12v DC|DC charger setup for LiFePO4 battery - check my work please

[itllgrowback]

Hi folks - new guy to the forum, but not new to tinkering with my trucks. I've done some (mostly successful!) electrical projects in the past, including a nice distribution panel fed by the battery, like a lot of folks do. Right now, the only things that connect directly to the start battery apart from stock are the winch wiring (B+ to a solenoid, and the ground returning back to negative), and the 4ga B+ feed to my distribution panel on the opposite corner of the engine bay - that runs lighting, radios, and has spare capacity for the future.

Recently I've been troubleshooting my Dometic CFX3 55IM fridge, which just doesn't really keep up here in Phoenix even using their optional 10ga install kit - which does make for a clean install, but even using that, I had too much voltage drop in back for it to really be a sound setup.

In doing my testing, I found that I have significantly less voltage drop in back if I ground devices to the chassis close by, rather than running the ground back to the battery. The opposite was an assumption I've made for a long time, that it would be more efficient to run back to the battery directly; but my recent testing has shown the opposite, and the measured voltage drop is significantly less now. What that also opened my eyes to is that if I don't have to run two conductors, I can run a bigger one. So I think I'll use some 8ga that I have handy, and run a dedicated line from the battery to the Dometic HWK-DC and then from there use their 12ga power cord to feed the fridge. I'll run that for now and see how it performs.

But in thinking about all this, I'm tempted to go ahead and build a dedicated battery and charger setup to feed that fridge, to isolate my starting battery completely from that circuit. I'm looking at running 6ga (one conductor) from the start battery to a Victron 12|12-18 Charger in back near the fridge, grounded there, and then 6ga (two conductors) from there to perhaps a Dakota Lithium 54Ah LiFePO4 battery. 18A into 54Ah gives us 0.3C for the charge ratio, which is solid and conservative and should help the battery last many cycles. From the battery, I would do a short run of 10ga to the Dometic 12v port and use their heavier than stock 12ga power cord to supply the fridge.

So if you would be interesting in helping, take a look at the schematic I drew up, to see if anything seems amiss, or if there are things I should be considering.

I plan to use Blue Sea ANL fuseholders/fuses for the two 35A fuses, and the rough run lengths are 12' from the battery to the charger in back (6ga), a couple feet of cable from there to the new battery (also 6ga), and then another couple feet from the battery to the 12V Dometic port (10ga). Victron says that the 30A version of this charger draws a max current of 35A at the input - I've extrapolated that to be ~21A max input current for the 18A version. So it doesn't need to be fused at 60A - 35A should keep everything nice and safe and the 6ga will allow for efficient voltage delivery.

Thanks for any insight or thoughts!

 

[itllgrowback]

One question I have is whether there's any issue using the isolated ground version, when it's not strictly necessary. I've been advised by a tech at Dakota that I should use the non-isolated version because the chassis is a shared ground; but Victron only offers the non-isolated version in the 30A output size, which is too big for any battery I have the room for. The 18A model feeding a 54Ah battery is the perfect form factor for my space.

But my thinking is, if I'm grounding the input of the charger to chassis as the battery itself is, but then running two conductors to every component downstream of the charger, the isolated version makes sense.

 

[jonyjoe101]

If the rewiring doesnt help with your voltage drop problems, one thing that might help is a boost-buck converter. 2 years ago I had a similar problem with my 26L 12 volt fridge, on very hot days the compresser would start cycling back and forth. I had a 220ah lifepo4 and it wasn't able to keep up with it, thats when I added another 312ah of li-ion battery bank (which were of lower output voltage 12.6 volts) and that also couldnt run the fridge without the cycling problems. What happens is when the batteries are below 50 percent, any voltage drop will affect how the fridge runs. The fridge only ran during the day while it was charging with solar, during the night was when the compresser would start run for a few seconds then shutoff and a few second later it would restart.

I had small boost buck converters that I use to charge small lifepo4 packs, I decided to run the fridge through one of them and that was able fix the problem. At lower battery voltage the boost buck converter keeps the output voltage stable, at most you might see a .2 volt drop when the compresser starts. Even when my li-ion battery bank gets down to 10 volts, the fridge runs perfectly. I tried different output voltages but settled at 13.2 volts where the fridge doesnt cycle back and forth. I have been running my fridge through the boost buck converter for the past 2 years 24/7 and havent had any more problems even on the hottest days.

