Dc to DC Converter Between TV and AS

[cru-in]

Hi Team,

With my LI battery upgrade, I installed a Renorgy 20 Amp Dc to DC Power Supply (nice unit, about $120) between the TV and AS. It went in the AS, near the battery bank. It is multistage and LI Compatible. It works with voltages from the TV of 8 to 16 volts. Providing appropriate voltage for multistage charging of the LI Batteries. It is necessary to keep the higher voltage LI batteries from bleeding charge to lower voltage TV lead acid batteries.

Overall, I am happy with the performance. Via the Bluetooth Battery Monitor, I have seen it charging the AS batteries with 10+ Amps.

Even with lead Acid batteries, it might be a good thing to install. Helps overcome any voltage drop in the TV wiring and provides the appropriate voltage to the battery.

[Bill M.]

I have certainly been thinking about adding one of these. My truck puts no charge on my lead acid batteries when towing. It would sometimes be nice to capture a little charge when moving. Thanks for posting,
Do your instructions say wha the input amperage ought to be or what size wire the TV should have to the unit? Did you hook it to the hot wire in the 7 pin or run another wire with connectors?

[cru-in]

Originally Posted by Bill M.

I have certainly been thinking about adding one of these. My truck puts no charge on my lead acid batteries when towing. It would sometimes be nice to capture a little charge when moving. Thanks for posting,
Do your instructions say wha the input amperage ought to be or what size wire the TV should have to the unit? Did you hook it to the hot wire in the 7 pin or run another wire with connectors?

"Do your instructions say wha the input amperage ought to be or what size wire the TV should have to the unit? "

The DC to DC Power Supply will work so long as the input voltage is 8 to 16 volts on the input.

"Did you hook it to the hot wire in the 7 pin or run another wire with connectors?"

Pin 7 was already run to the bus bar that my battery bank hooks to. All I needed to do is disconnect it from there and run it to the input of the DC to DC Power Supply then run the output of the DC to DC back to the bus bar.

[waninae39]

i use a victron dc dc isolator between the TV an AS

it works very well

[nvestysly]

I like the idea of a DC to DC multi-stage charger but haven't taken the plunge yet. In the interim we installed a brute force charging system that simply utilizes the alternator of the tow vehicle to provide charging for the trailer batteries.

The 20A unit you installed seems a bit small to me. I have no basis for wanting more output except to say that our converter/charger in the trailer is rated at 50A so I'd like to use a DC to DC unit with a similar rating.

I noticed Renogy makes a DCC50S (50 A output) that looks like it has all the bells & whistles I might want. Allows me to upgrade my brute force charging methodology with a multi-stage unit, 50A output, a variety of battery types and the ability to add solar later.

Hmmmm…. interesting.

[rowiebowie]

Originally Posted by nvestysly

I like the idea of a DC to DC multi-stage charger but haven't taken the plunge yet. In the interim we installed a brute force charging system that simply utilizes the alternator of the tow vehicle to provide charging for the trailer batteries.

The 20A unit you installed seems a bit small to me. I have no basis for wanting more output except to say that our converter/charger in the trailer is rated at 50A so I'd like to use a DC to DC unit with a similar rating.

I noticed Renogy makes a DCC50S (50 A output) that looks like it has all the bells & whistles I might want. Allows me to upgrade my brute force charging methodology with a multi-stage unit, 50A output, a variety of battery types and the ability to add solar later.

Hmmmm…. interesting.

I installed the Renogy 40A DC to DC charger on my class b during my recent 200ah lithium upgrade. Stock cables from the engine alternator are 4ga. It occasionally taxes the 145a alternator output during initial idling at startup. Seems to do fine after warmed up.

Using tow vehicle wiring (which I assume is much smaller gauge) might benefit from the OP's smaller 20A unit.

[nvestysly]

Originally Posted by rowiebowie

I installed the Renogy 40A DC to DC charger on my class b during my recent 200ah lithium upgrade. Stock cables from the engine alternator are 4ga. It occasionally taxes the 145a alternator output during initial idling at startup. Seems to do fine after warmed up.

Using tow vehicle wiring (which I assume is much smaller gauge) might benefit from the OP's smaller 20A unit.

The OP's 20A and your 40A devices may do what I want them to do regarding DC to DC from the tow vehicle to the trailer but I like the MPPT function included on the DCC50S. The MPPT would allow us to add solar to the trailer at a later date if we decide to pursue that.

Looking at the 20A and 40A DC to DC units it's pretty clear they have outputs of 20A and 40A although I do see something in the specs from Renogy that indicates a 50% limit switch. Not sure if that limit can be overridden to get the full 20A and 40A output.

On the 50A Renogy the output seems to be 50A but it may be 25 from DC (alternator) and 25 from MPPT (solar). I'll have to call Renogy to discuss the details.

Regarding the heavier wiring... I've installed a brute force trailer charging system that utilizes 2 gage wire from the tow vehicle to the trailer. I don't want to hijack this thread but details of that 2 gage installation can be found HERE.

