DC-DC charger as an alternative to generator?

[Martinjmpr]

Also, you're still using those oooold FLA batteries! We get less than 2 years here in Phoenix from a car FLA battery before it doesn't charge right! I thought you had replaced with LFP ones. When you eventually DO, you're gonna want a 50A charger, so just get the 4AWG cable now, even the 20A is on the edge of needing 4awg at 30ft circuit already.

Well, no, not really.

The batteries in the trailer are LFP.

Inside the truck I have a home built "power box" that I made in 2019 to run the fridge. THAT currently uses a 90AH FLA battery but its also a boat anchor that weighs probably 80lbs. PITA to move. I put the DC-DC charger in the truck to keep THAT battery pack powered, not to charge the trailer batteries, which is why the DIP switches on the Renogy are set to the FLA setting and not the LFP setting. I actually did a write-up on my power box setup in 2019 or 2020 when I put it in, it's somewhere in the archives here.

However, now that I've "seen the light" with regard to LFP I'm going to be replacing the battery in the battery pack with an LFP, probably 100AH. Once I've got the FLA in my battery pack replaced with an LFP I'll change the settings on the Renogy to the LFP setting.

As far as updating to a 40 or 50A charger, I'm not sure I see that happening. I mean, think about it: My current 20A charger has been operating at ~40% capacity or less for the past 5 years and it still kept my FLA battery charged! If I can get it to operate at 100% it will be more than adequate. I mean, I see where you're going with the upgrades but I'm just not sure it's necessary and those heavy cables are enough of a PITA to deal with at 6AWG, I can imagine that 4AWG would only be worse, requiring bigger holes in the cab, bigger connecting ring terminals, etc.

 

[burleyman]

Yes, the amps seen on your bluetooth screen is the sum of amps in and out. Negative number means being discharged. Disconnect loads, and it is just charging amps coming in. That would be handy for the following charging test. You have already verified 14+ volts from your vehicle at the DC-DC.

Perform at your own risk. Disconnect one battery lead so that you are only charging one lifepo4. Make sure you have a fuse to protect the wire from your vehicle to the DC-DC input. Connect everything as in your posts above that show dipshit charging with the engine running.

Remove the wires from your DC-DC. With the engine running, brush them together and note the spark. If the fuse up front is not blown, then hold the wires together. If they get hot, disconnect. If they don't rapidly get hot, look at your bluetooth and note charging voltage and amps. If they improve, DC-DC problem.

Don’t give up on your wire size yet. For eight years, I’ve regularly charged a 30ah lifepo4, 20 feet away from the alternator-fed battery through a #10 wire with no DC-DC and a charging voltage up front at 14.1-14.6vdc. Fast, at near maximum 30 amp rated charging current.

I’m now in the same boat. Two hungry 100ah lifepo4’s to feed.

My present method you mentioned earlier. A 15 amp rated extension cord from front to back, permanently installed underneath. An inverter up front, temporarily attached with alligator clamps.

Not enough amps to suit me 120vac battery charger at rear, plugged into the redneck extension cord from up front. I’m shopping, and have charger choices narrowed somewhat.

Your high output alternator is nice to have. Charging at high amps just long enough to get by is also nice. Good luck.

 

[gator70]

I watched a video from AeonRv whereas they use a 120v converter from the alternator to charge the lithium batteries during drive time. They claim better charge performance.

 

[rruff]

^ Converting from 12V to 120VAC and back to 24V, is better than what? You can have a good amount of wiring loss before you exceed what you lose in AC-DC conversions.

 

[Martinjmpr]

Well, my 100AH LiFePo "smart battery" has been ordered and should be here shortly. That will go in the trolling motor box that I use for a portable power source. Since it has the Bluetooth monitoring app I should be able to get a real-time measure of the number of amps that my DC-DC charger is actually putting out.