I had to use 2 converters a boost (to boost up the voltage to 18 volts) and then a buck (to lower the voltage to 13.2 volts). When your using a 12 volts dc source, a boost or a buck by itself won't work due to the input and output voltages are to close to each other. With a boost buck you output all the amps the converters can produce without too much voltage drop.
They also sell all-in-one boost buck converters which should work also.

boost buck converter

 

[itllgrowback]

Interesting! Thanks for the input.

[snip]... during the night was when the compresser would start run for a few seconds then shutoff and a few second later it would restart.

That sounds a bit like my original problem; as the battery became discharged, the initial compressor surge would cause enough voltage drop for the fridge to see voltage below the cut-off limit, and the compressor would cut off. Then the voltage would immediately rise again enough to try again, and the same thing would happen.

I have been running my fridge through the boost buck converter for the past 2 years 24/7 and havent had any more problems even on the hottest days.

Do you mean that the buck/boost converter is between your batteries and the fridge? I think the Victron charger will essentially do the same as a combined Buck/Boost (8-17V in, 10-15V out), with the added benefit of three-stage charging - so it needs to feed the battery rather than the fridge directly - but it sounds like in your case, you use the buck/boost in between the battery and the fridge, is that right?

Do those converters run cool enough that you can keep them in those bins? That's awesome. I've heard the Victron runs hot so it's another consideration.

Here's a little buck/boost on Amazon - this one's fixed at 12v output, but is this like what you're thinking? One of these between the positive cable and the fridge?

 

[itllgrowback]

I'm also hemming-and-hawing about whether I need to spend the money on 6ga wire for a system that should only draw 25A maximum. It's about 15' in total from the AGM through the charger, to the LiFePO4, and it feels like 8ga should be plenty according to charts like the attached.

BUT, 10ga over that distance should also be plenty, to run my fridge which is rated at 8.9A - and the voltage drop was a problem. So I lean towards going over on capacity to remove potential headaches.

But then I go back the other direction, since the Victron takes input voltage and converts it to a variable output, I think as long as the input voltage is well within the range, slight voltage drop on the input isn't as much an issue (when we're still within recommended wire size guidelines anyway). And it seems like 8ga should provide plenty of input voltage, especially since the charger will only be running while the alternator is (13.5-14.7V typically - the charger output will ideally be 14v so it's not a huge change anyway.). Plus, the max wire capacity for Victron's 30A model at the screw terminals is 6ga; I'd be using their 18A model.

Any thoughts?

EDIT: I'd still love to hear any theory talk, but for the application, I decided to get 6awg 60V cable and stop wondering. Sometimes my indecision is the biggest obstacle. "Analysis paralysis" is real.

 

[jonyjoe101]

I placed the boost buck converter between the battery (my 312ah li-ion battery pack) and the fridge. From the converter to the fridge is less then 10 feet. The converter being in between the battery and fridge makes sure the fridge only gets 13.4 volts, no matter if the battery is being charged or discharged.
In my case I cutoff the cigarette plug from the fridge and used xt60 connectors for a more secure connection.

I suspect the higher voltage I use 13.4 volts compensates for any voltage drop to the fridge, and the 12 gauge wire I'm using is large enough to provide the necessary amps to the fridge. The converters get warm during use, and I had to use a small fan.

I tried different voltages 12.6 volts, 12.8 volts, 13 volts, 13.2 volts and settled with 13.4 volts.
The voltage stabilizer from the amazon link only outputs 12 volts, I didnt test the fridge at 12 volts but I suspect it would be too low for a fridge. Technically it should work but in my case I was getting the fridge compresser to cycle back and forth at 12.6 volts. On my fridge I have a switch low/hi battery voltage cutoff, no matter where it was set, it didnt help.

As far as wire gauge I been running 10 gauge (pure copper) from my charge controller to my batteries (about 5 feet). In full sunlight it passes up to 27 amps. The wires don't even get warm. For the fridge 12 gauge should be good if the voltage is high enough. The lower the voltage the larger the gauge you need. Getting pure copper wire is the important thing, its more expensive but its worth it.

