DC-DC Charge install Reflection 29RS

[Scott'n'Wendy]

Again because you can NOT create power (without a power source).

The alternator is the power source. The alternator varies it's output depending on the demand placed on it. Within it's limits. If you change the demand via the dc-dc charger, it will up the output.

[BLoflin]

I guess that "boost" was the wrong word to use. My understanding is that the charger will "draw" more amperage from the tow vehicle instead of just the regular 5 to 7 amps. This has been a good discussion though on how and why.

Brian

Yes, correct. That in fact is the concern that needs to be understood, if you are trying to apply say 240W to charge your battery (e.g. something like 15V at 16A) you have to be careful as if your truck is supplying say 12V at the trailer connector then your alternator and the wiring to the connector has to be able to supply and transport 20A and the truck wiring to the connector needs to be sized accordingly.

This is in fact what concerned/confused me about the OP. As it appears that the solution to only being able to "trickle" charge anything out of the vehicle 12V trailer connector (that was limited in current due to vehicle wiring, fusing, alternator output and demand, etc.) was to use the Renology part to get good battery charging currents by being "boosted". In fact it is the OPPOSITE. The battery charging voltage will be boosted at a corresponding lowering of the charging current (or else you may be over maxing the current draw from the vehicle).

If you have a vehicle alternator that is sized big enough to supply an additional 250W to your trailer (after accounting for all it's demand from the vehicle itself) then you need to possibly add a bigger wire from the alternator to the connector (or at least from the fuse box to the connector) and possibly upsize the fuse in the fuse box. But possibly you will be ok. See if you can find in your Vehicle Owners manual to see if that Trailer Connector 12V has a specific fuse and see what the size is. If it is 5 or 10A fuse, that will be a problem. You could change the fuse, but that would imply they built the vehicle with a small wire. If that fuse is 30A then you should be good to go, as the wiring should be sized accordingly.

[BLoflin]

I think you are missing the big reason for the low current on the charge circuit. Battery charging current is proportional to the charging voltage. The long run from the alternator to the trailer battery through a small wire results in significant voltage drop making charge current low as well. By boosting the voltage at the trailer battery, charge current is increased, but still limited by the capacity of the DC-DC converter and the vehicle charge circuit.

Available power isn’t the limiting factor on stock setups. It’s voltage.

Hmmm, but that isn't how electricity works at all. The equation is V=I*R. Also Power has to be conserved. Power In = Power Out - losses (thermal, etc). For Power Out to be more than Power In then Power needs to be created (via a power source, like a fuel or chemical reaction, or in case of the vehicle alternator, fuel into an engine and an engine turns the Alternator).

For example lets say the Alternator voltage is 14V (just to keep it simple, no decimals) at the Alternator. Lets say when you pull 6 Amps out at the trailer connection the Voltage drops to 12V. That means the wire resistance to the plug is 0.333 Ohms (14 - 12V / 6A). You drop 2V due to resistance along the wire. If you try and pull 12A then the voltage at the trailer connection drops to 10V.

Also the Power is consistent: At Alternator is 14V x 6A = 84Watts. At Connector is 12V x 6A = 72W + Power loss in the wire resistance is 2V x 6A = 12W which totals 84W.

Instead if you are trying to pull 12A then power at Alternator is 168W and power at connector is 120W and the power loss in the wire is 48W.

Now for simplicity lets assume the DC-DC Converter (Renology part) is sitting right at the Trailer Connector (i.e. no additional wire with additional resistance).

The Convertor will take that 10V with 12A input (i.e. 120W) and step it up (convert it) say to 15V to be able to start a Bulk Charge cycle for the battery (again using rounded numbers for math simplicity). Then the available current DROPS to 8A (actually a little less than 8 as there are some losses in the Renology circuits), 8A x 15V = 120W. Again because you can NOT create power (without a power source).

The OP spoke of "boosting" the power. You can not. You can boost the Voltage in a Converter, but with the corresponding LOWERING of the current. Or conversely you can boost the Amps by lowering the Voltage ouput, but of course that would not be helpful for charging batteries.