If it's not getting sufficient amps then I have the 6AWG cable I can run from the battery to the DC-DC charger. If it IS getting sufficient amps then the likely culprit keeping the DC-DC charger from adequately charging the trailer batteries is probably the 20' 8AWG charging cable I made. I might look to upgrading that even to 2 or 4 AWG.

 

[gator70]

^ Converting from 12V to 120VAC and back to 24V, is better than what? You can have a good amount of wiring loss before you exceed what you lose in AC-DC conversions.

Ask AEONrv as they claim otherwise

 

[rruff]

...then the likely culprit keeping the DC-DC charger from adequately charging the trailer batteries is probably the 20' 8AWG charging cable I made.

Wire losses are very easily determined... should be no mystery there...

 

[Martinjmpr]

@Martinjmpr
Meh I downloaded the manual for the OLD 1212 20.

Here are the potential mistakes you may have made and issues I see:
1. Did you wire an ignition source to the D+ terminal? If not, charger won't work.
2. Did you make a mistake and wire into the LC terminal instead? Instant derate of unit to 10A max.
3. Did you set all the DIP switches correctly for LFP and proper voltages?
There are 5 of them that must be all set per their table to adjust output voltage from 12.6 to 14.6 volts.
Here is what I'd pick:
Dip sw5 OFF for lithium, on is for lead acid
I would use 14.2V as the Absorption or "bulk" charge rate. You could use 14.4V for faster, but 14.6V can cause cells to overvoltage and the BMS will disconnect the battery, dont use 14.6v.
Here are the settings for 14.2V:

Sw1 on
Sw2 off
Sw3 on
Sw4 on
Sw5 off

Let me know what you had set.
Also, look at the LED fault light on the 1212 20 dcdc unit, is it on? There is a troubleshooting guide on pg 20 of manual for that...

OK, next update. As I previously stated back in 2020 I made a "power box" using a trolling motor battery case and a 90AH FLA battery. It was a boat anchor (literally weighed about 75lbs) but it did a decent job keeping the fridge charged up, and it was attached to the Renogy 20A DC-DC charger in the truck.
Since it was an FLA battery the DIP switches were set to the FLA settings.

UPDATE 1/1/2025: I finally got a 100AH LiFePo battery for the "power box." Like the two batteries I got for the trailer, this one has a Bluetooth equipped BMS that lets me look at the SOC and other relevant data.

SO, now that I have the LiFePo, I need to change the DIP switches. HOWEVER from my manual I am getting some slightly different info than what you have posted above.

Here is my manual showing the relevant pages:



Now, unless I'm reading this wrong, to get 14.4v of "switchover/constant voltage" I need to set S1 AND S2 at ON.

Then with S5 set to "off" for Lithium, the only thing I don't understand is what "type 1" and "type 2" mean. Can anyone help out here?

If I want charging mode voltage at 14.4 it should be S3 OFF and S4 ON, yes?

Again it's the "type 1" and "type 2" that is throwing me. I don't see how to set a "type" using the 5 DIP switches.

If it helps, this is what the manual for the battery shows:



So 14.4v would seem to be the optimal voltage for charging, right?

Final note: It seems my concerns about the 20A DC-DC charger not providing sufficient amperage were wrong. Right after I charged up the new LiFePo battery I plugged it into the DC-DC charger and pulled up the BMS screen on my phone. This is what it showed:



Note that on the "current" display (second from left, top row), it is showing 20.35A going in. So the 20A DC-DC charger is indeed putting 20A into the battery. Not sure why my earlier efforts didn't work but it could possibly have been the cable I used.

I have not yet upgraded the 8AWG cable connecting the battery to the DC-DC charger. I have the cable, I just haven't done the swap yet. Waiting for warmer weather for working outside since this requires me laying in a creeper under the truck.

In any case, if I can get a steady 20A out of the DC-DC charger then there's no reason I shouldn't be able to "top off" the trailer batteries as needed.

