Renogy DC-DC Charger UPDATE 2 Test

Hi BFL13,

It should not hunt--were the batteries down a bit?

This guy's 40 amper is not cycling, so I have a problem somewhere. It could be a "protection" thing turning it off every time it gets going maybe.

https://www.loveyourrv.com/renogy-40-amp-dc-dc-charger-installation-and-demo/

Found it! It was something to do with how the input and output neg terminals share the same chassis ground. I found 24ish volts across the output terminals, which is why it was cycling. :(

I swapped neg wires around until I hit upon a right way, and now it behaves.

Mystery how you can get 24ish volts with the positive wires the same way but just do whatever I did (don't know what I did) with the neg wires and get 14ish volts like it should.

With it finally working, with the truck running, I had 14.3 volts in the truck and 10.87 volts back in the camper at the DC inputs! (DC-DC will work down to 8 volts ISTR) That was using the 7-pin wires and no idea what the amps were on the input -maybe 25? Under 30 anyway since it was fused with 30a in Stud 1. Lots of voltage drop in those wires and connections.

Output was 14.1 and house batts 13.8ish, which would be going up over time to 14.7 (at the charger) with amps steady at 18ish in the Tri (so a 2a load +18= 20)

So it works and no amps tapering like with regular 7-pin. (down to 4.6 amps five minutes after starting the truck)

Had you followed the instructions? Was it not clear what to do? We all want to avoid this issue when we upgrade.

Very glad to hear it is working.

time2roll wrote:
Had you followed the instructions? Was it not clear what to do? We all want to avoid this issue when we upgrade.

Very glad to hear it is working.

I thought I followed the instructions, but I have so many neg wires all eventually going to the Trimetric's shunt on the neg house battery post, there must be something weird happened.

I swapped around so a more direct route to the input from the 7-pin neg that is also grounded to the camper chassis (but so was the wiring zoo it was on before and it didn't work right). Plus a more direct route from output neg to Trimetric shunt, but that has the battery ground to camper chassis with it too same as the more indirect route it had, so shouldn't matter. Beats me!

I can't say what I did that made it work right.

On my 5er before, the 7-pin started at 10 or so amps but quickly tapered to 5 amps area (in 5 minutes or so after starting the truck) and the same thing happens with the truck camper (same 2003 Chev truck). So this gizmo looks like it will maintain more like 20 amps (minus any 12v load in the camper) to the house batts using the same 7-pin input.

I intend to tidy up the wiring from Renogy to house batts with fatter wire as needed. Have to see if there is a better way to get the 7-pin grounded to the camper chassis as well as being the input to the Renogy.

The way it is now it might never get near 14.7v Vabs as seen at the battery, but I should be able to get pretty close.

This DC-DC idea is a good one if you need "alternator charging" that your solar won't do anyway while driving. Also good for LFP alternator protection if that applies to your set up (been some sceptics about that--not a clue by me). Help keep the fridge on 12v or 120v if you do that while driving I guess.

I want it just for when parked and not enough solar, because we can't carry a generator in the TC set-up, so the truck engine idling and the Renogy can at least get something done in a pinch.

Good to hear you got it fixed,
Sounds like the charging outputs solar and DC to DC converter, were somehow in series instead of parallel
Many solar controllers are positive connections (input-panels, output charge) are common ( connected together) on the controller and it is the negative connections input and output that are isolated separately, (charging control flow is thru the negative side, not positive side ! Of the solar controller, this is opposite of vehicle wiring we are used to
This might be why you had a problem, the DC to dc charger AKA voltage booster, is wired positives Isolated and negatives common, like vehicles
Thinking all the negatives would be common and at zero volts, you ended up wit two devices in series for double voltage, until you moved negatives around and took one off the Trimetric shunt and broke the series circuit between the two charge systems, Renogy and alternator in series, instead of Renogy replacing the alternator, an inline boost operation for DC, like an autoformer does for 120vac
That's my analysis based on your description, and first hand experience with a solar controller that was mfg input output positive connections common, negative connections separated/isolated, charge controlled by the negative

Since charge current into a battery has to travel thru both wires electrically , the control can be thru either side of the circuit, and since the charger is not in the load circuit there is no problem, the material design type of the power control devices determine whether it's positive control or negative control germanium, Silicon , npn pnp, etc.. Engineer's choice or budget been counter, or maybe what the purchasing dept knows they can get, and tells the engineer, we can't the ones you want, redesign this device, we can get from our main supplier, we can't source your choice at the profit we want to make, or in the number we want to manufacture

Any way that's my thoughts on what happened on your first try, you were able to correct it , and tells us about it

The you tuber "Foresty Forest" from Alberta has just built out another van. He uses a Renogy DCDC charger. He installed a high output alternator on his GM 6 litre engine. In this video at about 2 minute mark he said the system charges the lithium battery at about 1ah per minute.

Foresty Forest

" The DC-DC share a common negative ground meaning that there should be only one common ground point between all batteries and electronics typically seen in either chassis/body ground, a canopy, a trailer, or even the vehicle battery negative connection. In most cases, connecting
the starter and house battery directly to the DC-DC is enough for a grounding application. You will not ground the body of the DC-DC. In the illustration below, the two batteries are connected
to the same chassis ground point."

From the manual. They show the starter and house batts in a MH both being grounded to the frame, one at each end of the MH.

With the camper or a trailer, you get the common ground through the 7-pin and the starter and house batts are grounded to different frames.

The 7-pin works right with the camper's clearance lights etc, so my problem was internal to the camper, but ? Mr Wiz's idea is one possible, although I believe the solar controller to battery was disconnected (array too).

Whatever I did to fix it got the groundings sorted out it seems. Full marks for Renogy that the unit's "protection" saved it. ( On that, ISTR reading somewhere that there is a fuse inside it you can get at they don't mention in the manual, but can't confirm if that is true)

Hi Mr Wizard,

Having the dc to DC charger in series with solar would "give" 24 volts at the battery bank terminals. It would not show 24 volts on the dc to DC terminals. Nor would it cause the dc to DC unit to "hunt".

BFL13 is quite lucky he did not blow the computer boards in the various house equipment.

Some how the batteries ended up in series instead of parallel.

Not sure about the wiring of the Renogy but would assume it is similar to my Victron 12-12 Orion-Tr 30amp. The "-" connection to the source and load are isolated on the device, however can be connected together and do get connected together when the source and load share a common "-" ground.

Question for you guys
Anyone find a way to adjust the output/load current of a DC-DC converter?
Temporarily lowering the charge voltage does the job but is not as "clean" as desired.
The Bluetooth app does not offer any current adjustment that I can find only a custom voltage charge profile.

I charge a 200amp LiFe four cell prismatic home brew battery which will easily take the full 30amp from the DC-DC charger thus consuming 40+ amps from the source. My desire is to throttle back the charge current at times when the solar is not doing enough to keep up. The solar charges a 200amp PB battery bank. Don't care if it takes longer to charge the Li. I do not want to see my Pb bank get discharged while charging the Li bank. Yes I understand I could set the source cut off voltage high enough to eliminate the possibility of too much discharge but again it is not a "clean" solution and would cause the cycling on/off issue.