One thing I learned from trying to charge my lifepo4 from the alternator to the back of my van (less than 10 feet), I was going to charge directly from the cigarette output and was using 16 gauge pure copper wire. Since the cigarette output can at least handle 15 amps, I thought thats what I was going to be getting but with the engine running the battery (which was at 20 percent) was charging at a measly 3 amps (measured from dc wattmeter), which was insignificant for my 220ah battery. That was the first and last time I was going to use the alternator as a method to charge my batteries.

Here is an updated picture I took today. The output voltage is 13.4 volts instead of 13.2 volts. I also had replaced the larger fan with a smaller 60mm fan. The fan isnt running at full speed, which is too loud even for such a small fan. The 3.29 amps is what the fridge is using when running, it might get up to 6 amps during compressor start up.


xt60 connector, black wire goes to fridge. The 0.04 amps is what the fridge is using when compressor is not running.

 

[itllgrowback]

OK, I pulled the trigger on a setup and it's in the works. I'll go ahead with 6ga cable from the B+ on the starting battery, run that back to the Victron 18A DC/DC Charger, and ground it there via 6ga to chassis. Then from the charger output to the LiFePO4 (a Dakota Lithium 54Ah), I may go with 8ga direct from charger to battery if I don't want to mess with 6ga in that short run.

As for the Dometic wiring, I think I'll abandon the Dometic-proprietary connection as well as the 12v accessory port, and instead get a panel-mount Anderson Powerpole setup, and feed that with 10ga cable from the Dakota battery. Then I'll cut the 12ga Dometic power cord to length and put powerpoles on the end of that so there's no wasted length and fewer connections.

On the front face of the cabinet I need to build to house all this, I'll add two more easily-accessible APP connections, and a simple volt meter for the accessory battery.

Building the cabinet now; I've got the battery and charger in hand, waiting on the 6ga cable which should arrive tomorrow, but I'll take my time putting this together cleanly.

Testing the fridge on the new battery, it ran at 37 degrees for over 24 hours and dropped only 1 volt, so that should give me something like 3+ days before hitting the Low battery protection cutoff in similar weather, and that's a huge relief - and also being separated entirely from the starting battery for its power is nice. My AGM was having trouble staying fully charged in my short commute, which should be solved by not also running the fridge off it.

 

[01tundra]

I'd be careful guessing at the 12/12-18 input current draw.

Mine put out 25A+ to the batteries consistently, when I asked Victron about it they stated that they "underrated" it.

My new 12/12-30 puts out over 30A as well.

I run a 60A Blue Sea breaker on the primary side and a 60A Mega fuse on the secondary, per the Victron manual. You're protecting the wire and 6 AWG can handle up to 60A. A 35A fuse will be cutting it close and really gains you nothing by under sizing it, other than possible nuisance trips.

This was from my 12/12-18 -

 

[itllgrowback]

Thanks for that good input! I've actually finished building my setup and it's humming along well. Working on a write-up to document everything.

Mine seems to output just slightly over 18A at times, but it's sized for higher if need be. I ended up using a 50A ANL fuse on the 6ga feed, and another inside the cabinet protecting the battery from a charge current overage. The feed from the charger is 8ga but less than a foot long; the 50A fuse might be a touch oversized for that 8ga feed, but it should be OK.

 

[01tundra]

Mine is in a truck/travel trailer application so I ran 4 AWG +/- from the truck battery back to the trailer with 175A Anderson plugs at the tongue and set power posts inside the trailer to take the wiring from 4 AWG to 6 AWG at the Dc charger.

 

[itllgrowback]

Very clean install! Looks outstanding.

[tmb]

 

[itllgrowback]

I finally did a write-up for my own system, and you can find it here:

Custom LiFePO4 Accessory Battery Cabinet and DC|DC Charging System - FJ Cruiser

I recently spent some time troubleshooting why my Dometic CFX3 55IM fridge wasn't performing like I thought it should. The long and short of that was voltage drop and the limits of the one battery; even with their upgraded 10ga hardwire kit and a good AGM battery, the setup was simply not able...

expeditionportal.com