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Discussion Starter · #1 ·
I have my van fully built so adding a DC to DC charger is possible, but not ideal. Also fairly expensive. Particularly for the rare situations where my solar can't keep up with my usage.

So... was thinking is there any reason I can't mount a small inverter up front and run a 120v power cord to my existing inverter/charger that's all ready to charge my batteries?

Figure a small 300w inverter could be mounted under the cup holders and permanently hooked up to the battery. I could then plug into that.

I know there are a lot of losses going from 12v to 120v and back to 12v, but that shouldn't be a big deal. With 300w I should still get 20 amps or so per hour charge. Probably could up that to a 500w inverter if it fits.

Thoughts? Am I missing anything? Yes, in my build I'd have to run a 120v extention cord from front to back but that's ok. It would only be while driving and only when absolutely necessary. Most of the time solar is fine. I figure this would cost $100 vs much more for a DC to DC charger and large Guage wire id have to run under the van.
 

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It’s theoretically possible, and I’ve tried it with twice, with two different inverters feeding my multiplus. The first time was with a cheap non-psw inverter. The 2nd time I used a nice Victron unit. I never got it to work consistently. The inverters feeding the multiplus always tripped fairly quickly.


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You'd need to use an inverter large enough to provide the draw your other charger expects. The losses aren't a big deal. Orton did this awhile ago to create his own dc-dc charger using 2 inverters. The first also heated water while driving. 300w is probably going to trip or cycle constantly. 500w may just be large enough depending on what kind of power your charger draws (probably producing 30-50a dc with like ~4-7a ac draw?). A ciggy plug device is not going to cut it. Even if the inverter says it has a higher maximum output, the ciggy plug does not. Generally 10a maximum

Edit: disregard the ciggy plug thing since I see you did specify direct connection to battery. Leaving it there in case it's applicable in a different case.
 

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Discussion Starter · #4 ·
Adding one more data point, my xantrex inverter/charger allows me to change the charge rate. I can dial it back to compensate for whatever size inverter I buy.
 

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I have my van fully built so adding a DC to DC charger is possible, but not ideal. Also fairly expensive. Particularly for the rare situations where my solar can't keep up with my usage.

So... was thinking is there any reason I can't mount a small inverter up front and run a 120v power cord to my existing inverter/charger that's all ready to charge my batteries?

Figure a small 300w inverter could be mounted under the cup holders and permanently hooked up to the battery. I could then plug into that.

I know there are a lot of losses going from 12v to 120v and back to 12v, but that shouldn't be a big deal. With 300w I should still get 20 amps or so per hour charge. Probably could up that to a 500w inverter if it fits.

Thoughts? Am I missing anything? Yes, in my build I'd have to run a 120v extention cord from front to back but that's ok. It would only be while driving and only when absolutely necessary. Most of the time solar is fine. I figure this would cost $100 vs much more for a DC to DC charger and large Guage wire id have to run under the van.
I asked a very similar question on the Victron technical site. Apparently there is a big problem synchronizing the two AC signals when you attach an inverter to MultiPlus or other inverter charger. Only really clean power sources will work without derating the Victron inverters to take dirty power.

Also ... a 300W inverter will take about a 30 amp draw. That will require special wiring and fusing in any case, so it's probably not a super easy install in any case.
 

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Discussion Starter · #6 ·
Hmm. Interesting point on the clean power issue. Wonder if xantrex is the same? Certainly would take out any non pure sine wave inverter option.

Regarding fusing, I'm sitting in a foot of snow 20 miles from my van so imagining all this in my head. I figure there is a way to get this done cleanly.
 

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It’s theoretically possible, and I’ve tried it with twice, with two different inverters feeding my multiplus. The first time was with a cheap non-psw inverter. The 2nd time I used a nice Victron unit. I never got it to work consistently. The inverters feeding the multiplus always tripped fairly quickly.


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It is going to trip unless you set the multiplus to draw much less because the chargers on the multiplus are very large. You either do this through the Mk3 VE Bus to USB adapter with victron connect software or the Multicontrol panel. You can adjust the maximum AC power the multiplus will draw using either method. If you want to also use shore power my recommendation would be the multicontrol panel. Changing the settings via a laptop and Mk3 all the time would be a big hassle.
 

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Orton's website with documentation is here:

I am doing this to charge a 1.1 kwh 48v lithium ebike battery via the alternator while driving, the battery just uses a 3 amp 110v charger (takes forever to charge) and im running it off an inexpensive cigarette lighter plug inverter. It works! I use the 48v lithium ebike battery to power my van electrical system with a 48v to 12v buck converter when i need it.

I have a 20 amp pro mariner (same company as sterling) charger permanenty mounted for shore charging that I plan to wire to charge instead of using pure alternator charging. The promariner charger has specific AGM charging profile and 20 amps is plenty for my single 100ah AGM.
 

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Why not just run power from your alternator directly to your battery with a fuse and an on/off switch. Should be fine for intermittent use.
Dissimilar batteries, Dissimilar charge rates, blah blah blah. Your not running it all the time.
 

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Orton's website with documentation is here:

I am doing this to charge a 1.1 kwh 48v lithium ebike battery via the alternator while driving, the battery just uses a 3 amp 110v charger (takes forever to charge) and im running it off an inexpensive cigarette lighter plug inverter. It works! I use the 48v lithium ebike battery to power my van electrical system with a 48v to 12v buck converter when i need it.

