DC-DC charger as an alternative to generator?

burleyman

Active member
Looks great. I've also seen that my camper's inverter (charger) outputs 13.3vdc most of the time. Not enough.

I'm curious, can you tell if your engine grunts or changes rpm when beginning charging? Mine doesn't, but some have in my past experience.

Waiting around for Lifepo4 prices to fall and to also include bluetooth monitoring is definitely a plus.

More redneckedness years ago. Permanent inverter behind front grill, permanent extension cord to rear. 12v power supply adjusted to 14.4v installed in box on camper tongue. Charging while driving. Inverter turned on and off with continuous duty relay via a toggle switch.

Thanks for the posts and pictures.
 

Martinjmpr

Wiffleball Batter
Looks great. I've also seen that my camper's inverter (charger) outputs 13.3vdc most of the time. Not enough.

I'm curious, can you tell if your engine grunts or changes rpm when beginning charging? Mine doesn't, but some have in my past experience.
I didn't notice any change in idle speed on the truck while the inverter was running. It's a 230A alternator and the inverter is only pulling about 33A so I can't imagine it would have much effect. Obviously I'm not running a lot of other stuff at the same time I'm charging the battery.
 

Martinjmpr

Wiffleball Batter
More redneckedness years ago. Permanent inverter behind front grill, permanent extension cord to rear. 12v power supply adjusted to 14.4v installed in box on camper tongue. Charging while driving. Inverter turned on and off with continuous duty relay via a toggle switch.

As we used to say in the Army, "If it's stupid but it works, it isn't stupid."

My only concern about what you posted above is having the inverter permanently mounted behind the grill. Did you do anything to waterproof or protect it? I would be worried about rain, dirt, etc, getting into the electronic parts of the inverter.
 

Martinjmpr

Wiffleball Batter
Assuming 100% efficiency, 33A x 14.2V = 487W divided by 746W per hp = 0.63hp.
Cheers,
Peter
OKA196 motorhome

Also you have to remember that using the inverter to charge the camper battery is something that only happens when we are stopped. I don't use this setup when we are moving down the road.

In theory I suppose I could use a setup something like this while moving (as Burleyman did above.) As long as all the sensitive electronic parts could be protected from the extremes of weather I think it would work but in my case, at least, it's simply not necessary.

This coming Summer will be our first "full" summer with the trailer and our first Summer with the LiFePo batteries (I didn't install them until November of last year.) Now that I have a way to monitor the power draw (via the Bluetooth app for the batteries) I'll have a better idea of how much power our 12v fridge actually draws (the 12v fridge is the main power draw in the trailer, everything else is negligible.) Our previous trailer had a 3 way (120v/12v/propane) absorption fridge so getting used to this new-to-us large compressor fridge will have a bit of a learning curve. The fridge itself also needs some adjustment as it is over-cooling by quite a bit and freezing our food (and of course drawing an excess amount of power while it does.)

As with anything else in life, learning how to make everything work is a constant process.
 

rruff

Explorer
Now that I have a way to monitor the power draw (via the Bluetooth app for the batteries) I'll have a better idea of how much power our 12v fridge actually draws (the 12v fridge is the main power draw in the trailer, everything else is negligible.)
I bought a cheap battery with BT last fall, but it doesn't seem very accurate regarding power. A calibrated shunt and meter are needed I think. The one I bought was ~$35. You need to measure energy used over time, not just what it consumes when it kicks on. For room temp conditions a portable fridge typically consumes ~350 Whr/day but some are a little better than others.
 

Martinjmpr

Wiffleball Batter
Well, the project is finally "Done" done.

Obviously having to connect the 120vAC battery charger to the camper using alligator clips was not a viable long-term solution. I didn't want to have to pop the top off my battery cover every time I charged them so I made a small "pigtail" with a couple of ring terminals and an Anderson connector that will hang outside the battery box and allow me to keep the battery box closed and locked (double padlocked.)

2025-02-19 13.06.49.jpg

Of course, this meant that I could no longer connect the charger to the batteries using the alligator clips so I snipped them off and put an anderson connector on the charger.