[cru-in]

Originally Posted by rowiebowie

I installed the Renogy 40A DC to DC charger on my class b during my recent 200ah lithium upgrade. Stock cables from the engine alternator are 4ga. It occasionally taxes the 145a alternator output during initial idling at startup. Seems to do fine after warmed up.

Using tow vehicle wiring (which I assume is much smaller gauge) might benefit from the OP's smaller 20A unit.

Consideration of stock TV wiring is exactly why I went with the 20A. Would have liked the 40A but went conservative.

[uncle_bob]

Hi

The 7 pin outlet on the tow vehicle is likely fused at 30A. At the 80% level (= the max to design for), you would be pulling 24A over the circuit.

If your DC-DC is taking in 11V and putting out 14V, your input current will be about 27% higher than the output. Toss in a 90% efficiency and it will be higher still. All of that gets you to a converter that puts out something in the < 17A range.

With a 60' loop of #10 wire (hot + return), you have a resistance of 0.06 ohms. At 17A that would give you a 1V drop. A loop half that size puts you at a half volt. If it is number 10 the whole way, 11V is a very real possibility.

Bob

[waninae39]

we have use a Victron orion device for the past few years
https://www.victronenergy.com/dc-dc-converters

it is a good protection device for both the AS and the TV

[uncle_bob]

Originally Posted by waninae39

we have use a Victron orion device for the past few years
https://www.victronenergy.com/dc-dc-converters

it is a good protection device for both the AS and the TV

Hi

It also turns on by sensing the input voltage rather than needing a signal from the ignition circuit ....

Bob

[cru-in]

Originally Posted by uncle_bob

Hi

It also turns on by sensing the input voltage rather than needing a signal from the ignition circuit ....

Bob

The Renogy does not need a signal from the ignition circuit either. It does have a separate sense input that can be used for several purposes:
1) Start the unit based on an input from the ignition circuit. (as you mention)
2) Start the unit upon sensing voltage at the input to the DC-to-DC Supply (simply wire it to the input)
3) As an control (switch, timer, whatever) for the operation of the DC To DC power supply.

Mine is wired to the input and starts upon sensing voltage at the input to the DC to DC charger.

Having it operate with an input from the ignition circuit would be useful if you TV provides 12 volts even when off. It would prevent the TV Battery from being drained.

In time, I will probably wire mine to operate when I turn on the trailer driving lights. Anytime I tow, the driving lights are on as they power the rear view camera. The trailer driving lights are already wired to the trailer. The wiring passes near the DC to DC power supply. So it would be easy to do. This would shut the Power Supply off when I stop (and turn off the driving lights). Preventing the battery drain from my dodge truck. I miss the relay on our ford.

I see the sense input as one of the advantages of the renogy unit. It allows lots of flexibility.

[cru-in]

Originally Posted by uncle_bob

Hi

The 7 pin outlet on the tow vehicle is likely fused at 30A. At the 80% level (= the max to design for), you would be pulling 24A over the circuit.

If your DC-DC is taking in 11V and putting out 14V, your input current will be about 27% higher than the output. Toss in a 90% efficiency and it will be higher still. All of that gets you to a converter that puts out something in the < 17A range.

With a 60' loop of #10 wire (hot + return), you have a resistance of 0.06 ohms. At 17A that would give you a 1V drop. A loop half that size puts you at a half volt. If it is number 10 the whole way, 11V is a very real possibility.

Bob

Hi Bob,

Nice quick analysis. Thanks.

While I placed my DC to DC in to isolate the LiFePo from the Lead Acid, I wish I had done it years ago to simply charge the trailer's good ole Lead Acids.

At $125, and a pretty easy install, IMHO it is a very worthwhile upgrade.

[uncle_bob]

Originally Posted by cru-in

The Renogy does not need a signal from the ignition circuit either. It does have a separate sense input that can be used for several purposes:
1) Start the unit based on an input from the ignition circuit. (as you mention)
2) Start the unit upon sensing voltage at the input to the DC-to-DC Supply (simply wire it to the input)
3) As an control (switch, timer, whatever) for the operation of the DC To DC power supply.

Mine is wired to the input and starts upon sensing voltage at the input to the DC to DC charger.

Having it operate with an input from the ignition circuit would be useful if you TV provides 12 volts even when off. It would prevent the TV Battery from being drained.

In time, I will probably wire mine to operate when I turn on the trailer driving lights. Anytime I tow, the driving lights are on as they power the rear view camera. The trailer driving lights are already wired to the trailer. The wiring passes near the DC to DC power supply. So it would be easy to do. This would shut the Power Supply off when I stop (and turn off the driving lights). Preventing the battery drain from my dodge truck. I miss the relay on our ford.

I see the sense input as one of the advantages of the renogy unit. It allows lots of flexibility.

Hi

You might want to measure the parasitic load from the unit when it's run that way .....

Bob

[qwertyd10n]

Originally Posted by cru-in

Hi Bob,

Nice quick analysis. Thanks.

While I placed my DC to DC in to isolate the LiFePo from the Lead Acid, I wish I had done it years ago to simply charge the trailer's good ole Lead Acids.

