DC-DC charger as an alternative to generator?

[gator70]

Yes, this is what's making me lean towards alternator charging. Trying to keep things as simple as possible and with as little equipment as possible.

Just went out and started my truck and checked the voltage at idle. Seems to stay right around 14.4v.

Or generator, they are cheaper and output twice the charge

 

[rruff]

TL;DR: Would like the ability to be "off grid" for at least 10 days and need the ability to keep batteries (2 x 100AH LiFePo4) at a reasonable SOC for that period.

ETA: Biggest draw is the 12v refrigerator/freezer.

Typically a fridge will use ~300Wh per day, but more if it's hot. Do you ever measure the energy you consume? I have a cheap monitor for this.

A LiFe 100A 12V battery will have at least 1200Wh of capacity, and draining them that low does not hurt them. IOW, there is no reason to charge them unless they are near empty. They weigh about 20 lbs, and I bought one recently with BT and cold temp cut off for $170 shipped. 3 of them should be enough to run your fridge for 10 days. Maybe add a 4th and call it good.

I'm just thinking that a DC-DC charger that you mainly only use while driving would be a good solution. Running your vehicle regularly when parked just to top off batteries, not so much. But having that as a fringe emergency measure would be fine.

 

[Dave in AZ]

one recently with BT and cold temp cut off for $170 shipped

Which was that, and do you recommend? I was about to buy one Will Prowse just tore apart and liked, WattCycle, but the day after his review price went from $207 to $250. Looking for a decent BT 100Ah mini. Thx

 

[rruff]

I haven't tested it as a bought it right before a trip, and now that its all snug and cozy I probably never will. It is a mini and the BT works... although it doesn't seem to get the watts right. Supposed to be a JB BMS. I like having BT for trouble shooting bad cells.

It's even cheaper now, $160 shipped with coupon. Since Amazon links are wacky I won't try, but search for this.

Elefast 12V 100Ah Bluetooth LiFePO4 Lithium Battery​

 

[Martinjmpr]

So a quick update: We are on the road now, currently in Key West camping at a site with no electric hookup.

Solar panel is on the “wrong side “ of the roof to get direct sunlight and at this point it wouldn’t make sense to try and rotate the trailer. Nevertheless according to the controller we are getting around 6 -7 AH per day off the solar (single 100w panel attached to the roof.)

Batteries got down below 20% last night according to the batteries app so I attached a charging cable from my Renogy 20a DC-DC charger.

But for some reason I can’t figure out, the batteries are only getting 4-8a each.

Screen shot of the app showing what I see:

I have two batteries so I can understand that the amperage will be halved but I would have expected each battery to be getting close to 10a from my 20a charger, right?

Not sure what I’m missing. Even if the fridge compressor is running that’s typically no more than 3.5 to 4a spread out over 2 batteries so if m DC-DC charger is actually putting out 20a, there should be a net 16a going into the two batteries, right?

And before anyone asks, yes the DC-DC charger is set for LIFEPO battery type.


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[burleyman]

Has it worked before, or is this a first time use? Do you have an activation (really small wire) connected to your vehicles 12v or another really small output current reducing wire connected somewhere?

Possibly disconnect the solar panels and see what happens.

Don't try the old jumper cable trick. Those hungry lifepo4 batteries look somewhat like a short to your vehicle's charging system and could be hazardous.

Do you expect a reply from someone getting set up to blow out camper water lines while you bask in FL?

 

[Martinjmpr]

To answer your first question, I have used the DC – DC charger for about four years, but up until now I have never been able to measure the output current. I got the DC – DC charger to keep my 12 V battery pack charged, that I plug the Truck refrigerator into. it has worked fine for that purpose for more than four years now.

So I know for certain that the charger is putting out power, but I don’t understand why it only seems to be putting out 5 A instead of 20.


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[Martinjmpr]

So let me throw another question out here for the group.

Let me start off by saying, I don’t think this will work. What I don’t understand, is WHY it won’t work.

Here goes: according to what I found online, because my F150 has heated seats, and the 3.5 V6, it comes from the factory with a 240 amp alternator.

Now this sounds stupid, but what if I got a 1000 W pure sine-wave inverter and attached it to the battery of the truck, so that it was receiving power when the engine is running and the alternator is on.