2025-02-19 12.59.49.jpg

But, since I could envision sometimes needing the alligator clips, I went ahead and put them on an Anderson connector as well. So if I need alligator clips, I'll just click them onto the connector on the charger. I hooked it up to "bench test" and make sure it worked.

2025-02-19 13.49.49.jpg

26.2 amps going into the battery.

Next it was time to hook up the whole system with the inverter. Here I ran into a curious situation.

Here's a photo of the charger. It's hard to see due to the sunlight, but the charger is showing that it's putting out 29.9

A to the two batteries:


2025-02-14 15.46.43.jpg

And yet...when I pull up the BMS app on my phone, this is what I see:

2025-02-19 14.18.12.jpg

Now previously when I tried this setup, I was getting 15A to each battery, which makes sense since the charger is a 30A charger.

But only showing 10A when the charger shows that it's putting out a full 30A was puzzling to me. Literally the only thing I did differently was use Anderson connectors.

After about 8 minutes the result was still the same: The batteries were only receiving just under 10A while the charger was putting out the full 30A:

2025-02-19 14.26.37.jpg

I racked my brain trying to figure out what happened. Had I failed to tighten up one of the bolts? Was a loose bolt causing the amp loss? I even felt the cable with my hand to see if it was heating up, thinking that energy had to be going somewhere.

I decided I would have to open up the battery box to check the connections. But I didn't want to do it today because it was SO DAMN COLD OUTSIDE! It only got up to about 18f (-8c) yesterday, just bitter cold. My hands were numb and I was tired of being outside so I said "I'll check it when it warms up."

It wasn't until I was getting ready for bed that I realized something: These are SELF HEATING batteries. The display showed a battery temperature of 10c/50f but in fact it was well below freezing outside.

So my conclusion is that the reason the batteries were not getting the full 30A of charge is because a portion of that energy was being bled off to keep the batteries warm with the built in self-heater.

It's another bitter cold day today, but it's supposed to start warming up tomorrow. Once the weather gets above freezing, I'll check the charging capacity again, my guess is that it will go back to the batteries able to get a full 30A out of the charger once the self-heaters aren't being used.
 

Martinjmpr

Wiffleball Batter
Lets hope that exposed pigtail wiring is properly fused.
Otherwise the battery gods will be mad.

Eh, I don't know that I see that as being necessary. I mean I COULD put a big 40A or 50A fuse block on it where it connects to the battery but why? I mean, I guess in theory the Anderson connector could break if I hit a bump and ground the positive to the trailer frame but I think that extremely unlikely. Or the insulation on the 8AWG cable could wear through. But I think either of those is pretty unlikely to happen with the way we use the trailer.

The whole pigtail assembly is maybe 18" long. It's not going to whip around and get caught underneath the frame of the trailer.

EDIT: After giving it some thought I think a fuse would probably be a good idea. I'll wire in an in-line fuse holder and put a 40A blade fuse in there. Thanks for the advice!
 
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Pacific Northwest yetti

Expedition Medic
A fuse/ or you could use a resettable breaker is a good idea. I will say I have Andersons front and rear of my 2004 farm truck. And they have been there for 200kmiles and never had an issue. But hope it not a great plan. And a fuse/breaker is cheap insurance
 

burleyman

Active member
Yes, it was your review that caused me to purchase two Elefast lifepo4 batteries as mentioned in a prior post. Thanks.
 
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burleyman

Active member
Martinjmper, the 13.32 volts seen on your charging screenshot is curious. My testing with an adjustable voltage power supply shows low charging amps until charging voltage seen on the bluetooth screen rises to about 13.8vdc. My batteries don't fully charge without voltage in the low fourteens.
 
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burleyman

Active member
^^^^^Huh Oh! Wrong info in that post. The bluetooth voltage on mine is the voltage of the battery at present. It is NOT the charging voltage applied as mentioned above. You would need to test the charger voltage measured at the battery terminals to see if the charger voltage is high enough for charging. Smart charger maybe not. Adjustable charger below at 14.3vdc. 14.1vdc at the battery terminals

The pictures below show the difference. Values aren't identical, but ballpark.
 

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