Also, again the 20A rating on the Renology is NOT the guaranteed current output, it is the highest current (and also highest power rating is nominal 250W based on the 20A and 12V). They also have a 40A (500W) and a 60A (750W). None of these can create power or boost current if you are not delivering the corresponding input current and voltage.

[Country Campers]

I guess that "boost" was the wrong word to use. My understanding is that the charger will "draw" more amperage from the tow vehicle instead of just the regular 5 to 7 amps. This has been a good discussion though on how and why.

Brian

[howson]

BLoflin -- technically you're right. The Renogy cannot (and does not) "boost power".

The rest of this post may make the eyes of the reader roll back in their head--so why read it and try to comprehend the math? Because it is important to understand why anyone doing this mod must ensure the 7-pin connection and associated wiring in their vehicle and trailer can support the higher amperage when using the Renogy.

Power is measured in watts (the product of voltage and amperage). So...power out must equal power in (assuming no loss in the Renogy itself). So watts in = watts out. What can change (and does change) is the volts and amps.

Since power out = power in, and we'll assume the truck's output voltage (the input to the Renogy) is constant, the amperage from the truck through the 7-pin must increase. How does the amperage increase from the truck? The apparent resistance of the circuit drops. (Voltage constant but amperage increase = resistance decreases.)

Example below is not representative of actual numbers (but they are close):

Stock Configuration
Truck Voltage = 12.9vDC
Measured amperage = 9 amps
Wattage to battery (12.9 * 9) = 116 watts
Apparent resistance of the circuit (12.9 / 9 ) = 1.43 ohms

20A Renogy
Truck Voltage = 12.9vDC
Truck Amps = ??? (will be calculated)
Renogy Output Voltage = 13.6
Renogy Output Amps = 20
Wattage to Battery ( 13.6 * 20 ) = 272 watts

Note in the next equation the Renogy's output wattage is used to calculate the truck's amps because power in must equal power out (again, assuming no losses).

Truck amps ( 272 / 12.9 ) = 21 amps
Apparent resistance felt by truck (12.9 / 21) = .61 ohms

To the best of my knowledge, the forum members that have done the 20A Renogy mod using their vehicle's OEM wiring confirmed the wiring (and fuse) are capable of carrying the increased amperage output.

I hope this helps anyone reading this thread understand...

Howard

[Jkwilson]

I'm new to the RV scene (first TT coming in at the the month), so I'm trying to learn everything.

That said I'm confused by this post.

The Renology (or any device) can NOT boost amperage unless voltage is changed in the process (i.e. going from 24V 5A then a DC to DC converter could go to 12V 10A). But the power remains the same.

To create additional current at the same voltage, the device would have to be a power creation device, and would need a power conversion "engine" (i.e. using a fuel, or a chemical reaction process).

This Renology device will take the Voltage supplied at the 7 pin and transform it to the required 3 voltages required during the charge phase of your specific type of batteries. The stages are Bulk, Boost and Float, and are different for each type of battery (i.e. Lithium vs AGM, etc.).

But it does not take the max (as stated in the OP) current of 5 or 6 amps coming from the vehicle power thru the 7 pin connector and boost it to 20 amps.

The 20 amp rating on the Renology device is specifying the max current this device can handle (not create).

To say another way:

Taking the 12V (nominal) pin from the trailer connector can power 12v devices (up to a power limit). If this was tied directly to your trailer battery you might get some charging, but would not be very effective.

The Renology device takes the 12V and changes it to the different required voltages to charge a battery. For example on a Lithium it might be Bulk Voltage will be 14.6V until about 80% charges, then drop to Boost (adsorption) of 14.4V then finally when at 99% charge drop to Float of 13.6V

But it cannot "boost" the power/amperage.

I think you are missing the big reason for the low current on the charge circuit. Battery charging current is proportional to the charging voltage. The long run from the alternator to the trailer battery through a small wire results in significant voltage drop making charge current low as well. By boosting the voltage at the trailer battery, charge current is increased, but still limited by the capacity of the DC-DC converter and the vehicle charge circuit.

