer batteries from the tow vehicle and make up for the long wire runs, match the vehicle charging system to lithium trailer batteries and limit the power taken from the tow vehicle. It can take a lower DC voltage and turn it into a higher voltage, and have a multi-stage charge program. Not always needed for AGMs or flooded batteries, but definitely needed for lithiums. It can also be used to shut off the charging power draw from the tow vehicle without unplugging, in other words, isolate the trailer.
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Old 01-13-2022, 07:32 PM   #6
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I intercepted the 7-pin charge line in the closet and ran 8awg from that to the dc-dc charger. That way my electric jack would continue to work by running off the house battery. More details can be seen in my solar install video here: https://youtu.be/5lzwRLJOBI8
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Old 01-14-2022, 10:09 AM   #7
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I ran a separate line consisting of two 6 gauge wires from my pickup's battery to the charger which is located inside my Scamp 19. I used a Anderson plug to disconnect when the trailer is unhooked. I am using the Victron Orion-TR Smart 12/12V-30A Isolated Charger. Am running the Battleborn 200Ah 12V LiFePO4 Heated Battery Kit – 2 Batteries. My 7 pin battery plug was not disconnected.

The Battleborn battery's built in management system will handle any charging voltage issues.
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Old 01-14-2022, 11:15 AM   #8
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Originally Posted by parmm View Post
I ran a separate line consisting of two 6 gauge wires from my pickup's battery to the charger which is located inside my Scamp 19. I used a Anderson plug to disconnect when the trailer is unhooked. I am using the Victron Orion-TR Smart 12/12V-30A Isolated Charger. Am running the Battleborn 200Ah 12V LiFePO4 Heated Battery Kit – 2 Batteries. My 7 pin battery plug was not disconnected.

The Battleborn battery's built in management system will handle any charging voltage issues.
This is very similar to my setup. I also have a Victron Orion 30A, dc-dc charger, but it is not yet installed. I'm charging by simply plugging in my Anderson plug as needed. A set of #4 wires run from the truck battery, through an 80 amp breaker, and back to the rear bumper, where the Anderson plug is. And then on as #6 wires inside the trailer. I'm also running the power through the on-board shunt to read the amps being delivered.

I have 480 amp hours of lithium, (4) VPR 120 amp hour in parallel. They don't want voltage over 14.6 and I do not want to rely on the internal BMS to limit this, so I wait until the truck charging voltage drops below 14.6, and then plug in. This is a temporary hack, but it works until I install the Victron.

There is a small conflict with this kind of charging. If the solar (460 watt system) is charging when I plug in the truck, the solar charger and Go Power system can think the batteries are fully charged by mis-interpreting the apparent battery voltage. But other than that, charging with an Anderson style system from the truck is a wonderful way to get some serious amps to the batteries. This really helps in extended bad weather and, obviously, works at night. So I can top off the batteries simply by driving to the next location. I've been doing this for the past two years with two different trailers and two different trucks.

The Orion will go in as soon as we get back from this trip. I plan to control it by adjusting its threshold voltage to sense when the truck is running. Is that how you run yours? I see no reason to leave the plus wire from the seven pin connected as it will not provide the same voltage, or even very useful amps as compared to the Orion. Plus, if needed, it could be used as a signal wire for the Orion.
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Old 01-14-2022, 02:14 PM   #9
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Originally Posted by parmm View Post
I ran a separate line consisting of two 6 gauge wires from my pickup's battery to the charger which is located inside my Scamp 19. I used a Anderson plug to disconnect when the trailer is unhooked. I am using the Victron Orion-TR Smart 12/12V-30A Isolated Charger. Am running the Battleborn 200Ah 12V LiFePO4 Heated Battery Kit – 2 Batteries. My 7 pin battery plug was not disconnected.