I do still have a few experiments to run but we head out for the Gulf Coast of Texas next week. However on this trip we will likely have electric hookups the entire time so it won't be a real boondocking test for the batteries.

 

[Dave in AZ]

OK, next update. As I previously stated back in 2020 I made a "power box" using a trolling motor battery case and a 90AH FLA battery. It was a boat anchor (literally weighed about 75lbs) but it did a decent job keeping the fridge charged up, and it was attached to the Renogy 20A DC-DC charger in the truck.
Since it was an FLA battery the DIP switches were set to the FLA settings.

UPDATE 1/1/2025: I finally got a 100AH LiFePo battery for the "power box." Like the two batteries I got for the trailer, this one has a Bluetooth equipped BMS that lets me look at the SOC and other relevant data.

SO, now that I have the LiFePo, I need to change the DIP switches. HOWEVER from my manual I am getting some slightly different info than what you have posted above.

Here is my manual showing the relevant pages:

View attachment 864987

Now, unless I'm reading this wrong, to get 14.4v of "switchover/constant voltage" I need to set S1 AND S2 at ON.

Then with S5 set to "off" for Lithium, the only thing I don't understand is what "type 1" and "type 2" mean. Can anyone help out here?

If I want charging mode voltage at 14.4 it should be S3 OFF and S4 ON, yes?

Again it's the "type 1" and "type 2" that is throwing me. I don't see how to set a "type" using the 5 DIP switches.

If it helps, this is what the manual for the battery shows:

View attachment 864988

So 14.4v would seem to be the optimal voltage for charging, right?

Final note: It seems my concerns about the 20A DC-DC charger not providing sufficient amperage were wrong. Right after I charged up the new LiFePo battery I plugged it into the DC-DC charger and pulled up the BMS screen on my phone. This is what it showed:

View attachment 864990

Note that on the "current" display (second from left, top row), it is showing 20.35A going in. So the 20A DC-DC charger is indeed putting 20A into the battery. Not sure why my earlier efforts didn't work but it could possibly have been the cable I used.

I have not yet upgraded the 8AWG cable connecting the battery to the DC-DC charger. I have the cable, I just haven't done the swap yet. Waiting for warmer weather for working outside since this requires me laying in a creeper under the truck.

In any case, if I can get a steady 20A out of the DC-DC charger then there's no reason I shouldn't be able to "top off" the trailer batteries as needed.

I do still have a few experiments to run but we head out for the Gulf Coast of Texas next week. However on this trip we will likely have electric hookups the entire time so it won't be a real boondocking test for the batteries.

On
On
Off
On
Off

14.4v is fine.
Your manual is a bot different than the one I downloaded off their website when assuming your exact model #, is why the difference.

Just look at the charge voltage for type 1 and type 2 lithium. You obviously have a type 2. Type1 are older chemistries with lower voltages, old school lithium, not LFPs.

 

[gator70]

What's wrong with using a 1000w sine wave converter powered by the alternator, plugged into shore power while driving?

 

[Martinjmpr]

What's wrong with using a 1000w sine wave converter powered by the alternator, plugged into shore power while driving?

That seems a little "redneck" to me, converting 12v to 120v and then running it through the converter to convert it back to 12v.

Besides the issue of having to run 30+ feet of 1AWG cable from the truck to the trailer.

Easier to put a DC-DC charger in the trailer but I would still have to run heavy gauge cable from the truck battery to the trailer.

 

[Martinjmpr]

OK, I'm going down a different rabbit hole so I'm going to start a new thread specifically for the Renogy DC-DC charger since that appears to be the issue I'm dealing with now. Thanks for all the input!

 

[Martinjmpr]

OK I'm happy to report full success on this project.

When I started, I thought the best way to go would be to have a DC-DC charger attached to the truck's battery and then have that go into the camper batteries.

However, after a lot of back and forth (some of which you can read about in the companion thread here) I decided to go with a 2 stage setup: An 1100W inverter attached to the truck's battery, and then plug my regular 120vAC 30A "smart charger" into that for the camper battery.