At $125, and a pretty easy install, IMHO it is a very worthwhile upgrade.

cru-in,

How did you wire the negative (-) wires on your Renogy 20A DC-DC Battery Charger? The positive (+) side seems pretty straight forward - take the charge wire output from your 7 pin where it connects to your DC Panel bus bar and use that as the input to the charger and then run from the charger output to that vacated position on the bus bar. However the negative leads are a little less obvious to me... did you tap into the ground wire on the 7 pin for the input and do the same thing as above with the negative output?

Also, did you connect the D+ cable on your charger? If so, what did you use for this?

Cheers,
qwertyd10n

[dmatzinger]

I have been interested in the DC/DC converter issue for a while. Hopefully my issue is close enough to this thread (and timely enough) be appropriate.

I have a Bambi 16 Sport with the Zamp solar option and AGM batteries. As I understand the issue, while the solar charger and AC converter are both 3 stage and will provide adequate over-voltage for charging as required, the TV will not, hence the need to boost the voltage if the car is to fully charge the AS battery in the absence of sunlight or shore power. Normally there are two chargers (AC/DC converter and solar charger), and the DC/DC converter would be a third. What confuses me is how the three chargers would potentially interact if connected in parallel to the battery. I assume each one would be sensing battery charge level and trying to maintain the appropriate voltage and current for the state of charge of the battery. Do the devices not currently being used need to be isolated from the battery for charging to work correctly? Because the solar charger connects to the battery circuit between the cutout switch and the battery, the solar charger and the output of the AC/DC converter are connected in parallel whenever shore power is on and the cutout switch is closed to allow battery charging from shore power. If I added a DC/DC converter/charger directly to the leads from the battery, it would potentially be in parallel with the other two (or at least the solar charger, since shore power would be disconnected when the TV was powering the DC/DC charger). Are these potential problems, or am I just worrying about nothing?

[DaveP]

I just disconnected the positive lead from the TV to the trailer bus bar and left the negative connected. No chance of the lithiums discharging your TV battery (or over-drawing the alternator).

Originally Posted by qwertyd10n

cru-in

How did you wire the negative (-) wires on your Renogy 20A DC-DC Battery Charger? The positive (+) side seems pretty straight forward - take the charge wire output from your 7 pin where it connects to your DC Panel bus bar and use that as the input to the charger and then run from the charger output to that vacated position on the bus bar. However the negative leads are a little less obvious to me... did you tap into the ground wire on the 7 pin for the input and do the same thing as above with the negative output?

Also, did you connect the D+ cable on your charger? If so, what did you use for this?

Cheers,
qwertyd10n

[ryanc55]

Originally Posted by dmatzinger

Are these potential problems, or am I just worrying about nothing?

It's fine. During the Constant Current phase of charging (AKA bulk), the voltage that the chargers see will be lower than the Constant Voltage target (AKA absorption voltage) so they will all be putting in all the current they can muster.

Once your battery gets to CV, the chargers will start stepping down current. (Actually a bit before since there will be some voltage drop on your charging cables/connections.) Since the multiple chargers won't be perfectly calibrated voltage-wise, some of them will likely drop off before the others. So in this sense they will "fight" but it's not a big deal, the battery is nearly full by then.

[uncle_bob]

Originally Posted by ryanc55

It's fine. During the Constant Current phase of charging (AKA bulk), the voltage that the chargers see will be lower than the Constant Voltage target (AKA absorption voltage) so they will all be putting in all the current they can muster.

Once your battery gets to CV, the chargers will start stepping down current. (Actually a bit before since there will be some voltage drop on your charging cables/connections.) Since the multiple chargers won't be perfectly calibrated voltage-wise, some of them will likely drop off before the others. So in this sense they will "fight" but it's not a big deal, the battery is nearly full by then.

Hi

What a typical charger does is to run up to some max voltage. Eventually it drops back from this voltage to go to a lower "float" voltage. Some chargers do this by current rather than voltage. Once the current drops off, they decide that things are fully charged and drop back to float. Many of the devices in our RV's fall into that second category.

If you are looking at things like solar chargers, their current output can be highly variable. Another charger trying to "make up" the difference between what the battery needs and what one is putting out will see a highly variable load. When that load drops to zero, the charger may well go into float. (and effectively be out of the picture).

Indeed the battery is nearly fully charged at this point. On a lead acid battery it's somewhere between 70 and 80% charged so indeed you are only giving up a very small amount of the capacity. That assumes you run to zero ... ( which can be the case on lithiums, not so much on lead acid). With a lead acid, this "not much" is about half of the useful capacity of the battery.....

Bob

[ryanc55]

Originally Posted by uncle_bob

Hi

If you are looking at things like solar chargers, their current output can be highly variable.
Bob

Ah yes, good point. I hadn't thought about solar chargers fooling the other charger into thinking the battery was full and then not finishing the job when the sun goes away.

So if possible it would make sense to set the solar charger's acceptance voltage ever-so-slightly lower than a shore charger so that it is the one that shuts off first.

And yes, I'm on LFP batteries so I get spoiled by that