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Renogy DC to DC charger...interesting

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Rbertalotto

Solar Enthusiast
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Nov 26, 2019
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(I Updated this post below....)


Here is the situation. 40amp Renogy DC to DC charger. Travel trailer with two 100ah Renogy Li batteries. Truck is Ram2500 with 220amp alternator. 4g wire to and back to charger located in rear of truck. About 20' each run of wire. This week camping in well shaded campsite. 420 watt of solar did not keep batteries charged. My battery bank was at 60%. Moving from one campground to another, three hour trip. I turned on the Dc to DC charger and hit the road. The shunt based Renogy battery monitor was only showing 15amps going into the batteries. After three hours of driving, the batteries were back up to 90%.
Why am I only seeing 15amps to the batteries? Any help appreciated.
 
Last edited:
Do you have the wire that halves charging current hooked up? The port next to the ignition sense wire, if hooked up, will drop the current in half.
 
Safe to assume that the two batteries you mentioned are paralleled? I would think so as I am pretty sure Renogy only does 12v on the chargers.
Was that 15amps constant the whole time? I ask as heat might come into play and the unit throttled back (would assume the Renogy units would).
You need to ensure that the Renogy unit is working. You might very well be getting 15amps from the charge wire on the 7 pin trailer connector and nothing from the Renogy.

If you want to test without the truck charging through the 7 pin, you can pull the 30amp fuse. Long short, you should be getting at least 15amps just from the trailer connector.

I would also check the voltage on the input side of the Renogy. Closer to 14v the better when the truck is running as I would assume, as most, the charger is sensing if the vehicle is running by the voltage, before engaging. You can bypass this by wiring an ignition feed into the charger. I always do this as I have yet to find a charger that senses the truck is running correctly (might be me).

Even better to run the ignition feed if you have upfitter switches in the Ram.
 
What type of connector are you using between the tow vehicle and the trailer?
Is it getting hot?
Is the 7-pin charge circuit disabled?
 
Answers: Anderson Grey connectors (50amps?)… Nothing is getting close to hot. Wires, connectors, charger and alternator is not showing any issues on my trucks dash board. My 7 pin charge is not connected on this truck. I do not have the half current connected. I double checked and DIP switches are correct for Li . Batteries are parallel. Thanks
 
Try again when the battery is deeply discharged, see if you get more current flow.
You could post the dip switch settings for us to verify.
 
Did you actually see 15amp current flow or are you just extrapolating from the SOC?
 
John Frum said:
Did you actually see 15amp current flow or are you just extrapolating from the SOC?
Click to expand...
I can see current from the shunt battery monitor. I have a DC clamp on amp meter. Once batteries get low again I’ll measure from truck battery to trailer.
 
Dip Switch setting…hard to see how I have unit mounted…looking at it from left to right…down-up-down-down-up
 
What is doing the voltage regulation ? As John Frum was eluding to, my first thought is that the batteries were in the absorb charging cycle when the current was 15 amps.

100 amp-hours is a small-ish battery bank after all and they cannot take that much current without the voltage rising fairly quickly.

boB
 
Rbertalotto said:
Here is the situation. 40amp Renogy DC to DC charger. Travel trailer with two 100ah Renogy Li batteries. Truck is Ram2500 with 220amp alternator. 4g wire to and back to charger located in rear of truck. About 20' each run of wire. This week camping in well shaded campsite. 420 watt of solar did not keep batteries charged. My battery bank was at 60%. Moving from one campground to another, three hour trip. I turned on the Dc to DC charger and hit the road. The shunt based Renogy battery monitor was only showing 15amps going into the batteries. After three hours of driving, the batteries were back up to 90%.
Why am I only seeing 15amps to the batteries? Any help appreciated.
Click to expand...
Since your batteries were at 60% state of charge that means you only needed to charge about 80ah at 12 volts. So the 40 amp DC to DC charger would only take 2 hours to fully charge. However, before that two hour mark once you got to about a 90% State of charge it would have went into float and the charger would drop the current from 40 amp charging to 15 amps or lower as it gets closer to 100%. There is a good chance you just didn't see it when it was pushing 40 amps.
 
A 40 amp DC-DC charger will use 50 (or more) amps on the input side. A 20' run of cable from the alternator to the charger (40' round trip) at 50 amps will have a 3.93% voltage drop at 13 volts. Then there's the output side of the charger. It isn't 100% clear from the description, but it sounds like you have another 20' run (40' round trip) between the charger and the Renogy LiFePO4 battery bank. Assuming the charger produces only 40 amps (I know you're seeing only 15 amps), you're seeing a 3.15% voltage drop on the output side of the charger.

I don't know how to aggregate the voltage drop for those two runs of cable, so I ran it as 40 amps at 13 volts for an 80' round trip which results in a 6.29% voltage drop.

That's just a "by the numbers" assessment. It's not good, but not horrible. Under 3% voltage drop is what most of like to see.

The suggestions so far that the battery state of charge prevented the charger from going full throttle, heat soak and missed observations are all valid. Throw in Renogy's low quality and there's a lot to work through.
 
"100 amp-hours is a small-ish battery bank after all and they cannot take that much current without the voltage rising fairly quickly."

I have two 100ah batteries in parallel =200ah
 
200 AH is bigger but as soon as I gets to Absorb or float, it won't take very much current to finish the charge up to 100%. Can't rush the absorb cycle by giving applying more current because you would over-voltage the batteries. I am just guessing that this is what is happening.
 
I have a Renogy 50 amp DC-DC with mppt hooked up the same way as the OP (I think). I have the DC-
DC and the battery in the back of the pickup bed. Initially, I was only getting 20ish amps when I should have been getting 50 amps.
As best as I could discern, I didn't have a smart alternator so therefore I didn't need to connect the smart alternator wire as per the directions.

I experimented and plugged the smart alternator wire into the Renogy and into a small 12 volt 4 AH battery. Doing this immediately jumper the amps up to specs-50 amps. I think the OP's problem is his lack of a power source on the smart alternator.
 
Sadie04074 said:
I have a Renogy 50 amp DC-DC with mppt hooked up the same way as the OP (I think). I have the DC-
DC and the battery in the back of the pickup bed. Initially, I was only getting 20ish amps when I should have been getting 50 amps.
As best as I could discern, I didn't have a smart alternator so therefore I didn't need to connect the smart alternator wire as per the directions.

I experimented and plugged the smart alternator wire into the Renogy and into a small 12 volt 4 AH battery. Doing this immediately jumper the amps up to specs-50 amps. I think the OP's problem is his lack of a power source on the smart alternator.
Click to expand...
What is this "Smart Alternator" wire you speak of. I don't have that option on the 40 amp DC to DC....?
 
Rbertalotto said:
What is this "Smart Alternator" wire you speak of. I don't have that option on the 40 amp DC to DC....?
Click to expand...
IIRC the 1212 series (20, 40, 60) are triggered by D+ and have no need for the smart alt wire. They know when the engine is running because D+ is powered.

The DCC (and new Rego series I just saw) are voltage-sensing, so they need help to know when the car is running and the alt voltage is just low.
 
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