DC-to-DC + Isolator

RT-NY · 09-06-2021, 05:45 PM

The Renogy DC-to-DC chargers are supposed to be connected to the car battery on one side and the coach battery on the other, and they have a "D+" signal input that switches them on only when the alternator is running. So they do not require an isolator.

But, it would much easier in my 1999 Roadtrek 170, if I just disconnect the positive cable on the coach battery (which is coming from the already installed isolator) and connect that to both the car battery side of the Renogy and the D+ signal. And then just continue on to coach battery from the other side of the Renogy, (I would connect the negative on both sides to the coach battery negative, which I believe is a common ground in any case). Has anyone here wired them up in this way? Can anyone think of any reason it wouldn't work?

hbn7hj · 09-06-2021, 08:06 PM

As I read it that will install the Renogy backward.

The coach battery is removed from the isolator and connected to the Renogy output. The Input of the Renogy is connected to the car battery positive along with the D+ line. The alternator line can be left where it is on the isolator or removed from the isolator and connected to the car battery positive. If you do that the line from the isolator to car battery positive can be removed.

CanuckRV · 09-06-2021, 08:15 PM

That's how I installed mine and it works fine.

RT-NY · 09-06-2021, 08:35 PM

Quote:

Originally Posted by hbn7hj

As I read it that will install the Renogy backward.

The coach battery is removed from the isolator and connected to the Renogy output. The Input of the Renogy is connected to the car battery positive along with the D+ line.

Can you explain why my proposed wiring is backwards? The input and D+ are still coming, ultimately, from the car battery but will only be powered when the alternator is running (because of the isolator). And the output is still going to the coach battery.

I know how it is supposed to be hooked up, but I would just be easier to mount it in the back by the coach battery and to leave the wiring under the hood unchanged.

RT-NY · 09-06-2021, 08:37 PM

Quote:

Originally Posted by CanuckRV

That's how I installed mine and it works fine.

Thanks! And your rig is also a Dodge RT?

hbn7hj · 09-06-2021, 08:42 PM

It will work without changing anything under the hood. You still have the isolator voltage drop but it will work.

RT-NY · 09-06-2021, 08:54 PM

Quote:

Originally Posted by hbn7hj

It will work without changing anything under the hood. You still have the isolator voltage drop but it will work.

Thanks! But won't the Renogy adjust the voltage according to its own 3-stage charging specifications in any case?

hbn7hj · 09-06-2021, 09:13 PM

Quote:

Originally Posted by RT-NY

Thanks! But won't the Renogy adjust the voltage according to its own 3-stage charging specifications in any case?

Yes, it will.

CanuckRV · 09-06-2021, 10:51 PM

Yes I have a 97d190p
I can take some pictures if you want.

RT-NY · 09-06-2021, 11:21 PM

Pictures would be great! I think that others would find it helpful as well because I have seen this question posted on other forums but could not find a clear answer.

rogerstg · 09-07-2021, 10:44 PM

I'd consider eliminating the isolator. Who needs an item in the system of no use, that can fail. Usually there is enough cable under the hood to "jump" the isolator (assuming it's near the battery).

markopolo · 09-08-2021, 01:04 PM

Bypassing the isolator would be easy. You can gain more control over the Renogy voltage trigger by using a low cost programmable DC comparator or charge control module. You can use a second module to automate the LC trigger for even greater control. See: https://www.classbforum.com/forums/f...way-11572.html

A '99 RT would have 8AWG or 6AWG at best. RT would have used either 30A or 50A breakers. A Renogy 40A for example could exceed 50A on the input side so you really don't want to waste power / add to the load on the wire by leaving the isolator operational (in my opinion).

RT-NY · 09-09-2021, 02:10 AM

Quote:

Originally Posted by rogerstg

I'd consider eliminating the isolator. Who needs an item in the system of no use, that can fail. Usually there is enough cable under the hood to "jump" the isolator (assuming it's near the battery).

The reason I was thinking of leaving it in place is just simplicity: it saves me the need to run a wire to the back of the van for the D+ on the Renogy. The isolator is mounted around the mid-point under the hood, so not right near the battery. I do not know how much work it would be the bypass it or if there might be unforseen implications in regard to the car charging system.

Quote:

Originally Posted by markopolo

RT would have used either 30A or 50A breakers. A Renogy 40A for example could exceed 50A on the input side so you really don't want to waste power / add to the load on the wire by leaving the isolator operational (in my opinion).