The first step for me was to get some heavy gauge cable to attach the inverter. A local welding shop just happened to have a left-over 14' section of 2AWG welding cable they'd sell to me cheap. I also had some leftover cable from my 2016 dual battery project on my 2004 Suburban.

I had to get a set of high-quality crimpers so I could crimp the cables together. Although I was tempted by the cheaper alternatives, I ultimately went with the expensive FTZ crimper, in a kind of "buy once/cry once" state of mind.

After that I got some SB175 Anderson connectors. Since they're rated up to 175A I figured if I wanted to I could make myself some plug-in jumper cables as well.

I cut a short section of cables (less than 2 feet) and put ring terminals on one end and the SB-175 on the other. It fits neatly in the engine compartment and the cable is short enough that it shouldn't interfere with anything else in there:



When not in use the cable tucks away in front of the battery:



And yes, I checked to make sure the cable is short enough that it can't interfere with the fan.

Next I had to use the 14' section of cable to make the power cable for the inverter. Ring terminals on the inverter side and the SB-175 on the other. So it came out to be roughly 7' (2m) long:



The idea I had is that all of this should fit neatly into a watertight plastic case I got at Harbor Freight. It's tight but it fits:



So when setup, it should look something like this:



I had thought about making longer cables but I didn't think it was necessary since carrying a 120v AC extension cord is easy.

I placed the 120vAC charger on top of the propane tanks so it wouldn't be sitting in the snow. As you can see I had to remove the top cover to the battery to connect the clamps. This is a temporary measure, eventually I'll snip the clamps off of the 120vAC charger and replace with an Anderson connector, and then I'll make a "pigtail" cable with the Anderson connector hanging out of the battery box. This way I'll be able to plug in without removing the battery box cover. (A future mod will likely see the battery box replaced with a bigger tongue box, but that's down the road a bit.)



It's alive! Display on the Inverter while running:



Looks like it's using 480 watts, so that's roughly 430 in power output (14.3 x 30A) and 50 for the inverter itself.

On the smart charger, this display shows charging amps:



29.9. It fluctuates slightly as it charges.

Battery display shows much the same. Keep in mind I have two batteries (one ending in 28 the other ending in 33) so each battery is getting roughly 15A of charge. This is Battery 33:



And battery 28:



TO BE CONTINUED....

 

[Martinjmpr]

CONTINUED FROM PREVIOUS:

After 15 minutes I checked the battery charge status:








Battery 33 went from 26% to 31%, and Battery 28 went from 28% to 32%, So by my math that's 9 AH into the batteries in 15 minutes (100AH battery so 1% = 1AH.) Pretty much the result I was looking for. This would mean that 1 hour of idling the truck could (in theory) put 36AH into the 200AH battery bank.

It also occurs to me that one advantage of this setup is that I'm using less than 500w on an 1100W charger. So if I wanted to go up to a higher amp charger, it's literally "plug and play."

As a side note, it occurred to me that if I wanted to I could simply plug the trailer's 30A outlet (with an appropriate 20A adapter) directly into the inverter and use the trailer's on-board charger to charge the batteries. So I tried that and here is the result:







5.68A on one battery and 6.14 on the other. I knew the on-board charger wasn't that great but I didn't know it was this bad. So less than 12A of charge between the two batteries. Based on this I'll use my external charger.

All of my stuff fits in two watertight containers that will ride in the truck (most likely.)

Anyway, I'm happy, this will now give me the option of keeping the battery on the trailer charged up and usable even if we don't have solar power available to us. It was a bit of a "journey" but I admit it was also a lot of fun figuring this stuff out.

 

[DiploStrat]

FWIW, AEONRV use a variation of this idea. 1kw+ inverter feeding the built in 3kW Victron Inverter/Charger. Been using this for a about 50 campers.