Unless something is off with my math, my 240 amp alternator should easily be able to power a 1000 W inverter, right? Even if you figure that the Truck is using 100 of those 240 A, that still leaves well over 100 A at 12 V (actually, when charging it’s typically putting out 13.5 to 14.5 V), which is well over 1000 W.

THEN what if I plugged my 30 amp 120 V AC smart charger into the inverter, an attach that to the 12 V battery?

I mean, it seems like the dumbest thing in the world to take 12 V of power, convert it to 120 V, and then convert it back to 12 V.

But the smart charger, rated for 30 A, even at a maximum charging voltage of 15 V, should be drawing no more than 450 to 500 W, correct?

So, an inverter rated for 1000 W should easily be able to power the smart charger, right?

As I said, there’s something that tells me this system would not work. What I don’t understand, is why it wouldn’t work?

What have I not taken into consideration, or not planned for?

This is basically the last thing I’m going to consider before I finally throw in the towel and just buy a generator.

Thanks for reading, and thanks for any insight anyone could give.


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[Dave in AZ]

So let me throw another question out here for the group.

Let me start off by saying, I don’t think this will work. What I don’t understand, is WHY it won’t work.

Here goes: according to what I found online, because my F150 has heated seats, and the 3.5 V6, it comes from the factory with a 240 amp alternator.

Now this sounds stupid, but what if I got a 1000 W pure sine-wave inverter and attached it to the battery of the truck, so that it was receiving power when the engine is running and the alternator is on.

Unless something is off with my math, my 240 amp alternator should easily be able to power a 1000 W inverter, right? Even if you figure that the Truck is using 100 of those 240 A, that still leaves well over 100 A at 12 V (actually, when charging it’s typically putting out 13.5 to 14.5 V), which is well over 1000 W.

THEN what if I plugged my 30 amp 120 V AC smart charger into the inverter, an attach that to the 12 V battery?

I mean, it seems like the dumbest thing in the world to take 12 V of power, convert it to 120 V, and then convert it back to 12 V.

But the smart charger, rated for 30 A, even at a maximum charging voltage of 15 V, should be drawing no more than 450 to 500 W, correct?

So, an inverter rated for 1000 W should easily be able to power the smart charger, right?

As I said, there’s something that tells me this system would not work. What I don’t understand, is why it wouldn’t work?

What have I not taken into consideration, or not planned for?

This is basically the last thing I’m going to consider before I finally throw in the towel and just buy a generator.

Thanks for reading, and thanks for any insight anyone could give.


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Yes, you can use an inverter to boost voltage up, to let you transport power in small wires, to point of use.

There are efficiency losses. They add up. They may still be acceptable vs cost of other equip, if your alternator is huge like you have.

Here is my 400W tacoma inverter, from my 130A Tacoma alternator , being used to charge a powerstation. Why? Because it was already there, I didnt have to run wires, and I could give a crap about losing 40W of power from conversions while driving.


I typed 5000 word answer, then gave up and deleted it, I can't tell what you're up to anymore, too hard to reread a 5 pages of posts.

Read and do this if you havent, it applies to sizing battery and inverter.

Power Required, Truck/Car camping

To determine what size powerstation or 2nd battery system you need for your camping or overlanding, the first thing required is to do an "energy audit" to figure out daily use, for both peak dc watts, 120Vac watts, and total Watt-hrs used for battery sizing. How you will recharge will determine...

forum.expeditionportal.com

 

[01tundra]

To answer your first question, I have used the DC – DC charger for about four years, but up until now I have never been able to measure the output current. I got the DC – DC charger to keep my 12 V battery pack charged, that I plug the Truck refrigerator into. it has worked fine for that purpose for more than four years now.

So I know for certain that the charger is putting out power, but I don’t understand why it only seems to be putting out 5 A instead of 20.


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What size wire do you have running from the vehicle battery to the charger and what's the approximate circuit length?

If your input wire is too small, you'll have excessive voltage drop and won't be able to pass the rated current through the charger.

That's why I have a 1/0 circuit ran from my truck battery back to the 50A charger in our trailer, we get full rated output current.

