Inverter on tow vehicle to charger in rv

[Rainier70]

I was wondering if anyone does this, and what the problems would be? I have tried search but got too much semi-related stuff to sort through.

I was thinking of about a 500 to 800 watt inverter off of my truck battery to power a 15 amp battery charger connected to my 220 amp hr golf cart batteries. The main question I have is: will a modified sine wave inverter run a digital charger or do I need a pure sine wave inverter?

The charger I am looking at is a Schumacher SC1359 15A 6/12V DOE Battery Charger

https://www.amazon.com/gp/product/B0789 ... 0DER&psc=1

[GTS225]

Oh, my. Now you're looking for engineering answers. It might be best to contact the manufacturer of the charger.

Roger

[absolutsnwbrdr]

You're over-complicating things. Search "charging while towing" on here, and you should see what you're looking for.

Theres no need to go from 12VDC at your tow vehicle alternator (battery) - to 120VAC at the inverter - back to 12VDC at the charger - to your 12V batteries in your camper. Skip all that nonsense and hook up directly - 12VDC to 12VDC.

You can wire a relay inline that will disconnect when the tow vehicle turns off, or there's devices that you can buy to do that - called battery isolators or battery separators.

*edit* I just looked up one that I used on a build a few years ago, and it was a Yandina 'Battery Combiner'. Worked great.

[daveesl77]

It is not necessary to do the inverter/charger when coming from your truck's 12v system to your camper's 12v system. Not a good idea to have a 120v line flopping around outside, keep it all at 12v. Just do the connection to where the truck can charge the trailer. There are a couple of safety things you should do.

1) Put in a fused system from the truck's battery system to the trailer connector.
2) Put in either a "charging control switch" or relay to shut down the connection when the truck is off.
3) Put in an isolator so you can keep the two systems apart.

Size your wiring to what you think the charge rate might hit, say 10amp. #10 or #12 wire will be sufficient, #8 better. The larger the wire, the less resistance you have for the run length vs the voltage/amperage. You might have a 100 amp alternator, but it isn't putting 100 amp into the trailer, so you don't need welding cables.

In many cases, if using a pickup, the truck may well have, built in, the complete power hookup for a trailer. My old F150 did, all I had to do was plug in the factory provided relay on the truck side, which automatically disconnected the line when the ignition was off. Check your owner manual

dave

[Andrew Herrick]

+1 what other people have said.

The other thing to know is ... due to voltage drop in the wiring between the tow vehicle battery and the trailer battery, a simple DC-to-DC parallel connection won't often fully charge the trailer battery. This is especially true if A) you're running AGM or Gel batteries and/or B) someone used 10-gauge wiring for the auxiliary charging line, which is allowed but really too small, as another posted has pointed out. Voltage drop in 12v cable is heinous.

If you want the best of the best vehicle-to-trailer charging system, you need a DC-DC battery charger, like Redarc. It mounts in your tow vehicle. Expect to pay $350+, though

[Pmullen503]

The RC world has developed chargers for Lipo batteries that are DC-DC. You'd need a 200W charger similar to this:https://www.amazon.com/Battery-Balance-Charger-Discharger-Digital/dp/B07HQ651SB/ref=sr_1_13?s=toys-and-games&ie=UTF8&qid=1545325567&sr=1-13&keywords=200W+lipo+charger Most have a lead acid setting. You could use the same charger with a 12V power supply when you have shore power.

[Andrew Herrick]

The RC world has developed chargers for Lipo batteries that are DC-DC. You'd need a 200W charger similar to this:https://www.amazon.com/Battery-Balance-Charger-Discharger-Digital/dp/B07HQ651SB/ref=sr_1_13?s=toys-and-games&ie=UTF8&qid=1545325567&sr=1-13&keywords=200W+lipo+charger Most have a lead acid setting. You could use the same charger with a 12V power supply when you have shore power.

So, I had never heard of charging an automotive battery with an RC charger. So I did some research. Here's a thread I found helpful:

https://www.rcgroups.com/forums/showthr ... rs-battery

It sounds like it CAN work, but it's not optimal. Not enough amperage to charge a battery quickly, and the wrong charging algorithm for most automotive lead-acid batteries.

PMullen, I'd love to hear more about your idea. Why do you prefer this DC-to-DC RC charger over an auxiliary 12v charge line (as recommended by others) and/or a dedicated 120v battery charger (when on the grid)?