I believe my 1999 has 50 amp breakers. But it only has a 137A alternator, so my understanding was that I should not use more than the 20A Renogy. Is that correct?

You are saying that a 40amp charger is going to pull 50A from the alternator? Why is that?

I understand that the voltage drop across the isolator means some waste of power, but I did not expect that it would be that significant.

hbn7hj · 09-09-2021, 02:38 AM

You could just put all three wires on the isolator center post. Leave the fourth wire as is.

I don’t know enough about the function of the fourth wire to understand what to do with it if you remove the isolator. I’m sure others do.

markopolo · 09-09-2021, 03:02 PM

Quote:

Originally Posted by RT-NY

.............. You are saying that a 40amp charger is going to pull 50A from the alternator? Why is that?..................

Page 9 in the manual: https://www.renogy.com/content/RNG-D...060-Manual.pdf

Quote:

When the battery charger is sending the rated amps, the input side
may experience a draw higher current draw by a factor of up to 50%.

It has to do with the unit having to use current to boost voltage. For example it can boost 13.6V to 14.4V but requires additional current to do that.

RT-NY · 09-09-2021, 03:42 PM

Quote:

Originally Posted by markopolo

It has to do with the unit having to use current to boost voltage. For example it can boost 13.6V to 14.4V but requires additional current to do that.

That makes sense. But I do wonder if that alone could account for as large of a draw as the manual suggests (50%!). In your example, if I understand the math correctly, the calculation would be (14.4V x 40A) / 13.6 V, which would suggest less then 2.5 additional amps. And I get a bit more than 14 volts at my house battery straight from the alternator.

In any case, your point is well taken, especially if trying to use a 40 amp charger with the stock alternator/wiring in a vintage Dodge RV. It would be best not to waste energy or push things too near to their limits. From what I recall, the rule of thumb in house wiring is 20 percent of rated capacity.

booster · 09-09-2021, 04:12 PM

Quote:

Originally Posted by RT-NY

That makes sense. But I do wonder if that alone could account for as large of a draw as the manual suggests (50%!). In your example, if I understand the math correctly, the calculation would be (14.4V x 40A) / 13.6 V, which would suggest less then 2.5 additional amps. And I get a bit more than 14 volts at my house battery straight from the alternator.

In any case, your point is well taken, especially if trying to use a 40 amp charger with the stock alternator/wiring in a vintage Dodge RV. It would be best not to waste energy or push things too near to their limits. From what I recall, the rule of thumb in house wiring is 20 percent of rated capacity.

They do say "as much as" so would be worst case of a severely pulled down alternator at maybe 11.5v or less and we have heard of 25% loss to inefficiency with B to B, so yours probably would be less, but I would count on at least 20% loss.

RT-NY · 09-09-2021, 05:56 PM

Quote:

Originally Posted by booster

I would count on at least 20% loss.

Am I recalling correctly that I saw a post in which you said you use a 40A charger with alternator that is not putting out too much more than my 136A, with a manual switch to avoid over-taxing the alternator? If I could manage a higher capacity charger then it would be worth disconnecting the isolator and setting it up right. What do you think the maximum would be with the stock Dodge 136A alternator?

booster · 09-09-2021, 11:52 PM

Quote:

Originally Posted by RT-NY

Am I recalling correctly that I saw a post in which you said you use a 40A charger with alternator that is not putting out too much more than my 136A, with a manual switch to avoid over-taxing the alternator? If I could manage a higher capacity charger then it would be worth disconnecting the isolator and setting it up right. What do you think the maximum would be with the stock Dodge 136A alternator?

I think that the 40 amp charger would be fine with a 136 amp alternator as long as the alternator can actually do that much. Some rebuilds don't actually make original spec so on older vehicles it can always be a question.

40+20 from the alternator to charge worst case plus another 20-40amps to run the van so still reasonable unless using a lot for the van, as in AC and blower on high, with the headlights on type thing. At that point a disconnect could give the alternator a break. The best way to tell for sure is to get a cheapo infrared thermometer and take the temp of the alternator lamination and diode areas. If under about 225*F should be OK.

RT-NY · 09-10-2021, 10:35 PM

I might try the Renogy 40A unit then--the Renogy also has the LC signal to limit the current, so perhaps with a double throw switch mounted to the dash to shut it off or switch between 20A and 40A.

So, would connecting the three main wires on the isolator to its center post work OK? If I am not mistaken, this would connect the alternator directory to the car battery and the house battery and bypass the isolator entirely, Would this effect the car wiring or charging system at all?

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