I've seen several people who utilized the factory vehicle harness get very little output from their chargers due to excessive voltage drop.

 

[Hnoroian]

Been reading since first posted and went through this a few years ago and wound up with a 100ah lifepo4. Started with solar (as I already had 200w), but we all know the trees were not helping the process, but when used in the desert it was fine. We have leaned on our 2500w inverter Gen, best of both worlds its dual fuel. I use a 20a charger to recharge the battery, It is both quiet and efficient.

 

[burleyman]

TL;DR. The last three paragraphs are important for me.

Crickets. Still traveling Mrtnjmpr?

I just purchased two of the Elefast batteries rruff posted above. I now know that the voltage seen on the bluetooth screen on mine is indeed the voltage seen from the charger.

Your screen shot showing 13.03 volts from your DC-DC charger is too low to provide enough charging current. Using a variable voltage power supply on mine shows very little charging amps until about the mid thirteens VDC. For some reason, your Renogy has low output voltage. It ain’t boosting. My bluetooth also shows battery temperature. If it’s too cold, don’t charge it.

Mjmpr said: “I attached a charging cable from my Renogy 20a DC-DC charger”. How? Trailer hitched or not?? 7 pin plugged to truck? Something other? I just replaced a 7 pin on a trailer cord because of a bad contact that limited charging current flow. The negative (ground) pin or metal must also support charging current. Also, what was said earlier about a trigger point/engine running/ignition sense input. Possibly on a small connector/plug on your dc-dc charger. It has to know.

Very much yes to Dave and another (disappeared) post. Typical extension cords from an inverter near the battery to the load works fine with enough alternator.
For me and my rural comrades, since the early nineties. 2000/4000 watts for a 120vac, 15amp source. Back then, mail a check and wait. Mainly for powering AC tools in the boonies before today's battery powered devices. Sometimes permanently installed. Just like being at home, except for having the engine running for some high start current things and an iffy battery. I've had one under a van for eight years. I installed two in engine compartments for others under duress. They gambled on the heat and won.

They were all Modified Sine Wave. I now own a 2000/4000 Pure Sine Wave inverter that will remain portable with connectors. Multiply or divide by ten. 120vac, 15 amps requires about 150 amps from the 12v battery/alternator. Big wire. Big, short ones to the inverter near the battery, and a typical extension cord to 120vac things.

After having a 120vac inverter or a small generator, the problem becomes how to charge the battery(s) if you don’t already have a camper/whatever with a shore power DC charging device (converter) for lifepo4. I’ve used inexpensive 30 amp switching power supplies with adjustable voltage output for years for lead-acid and 30ah lifepo4s. Adjust the voltage to the lowest setting, connect and turn on. Adjust the voltage while reading charge amps and increase to about 25 amps. Lead-acid will need some increased voltage as the battery charges. I’ve bought about ten so far, and they all adjusted up to the high fourteen volts DC. They can equalize a lead-acid and finish charging a lifepo4 from an non-lifepo4 camper converter.

For me, those 30 amp power supplies with no current limiting don’t work well with the 100ah lithiums without some monitoring and voltage adjusting. A minor tweak in voltage results in more output amps than the 30 amp power supply can handle. The Drok power supply in Dave’s video above seems to solve that with constant voltage and constant current adjustments.

Advice, please. Something about liquor and riding trains. I need/want up to about 100 amps of charging up to 15vdc or higher from shore power or a generator. A power supply with linear adjustments for constant volts and constant current are choice #1, for setting the levels wanted, followed by RV smart or dumb converters for plug and leave. Lots to choose from. Amazon reviews that say how well packaged and pretty it is don’t cut it. I’m not totally fixated on name brands. I’m old and jaded.

 

[gator70]

Dc to dc charger is too expensive to install.

 

[sancap]

Dc to dc charger is too expensive to install.

Could you explain? I installed mine in about 45 minutes using 6 gage wire, some terminals and simple circuit protection.

 

[gator70]

Could you explain? I installed mine in about 45 minutes using 6 gage wire, some terminals and simple circuit protection.

Did you add the second alternator - and the second lead acid battery?

As you know a direct dc to dc charge to lithium can be dangerous.