Available power isn’t the limiting factor on stock setups. It’s voltage.

[BLoflin]

I'm new to the RV scene (first TT coming in at the the month), so I'm trying to learn everything.

That said I'm confused by this post.

The Renology (or any device) can NOT boost amperage unless voltage is changed in the process (i.e. going from 24V 5A then a DC to DC converter could go to 12V 10A). But the power remains the same.

To create additional current at the same voltage, the device would have to be a power creation device, and would need a power conversion "engine" (i.e. using a fuel, or a chemical reaction process).

This Renology device will take the Voltage supplied at the 7 pin and transform it to the required 3 voltages required during the charge phase of your specific type of batteries. The stages are Bulk, Boost and Float, and are different for each type of battery (i.e. Lithium vs AGM, etc.).

But it does not take the max (as stated in the OP) current of 5 or 6 amps coming from the vehicle power thru the 7 pin connector and boost it to 20 amps.

The 20 amp rating on the Renology device is specifying the max current this device can handle (not create).

To say another way:

Taking the 12V (nominal) pin from the trailer connector can power 12v devices (up to a power limit). If this was tied directly to your trailer battery you might get some charging, but would not be very effective.

The Renology device takes the 12V and changes it to the different required voltages to charge a battery. For example on a Lithium it might be Bulk Voltage will be 14.6V until about 80% charges, then drop to Boost (adsorption) of 14.4V then finally when at 99% charge drop to Float of 13.6V

But it cannot "boost" the power/amperage.

[Country Campers]

Nice. I've been really happy with mine (the 60A version). Now even wondering if I should put in solar... but I don't think I can help myself.

Just say NO! It is hard to resist solar.

Brian

[TravelHabit]

Nice. I've been really happy with mine (the 60A version). Now even wondering if I should put in solar... but I don't think I can help myself.

[Country Campers]

I recently installed a Renogy 20 amp DC-DC charger, along with Solar, so here is the install.

The Renogy DC-DC charge takes battery voltage from the factory charge circuit of the tow vehicle and increases the power to what would be a max of 20 amps. Most tow vehicle charge circuits only have about 5 or 6 amps output and there are some videos and posts about the possible damage from trying to charge Lithium batteries with the wet cell batteries found in tow vehicles, or vice versa, so I decided to do the install to not only have added charge amperage to the Lithium batteries but also not damage the tow vehicle battery ( I am not totally sure that this is needed but it is what I wanted to do).

Here is the final install picture.



The input is on the lower right of the charger, out put on the lower left. The yellow wire from the switch , upper left, is the trigger wire required to turn on/off the charger. I wired this from the marker lights of the RV, from the junction box located under the pin box on a fifth wheel, to the switch and then to the charger. I did this because I always tow with the trailer lights on and if I do not want the charger on I can turn it off. Using the trailer lights as the trigger came from Guest Jim and information from his install, I just added a switch.

The input wires, lower right, are from the factory wiring that was located at the self resetting breakers located under the black plastic cover, below the fuse block that I had installed in the picture. There is a white wire with red stripe that I found was the factory charge wire, which I traced to the junction box along with the white ground wire. The junction box is where the 7 RV pigtail connects to the RV. The ouput side just goes to the battery positve, the negative goes to the shunt from the BMV-712. If you have a shunt in your system it is paramount that all negative/grounds go thru the shunt in order to get proper readings of usage and charge. It is also important to trace the wires to make sure that the correct wire is used. I have not tested this system yet as it has been winter but I feel confident that it will work. Jim has used this same system for a while and has noticed that the 7 RV plug does produce a little heat, gets warm to the touch from use, so this would probably be the max that one would want to use when using the stock wiring, I will keep an eye on this and this is another reason I added a switch. There is also a trigger input on the charger to allow the user to cut the amperage in half, that would be 10 amps so would decrease the draw on the tow vehicle wiring which I can add if needed. If it comes to cutting the amperage in half I will run a dedicated set of wires that will handle the full load.

Brian