The Battleborn battery's built in management system will handle any charging voltage issues.
Question, rather than run lets say 20ft #6 ground wire from your truck's battery, could one just run a ??ft ground from the input side of the dc charger back to the frame of the truck, since the battery grounds to the frame anyways?
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Old 01-14-2022, 06:59 PM   #10
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Question, rather than run lets say 20ft #6 ground wire from your truck's battery, could one just run a ??ft ground from the input side of the dc charger back to the frame of the truck, since the battery grounds to the frame anyways?
One could do that. It has been suggested. I don't think it is a good idea, but I've never done it. Before you do, compare the resistance of the frame to that of the wire, at let's say, 50 amps, and decide if it makes sense. A 20' piece of steel wire, riveted together, doesn't seem like a good conductor. Running two wires is not that much harder than running one. And the heavy amp draw of the starter, for instance, grounds through the block.
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Old 01-14-2022, 07:35 PM   #11
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There is a small conflict with this kind of charging. If the solar (460 watt system) is charging when I plug in the truck, the solar charger and Go Power system can think the batteries are fully charged by mis-interpreting the apparent battery voltage. .
I know my all Victron setup allows the dc-dc charger to charge from the tow vehicle and the solar controller to charge at the same time. The solar controller gets charge state from the BMV-712 and knows even though it sees the higher voltage, the batteries aren’t fully charged.
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Old 01-17-2022, 09:36 AM   #12
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I know my all Victron setup allows the dc-dc charger to charge from the tow vehicle and the solar controller to charge at the same time. The solar controller gets charge state from the BMV-712 and knows even though it sees the higher voltage, the batteries aren’t fully charged.
I have permanently mounted rooftop solar on the Casita, a 100ah AGM true deep cycle battery and typical battery usage, (fan, heater blower, water pump, LED lights etc…)

Question;
Is charging with 150watts rooftop solar while driving not enough to charge the battery while driving, say 8 hours a day (sans bad weather)? Is this setup sufficient to charge a LifePo battery?

If so then is a dc/dc system necessary?

Is the only advantage of a dc/dc connection to charge while driving in bad (severe cloudy, overcast or rainy) weather?

If switching to a LifePo battery would I “need” to add a dc/dc setup?
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Old 01-17-2022, 10:37 AM   #13
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I have permanently mounted rooftop solar on the Casita, a 100ah AGM true deep cycle battery and typical battery usage, (fan, heater blower, water pump, LED lights etc…)

Question;
Is charging with 150watts rooftop solar while driving not enough to charge the battery while driving, say 8 hours a day (sans bad weather)? Is this setup sufficient to charge a LifePo battery?

If so then is a dc/dc system necessary?

Is the only advantage of a dc/dc connection to charge while driving in bad (severe cloudy, overcast or rainy) weather?

If switching to a LifePo battery would I “need” to add a dc/dc setup?
If you switch to a lithium battery, you'll affectively have nearly twice as much capacity in the same physical size battery because you can discharge it down to about 5% if you have to, vs 50% for the AGM. The solar panels will not charge well in rainy weather. If you battery is low as evening sets in, and you are driving, the dc-dc charger will charge while driving.

So, it is better to have the dc-dc charger as well as the solar. In my case, the solar will not recharge the batteries completely in one day and if the weather is poor, the charge will trend down gradually over a few days to a week. I have 460 watts of solar and 480 amp hours of lithium. And I have a compressor fridge that is always on, plus winter furnace heat, etc. In bad weather the charge trends down and I do need to charge with the truck. In clear and warm weather, I never have to use anything more than solar. Each day that batteries reach 100% at about noon.

Also, if you stay with AGMs you don't need a dc-dc charger. Direct charging from the truck works just fine, but you will have to disconnect by unplugging the Anderson plug if you plan to park overnight. The dc-dc charger is needed with lithium to limit the voltage, provide a proper charging program, eliminate the float charge and to disconnect when the truck is not running.
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Old 01-17-2022, 11:12 AM   #14
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If you change the trailer power center to one that properly charges the Li battery, is there still any reason to have the DC-DC?
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Old 01-17-2022, 04:24 PM   #15
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If you change the trailer power center to one that properly charges the Li battery, is there still any reason to have the DC-DC?
The dc-dc charger is to interface the truck 12volt output to the trailer batteries directly. It is used when you are not plugged into shore power, but are towing, or charging while boondocking and with no solar.
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Old 01-17-2022, 05:36 PM   #16
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If you change the trailer power center to one that properly charges the Li battery, is there still any reason to have the DC-DC?
Depends on your tow vehicle's charging system. Some have newer tow vehicles have "Smart Charging". Just much safer to use a DC to DC battery charger, and your travel trailer batteries get a regulated charge.
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Old 01-17-2022, 06:28 PM   #17
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Thanks! I've never had a good understanding of electricity. For some reason I thought the charge from the truck 7 pin went through and was regulated by the power center before it got to the battery. Sounds as if I'll need the DC-DC if I switch to Li.
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Old 01-17-2022, 06:28 PM   #18
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Depends on your tow vehicle's charging system. Some have newer tow vehicles have "Smart Charging". Just much safer to use a DC to DC battery charger, and your travel trailer batteries get a regulated charge.