I have a 20 amp pro mariner (same company as sterling) charger permanenty mounted for shore charging that I plan to wire to charge instead of using pure alternator charging. The promariner charger has specific AGM charging profile and 20 amps is plenty for my single 100ah AGM.
You definitely sidestep one of the big technical issues when you don't connect to an inverter charger, but just to a charger.
 

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Discussion Starter · #12 ·
Charging 3 battle borne lithiums so don't really want to run a wire direct. Plus that would be a monster sized wire with fuses and the like. Would need to be lugged and connected all the time.

With the 120v extension cord I could just unplug it.
 

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I don't see the higher harmonic distortion of a msw affecting the charger much, or people would complain about using non-inverter generators. It'll use slightly more power and heat. It's just being converted again anyway. Also sounds annoying swapping charge settings for smaller inverter/shore, but would work. If I recall correctly (I didn't re-read despite the helpful link posted), orton used a msw to plug the psw charger into. This effectively creates step transformers allowing smaller gauge wire for a long distance. It's common it other applications. It's a good thought.
 

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This sounds overly complex instead of just running a 4-6awg wire with a fuse on the starter battery side and installing a DC-DC charger with a fuse or breaker on the output. You've got expensive equipment with the Xantrex and BB batteries. What's another $200-250 for a Kisae DMT-1230 (30 amp) or DMT-1250 (50 amp). You're converting DC to AC back to DC. A cheap 300-500w inverter is going to have an 85-90% efficiency rate, then the charger on your inverter/charger has a loss too, but not usually advertised.
 

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Discussion Starter · #15 ·
This sounds overly complex instead of just running a 4-6awg wire with a fuse on the starter battery side and installing a DC-DC charger with a fuse or breaker on the output. You've got expensive equipment with the Xantrex and BB batteries. What's another $200-250 for a Kisae DMT-1230 (30 amp) or DMT-1250 (50 amp). You're converting DC to AC back to DC. A cheap 300-500w inverter is going to have an 85-90% efficiency rate, then the charger on your inverter/charger has a loss too, but not usually advertised.
The problem for me is running the wires post build and finding space for the DC to DC converter. At this point I'd have to run permanent wires under the van. With an extension cord I can just run it when necessary. Which really isn't that often.

Probably just continue to forgo the whole thing. Worst case if the sun doesn't shine for a few days I'll find a campground.
 

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I don't think the conversion losses are an issue since it's the alternator rather than storage. I see the sense in keeping the equipment in the cab space and running smaller gauge and removable wiring. My only concern was if the second inverter couldn't keep up with the draw it's wasted effort. It's been done and gives you the option of 120v upfront also during the drive. It's commonly done in large scale commercial settings using transforms to up the voltage to 277 or 480 and then back down to power high loads far from the service. The benefits of smaller wires is not trivial.
 

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The big reason to do this USED to be that it was quite a bit cheaper to buy a 110v charger plus cheap inverter (maybe $175) vs. a sterling B2B. Now I see that the renogy 20a DC to DC charger is only $110. The renogy wasn't available a few years ago and the sterling units are at least $300ish.
 

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The bigger concern, in this instance, seems to be space rather than cost. Different needs arise to add or remodel than during original construction. I'd also be surprised if it wasn't more cost for the larger wire and charger than for a small inverter and the extension cord likely already being carried.

Putting the dc-dc charger up front needs much larger wire, plus likely oversized to combat the voltage drop since it should be located near the house batteries. Large wire, possibly smaller than with the charger upfront, still needs to be run if space was made to place it in the rear by the house bank. It doesn't sound like there is easy space to run this wire.

Inverters are also cheaper than they once were, especially smaller msw ones. Either case gives multistage charging. Maybe it's less efficient, but it's spare alternator juice not battery storage so it doesn't really matter in my mind. You're not 'wasting' anything. The dc-dc is probably the best option if you're starting from scratch. The second inverter is definitely an option for an after the fact add-on.
 

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Do you have the upfitter power connector? What's under your driver seat? My thought is you could splice into that 4 AWG wire under the driver side step like I did, but instead of running it back to a DC-DC you could put a battery switch and an inverter there. If you have access from the back to the underneath, it would simply become just a secondary inverter for AC use in the front of the living area. (Don't know enough about your layout; just 'thinking out loud').

I have two BB 100 Ah LifePOs and use a DMT-1250. It was explained to me that there is typically a current surge on ignition that can damage a LifePO so a typical isolator scenario common for AGMs isn't recommended. I don't know that this is true, but I got a decent price and wanted the solar option with the included MPPT. (Totally understand your build constraints though... sometimes I feel like I spend more time undoing and revising my build than getting it done!).
 

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Discussion Starter · #20 ·
Do you have the upfitter power connector? What's under your driver seat? My thought is you could splice into that 4 AWG wire under the driver side step like I did, but instead of running it back to a DC-DC you could put a battery switch and an inverter there. If you have access from the back to the underneath, it would simply become just a secondary inverter for AC use in the front of the living area. (Don't know enough about your layout; just 'thinking out loud').

I have two BB 100 Ah LifePOs and use a DMT-1250. It was explained to me that there is typically a current surge on ignition that can damage a LifePO so a typical isolator scenario common for AGMs isn't recommended. I don't know that this is true, but I got a decent price and wanted the solar option with the included MPPT. (Totally understand your build constraints though... sometimes I feel like I spend more time undoing and revising my build than getting it done!).
Thats not a bad idea. I do have access to under the driver's seat. I would have to pick up a negative lead somewhere as well.
 
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