DC-DC chargers in series?

[Country Campers]

I was thinking about adding another DC-DC charger after the Renogy 20 amp that I have installed. This would , in theory, take the amperage out of the 20 amp and into a 40 or 60 amp unit and then to the battery for charging. The reason I am thinking this is that I am using the stock wiring in my 2015 Chevy tow vehicle and instead of running heavier gage wire from front to back and then into the front storage of the RV to power a larger DC-DC charger I could still use the stock wiring to the 20 amp charger and then out of that to a 40 or 20 amp charger.

I have done a bit of searching to see if this is doable and could not find anything on the web where someone has done this, might not be using the correct wording.

Do the members think that this is possible?

I realize it may be cheaper to run new wire but I only have one alternator and not every one can run wires from the front to the back of their tow vehicle.

Brian

[ncitro]

I would not think this would work. The voltage level out of the current 20A DC to DC charger and the additional 40-60A charger would be the same, so the second charger would have nothing to step up from. There is no free lunch, or put another way you can't make something from nothing. I think to get that kind of power you'd need to run new wire to the truck battery.

[Country Campers]

But the first charger steps up amperage from the truck from 7 to 20 amps, wouldn't the second one step up amperage from the first?

Brian

[mallstott]

But the first charger steps up amperage from the truck from 7 to 20 amps, wouldn't the second one step up amperage from the first?

Brian

Are you saying that you get 12v 7 amps out of the 7 pin wire and step that up to 12v 20 amps out of the DC2DC charger? How does it do that?

-Mike-

[ncitro]

Country Campers So what the first does, is take whatever voltage the truck puts out, and steps it up to whatever you have it set for with the DIP switches. It does this for one of the following:

1. The current the charger is rated to (20A in your case)
2. The current its rated to divided in half (if the half load switch is flipped)

The DC to DC does not know anything about the truck wiring, or the capabilities of the trucks alternator. If the truck was wired with 18 gauge wire, it would happily try to draw 20A, and either blow a fuse in the truck, or melt a wire somewhere. It can't "make" current, it draws that from the truck. In fact its actually drawing more than 20A, since it needs to step up the truck output which uses some current as well. I have seen my 40A version trip the old 60A breaker I used to use before I increased its size.

Running it via the trucks 7 way was well tested by Guest as far as current draw and the trucks ratings so I would defer to him on the specifics there, but I am comfortable saying it probably will not support much more than the 20A charger. I am almost sure the wire in my truck is 10AWG, which would be rated to a max of 30A, I am not sure what it is fused to though.

[Jkwilson]

But the first charger steps up amperage from the truck from 7 to 20 amps, wouldn't the second one step up amperage from the first?

Brian

A DC-DC convertor can output no more power than goes into it. If fed 12.6V at 15A, it can put out no more than 189W regardless of the capacity of the converter. If the converter steps up to 14.6V, the output current will not exceed 12.94A. If output voltage goes up, output current goes down.

The reason a DC-DC converter increases charge current in this application is that the small wiring of the truck circuit results in significant voltage drop from the battery to the 7-pin. This voltage drop increases as current increases which results in a limiting effect on charging. The current into the battery is determined not by the capacity of the converter, but by how much the 7-pin voltage exceeds the trailer battery voltage. The DC-DC converter increases the voltage at the 7-pin which increases current flow both into and out of the DC-DC converter.

[Scott'n'Wendy]

But the first charger steps up amperage from the truck from 7 to 20 amps, wouldn't the second one step up amperage from the first?

Brian

First of all...don't try to cascade chargers..but for discussion....

It increases amperage by stepping up the voltage. With the higher voltage, electrical pressure so to speak, more current will flow to the trailer battery, but as Neil said, there is no free lunch. It is drawing that 20a from the alt. So with stock wiring, you are already at max charging current. Stock wiring on the 7pin charge wire in most trucks is 12ga. Some 14ga.
What happens Cascading dc/dc chargers? My guess? You would likely have the 20a unit 'choke' the 60a unit. The 20a unit would have internal circuitry designed to handle 20a and the 60a unit would be trying to draw 60a through it. likely frying the 20a unit.

[Country Campers]

Well thanks all for bursting my bubble.

It is great to have such a well knowledge group to lean on and save a person from doing something stupid

Brian

[howson]

Country Campers -- good technical discussion. Glad you asked the question!

[stvaughn]

A DC-DC converter is designed to be hooked up to the battery and take the excess power generated by the alternator and convert it to a usable output to charge the house batteries. The wiring has to be sized to accommodate the amps utilized.

[BLoflin]

I tried to have a similar discussion a couple of months ago, when I first joined. I probably went into too much detail, and caused confusion/arguing.

To net you CAN step up voltage with a DC to DC convertor, you can NOT step up current (basically with any device used in RV power systems).

You CAN get your "resistive" device (i.e. trying to charge the RV battery) to accept more current by raising the voltage (with the DC to DC converter), but again, you can not create more current than available from the original source.

V=IR always or I=V/R in this case meaning you can drive more current into your resistive device if you raise the voltage (this is how battery charging works), but for the full circuit view you have to remember the wiring from the Alternator output to the RV is also a "resistive" device.

[TedS]

Power out = power in v×i(out) = v×i(in) or i^2×r(out) = i^2×r(in)

Power is conserved.

[Scott'n'Wendy]

Power is conserved.

Reminded me of the law of energy conservation....
energy can neither be created nor destroyed - only converted from one form of energy to another

[TedS]

Been trying to read about and understand dc-dc chargers. They primarily take variable input voltage and output a controlled voltage appropriate to the battery to be charged. So putting these in series would be of no use; the final output voltage will be the same with no boost in current. Current will depend on the battery state of charge and the charger output voltage. If I understand correctly, they can be used in parallel. The vehicle alternator has to be capable of supplying the charger/battery current demand.

[Country Campers]

Been trying to read about and understand dc-dc chargers. They primarily take variable input voltage and output a controlled voltage appropriate to the battery to be charged. So putting these in series would be of no use; the final output voltage will be the same with no boost in current. Current will depend on the battery state of charge and the charger output voltage. If I understand correctly, they can be used in parallel. The vehicle alternator has to be capable of supplying the charger/battery current demand.

This is what I have learned as well, a lot of reading and questions.

My original thought was "boost" but it does not do that. It does increase the "draw" from the alternator or charging system in the tow vehicle instead of just allowing the tow vehicle to send what ever it wants to the trailer.

This has been a good discussion and learning for me, thanks all.

Brian