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Are DC to DC chargers really necessary.

DIDDLYV

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I am installing 2 310 AMP nominal 12 volt batteries in parallel to my 5th wheel. I am also installing 840 watts of solar panels hooked 2s then each pair running to its own EPEVER 40 Amp SCC. Given my 5th wheel is parked and charging a lot more than it is running down the batteries I am guessing the batteries will always be in a pretty decent state of charge. My Dodge Ram 3500 dually has a 30 amp fuse protecting the DC 12 volt connection to the trailer. I have isolated that hot line from the truck at the DC positive bus bar in the 5th wheel. I am tempted to install a 25 amp inline fuse in that line to introduce a point of failure at a lower amperage than will pop the truck 30 amp fuse.
If I start blowing fuses it will be readily apparent my initial assumption about the batteries being a a fairly high state of charge when the truck is hooked up will be in correct and I really do need a 20 - 25 amp max draw DC charge DC to DC charge.

What to those who know about this stuff think? Am I all screwed up in my thinking?
 
Assuming you mean you have 2 12V 310Ah batteries in parallel then that is a total of 2 x 12.8V x 310Ah = 7936Wh. With 840W of solar mounted flat on the roof (I'm assuming they are flat) then you might get at most 700W on a good day. With 5 hours a sun on average you might get about 3500Wh out of the panels in a day. So your panels, on a good sunny day, can only charge about 40% or so of your battery capacity each day.

If you need to use more than 40% of your battery each day or to cover less sunny days then you need another way to charge.
 
Assuming you are using the OEM connection through the 7-pin connector I don't think you will have any overcurrent issues. The long and relatively thin wire will self regulate the voltage to limit the amps to the battery. I thought my new LFP battery was a bit low recently and I was lucky to get 10 amps from the truck to the fifth wheel. Even if by some miracle the fuse opens... there is plenty of solar to keep power flowing into the RV battery until you can locate a new 30 amp fuse. Unless you are in poor solar conditions for an extended period, the battery is not going to get that low.
 
Assuming you are using the OEM connection through the 7-pin connector I don't think you will have any overcurrent issues. The long and relatively thin wire will self regulate the voltage to limit the amps to the battery. I thought my new LFP battery was a bit low recently and I was lucky to get 10 amps from the truck to the fifth wheel. Even if by some miracle the fuse opens... there is plenty of solar to keep power flowing into the RV battery until you can locate a new 30 amp fuse. Unless you are in poor solar conditions for an extended period, the battery is not going to get that low.
I just bought 12 of the 30 amp fuses. I to replace the one I blew somehow with no battery at all in the system. Who knows the battery positive and negative house battery cables may have touched or the positive cable touched the frame but i replace the fuse with one of the 2 I bought at auto zone and bought 10 more on amazon for about what I paid for 2 at auto zone.
Assuming you mean you have 2 12V 310Ah batteries in parallel then that is a total of 2 x 12.8V x 310Ah = 7936Wh. With 840W of solar mounted flat on the roof (I'm assuming they are flat) then you might get at most 700W on a good day. With 5 hours a sun on average you might get about 3500Wh out of the panels in a day. So your panels, on a good sunny day, can only charge about 40% or so of your battery capacity each day.

If you need to use more than 40% of your battery each day or to cover less sunny days then you need another way to charge.
Assuming you mean you have 2 12V 310Ah batteries in parallel then that is a total of 2 x 12.8V x 310Ah = 7936Wh. With 840W of solar mounted flat on the roof (I'm assuming they are flat) then you might get at most 700W on a good day. With 5 hours a sun on average you might get about 3500Wh out of the panels in a day. So your panels, on a good sunny day, can only charge about 40% or so of your battery capacity each day.

If you need to use more than 40% of your battery each day or to cover less sunny days then you need another way to charge.
I am mounting the panels with angle mounts, and really will probably only angle them if I am going to be in an area for an extended period of time. My fifth wheel does not have a factory ladder and lean up against it ladders are not real fun after 70 so you are probably right about the flat all the time. Most of my travel is in the summer time and I live in AZ so 5 hrs of sun may be a bit conservative. The big power users will probably be making 1 pot of coffee in the morning perhaps a few min of microwave and powering my CPAP machine at night. That draws 8 amps of 12 volts approx of 1.5 amps of 120. Given the power consumption of the inverter I would probably be better off running the CPAP of 12 volts.

I do have a 2000 watt generator and a 3700 watt generator that is still in the box for make up power. Just prefer not to drag them out all the time.
 
The best usecases for DC to DC chargers is in vans that get driven regularly during the winter when solar is poor. E.g. someone in a ski area

If there is plenty of sunshine you are much better off investing the same amount of money in solar panels. Much better bang for buck
 
The best usecases for DC to DC chargers is in vans that get driven regularly during the winter when solar is poor. E.g. someone in a ski area

If there is plenty of sunshine you are much better off investing the same amount of money in solar panels. Much better bang for buck
It seems like DC-DC charging is the hot ticket now. Kind of like a badge of honor. We got by with 170 watts on the roof to go with our 240 usable ah's of SiO2 batteries. OTOH, we didn't want "We got by" and installed another 300 watts on the roof. Now we can fully charge in inclement weather.

Over the years we have learned how to enjoy camping with 12v. We boil water for coffee, cook breads/other foods in our Weber grill, charge our two Chromebooks and iPad with 12v, and we don't watch TV but an occasional movie on our Chromebooks. I did just install a 2500 watt continuous sine wave inverter to run our 700 w toaster (we like toast and bagels). While we don't need all the amenities from home, at 72 want some comforts. We're all different.

With the experience of 15 previous campers over the years we've learned a lot. While the idea of a DC-DC charger looked attractive (I once ran 8 awg wire from our Odyssey back to our campers) we didn't want to install a wire from the alternator to our camper, much less manage another device. In addition, many times we stay for 8 or more days without moving the camper and don't want to run our truck any more than we want to hear a generator in the next site. We much prefer the solar panels that were easy to install and just plain work all the time for us.

Solar first.

Enjoy,

Perry
 
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We initially installed a DC to DC charger because we suspected our PV system wouldn't sufficiently replenish the house batteries during the winter. Redundancy was also a factor. Once we got everything hooked up however, we discovered that the PV system is more than enough to handle our current usage.

Thankfully, we are using a DC to DC charger with a built in MPPT so it's not like the effort to install was a waste. I suspect the DC to DC charger will see more use when the weather gets gloomy but as of now, we leave the circuit breaker from the alternator turned off.
 
I installed the Renog 40 amp DC to DC charger in my ram pickup truck. It is a fantastic piece of equipment as long as you were driving the vehicle. Obviously if you drive to a camping area and plan on spending a week or two the DC to DC charger is not going to do much. But on the other hand in my truck which only has 220 amp hours of battery I can run the truck on idle for about an hour and charge the batteries from 60% back up to about 90%. Because most new vehicles have Smart alternators which vary the voltage based on what the trucks battery is looking for a DC to DC charger is almost mandatory if you want to charge from the tow vehicle.
 
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