All vehicle charging systems are regulated. The smart vehicle charging systems act as a multi-step system. So, when the tow vehicle battery is fully charged the system goes into float mode. Upon watching my vehicle voltages, I see that it starts out at about 14.65 volts and holds that voltage for a few minutes to about 20 minutes. Then there is a period of apparent absorption charging at about 14.4-14.5 volts. And finally, a float charge of 13.? Fine for lead acid, but not correct or needed for lithiums.

Lithiums want no more than 14.6 and 14.4 is preferred. Then when fully charged, their float should be no higher than the rested voltage or be shut off.

If one is charging lead acid trailer batteries, the charging system is already designed and regulated to match their needed charge profile, so no dc-dc charger is needed. This is the way I charged mine for two years and with two different trucks. it was flawless. But one needs to have adequately sized wires to reduce line losses and one needs to remember to unplug if the trailer batteries will be heavily loaded.

With lithiums and their different voltage requirements, when compared to lead acid, there is a mismatch between the truck controller and the trailer batteries. This is where the dc-dc charger comes into play. It can take the available current from the truck and tailor it to match the lithium charging needs.

Victron has made a video of a charging system being damaged by "drawing too much power from and alternator". I call this false advertising. That "test" was performed under false conditions that are never seen in automotive charging systems and therefore fake.

The dc-dc charger also acts as a disconnect to eliminate running the trailer loads from the truck battery. In my case with the AGMs I simply unplugged the Anderson plug if we were staying overnight or longer.

As an experiment, I've been charging my lithiums, (4) 120 amp hour batteries, with my Anderson system with no dc-dc charger. This is OK only under certain conditions. I wait until the initial charging voltage is below 14.6 before I plug the trailer in. I unplug when we stop because the rested lithium voltage is higher than lead acid rested voltage. This could mean that the lithiums begin seeing the truck batteries as a load. I have now had a chance to see the amperage going to the lithiums and the response from the smart charging system when I plug the trailer in. I plan to install the dc-dc charger (30 amp Victron Orion) this week and stop running the system manually, but in the mean time, the charging system has been doing fine and bringing the lithiums up faster than the solar, or at night when there is no solar. The Victron will use the truck voltage to signal when to turn on or off. The installation could not be simpler in this case because all of the wiring is already in place. The dc-dc charger will simply go in the charging circuit, in the trailer, by cutting two wires and re-connecting them on the charger.
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Old 01-19-2022, 03:38 PM   #19
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I have been avoiding this thread because I didn't want to get into a "heated" discussion on how charging systems work.

The factory trailer plug on your TV is pretty much useless for charging your house battery. Your alternator looks at the TV battery for instructions and when it feels that battery is charged it drops down to 13. something to maintain.

Meanwhile you have a voltage drop between .5 to 1.0 volts at the house battery due to wire length and gauge. That reduces the charging voltage at the house battery to levels that won't do much. If you run a separate heavy gauge wire from battery to battery you will still encounter the same problems. It will be better but still not right.

A B-to-B charger with a heavy connecting wire "around 4 ga" takes the low voltage and turns it into whatever is appropriate for your house battery. If you want to charge your house battery while driving, a battery-to-battery charger is the only way to go.
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Old 01-19-2022, 03:57 PM   #20
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I have read that selecting Tow/Haul with Ford trucks (maybe others) will keep the voltage from dropping off, at least to some extent. Don't know if this is true and can't remember where I read it. This doesn't help the small wire situation, of course.
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