[MtnDon]

I think I see a potential upside to using an inverter in the TV and then a battery charger in the trailer for the trailer battery. If we run heavy gauge wires (pos and neg) from the TV to the trailer to charge the trailer battery is there not a danger of overcharging the TV battery if the trailer battery needs lots of charge?

Assuming the TV battery is near to full charge as a good TV battery should be most of the time, and that the trailer battery is low because we've been camping a day or two, when we take off down the road does the energy being put out by the TV alternator know to go mostly to the battery that needs charging? That would be the battery in the trailer. A battery that needs a good amount of charge needs a higher voltage applied along with the higher current. The TV battery, fully charged after driving a few miles, does not need as high a voltage nor much current.

The regulation system built into the TV alternator cannot do two different things at the same time, can it? So what happens? what battery does the TV alternator regulator read? It seems to me that one battery is not going to be getting its optimal charge. Will the TV battery be overcharged or the trailer battery be undercharged?

That's why I can see the potential value in what seems like an overly complicated idea. Thoughts?

[Andrew Herrick]

I think I see a potential upside to using an inverter in the TV and then a battery charger in the trailer for the trailer battery. If we run heavy gauge wires (pos and neg) from the TV to the trailer to charge the trailer battery is there not a danger of overcharging the TV battery if the trailer battery needs lots of charge?

Assuming the TV battery is near to full charge as a good TV battery should be most of the time, and that the trailer battery is low because we've been camping a day or two, when we take off down the road does the energy being put out by the TV alternator know to go mostly to the battery that needs charging? That would be the battery in the trailer. A battery that needs a good amount of charge needs a higher voltage applied along with the higher current. The TV battery, fully charged after driving a few miles, does not need as high a voltage nor much current.

The regulation system built into the TV alternator cannot do two different things at the same time, can it? So what happens? what battery does the TV alternator regulator read? It seems to me that one battery is not going to be getting its optimal charge. Will the TV battery be overcharged or the trailer battery be undercharged?

That's why I can see the potential value in what seems like an overly complicated idea. Thoughts?

The trailer battery will be undercharged. An auxiliary charging line will usually only get a trailer battery to 70-80% of total capacity, or just 40-60% of usable capacity. This is especially the case if you're using AGM or gel batteries which require higher peak charge voltages.

Lots of information online if you want to know more. See here for example: https://www.caravanworld.com.au/feature ... -explained

However, I don't think that problem makes a 120-volt inverter the best idea ... htere's a lot that could go wrong trying to run a 120-volt inverter off dual 12v power sources (alternator and starter battery) ... the best solution is an in-vehicle dual battery charger from a good manufacturer such as CTEK or Redarc.

[Pmullen503]

...........

It sounds like it CAN work, but it's not optimal. Not enough amperage to charge a battery quickly, and the wrong charging algorithm for most automotive lead-acid batteries.

PMullen, I'd love to hear more about your idea. Why do you prefer this DC-to-DC RC charger over an auxiliary 12v charge line (as recommended by others) and/or a dedicated 120v battery charger (when on the grid)?

I'm not saying that's the best way to do it, just better than trying an inverter with a 12V battery charger. There are good, robust systems for charging your RV battery from the TV so why reinvent the wheel?

That said, I use my RC chargers to charge my lipos as I drive and I know people who charge their deep cycles with their RC charger. It's pretty common to use deep cycle batteries to power RC chargers at the flying field so many chargers are equipped to charge the deep cycle when 120V is available. Mine has settings for gel, wet and AGM batteries so I assume the manufacturer has programed in the correct charge profiles for those.

[Rainier70]

Thanks for all the great replies.

The DC to DC RC charger looks interesting, but way too low of amperage.

And yes I do already have charging while towing. It is too slow charging unless I was driving all day and half the night.

I could use a direct connection with battery cables from the tow vehicle, but there are a couple of problems with that. One is if the TV battery is full and two is the unregulated charge to the RV batteries.

The way I am thinking is that the TV alternator will see the load of the inverter and adjust the amps for it. The charger will then adjust the volts and amps for the RV batteries being charged. I might be missing some points (or marbles) that I haven't thought about.

I have solar that charges the RV batteries regularly, but there are still those times when it is cloudy for longer periods than I like. I could just pack a generator and a charger.

This was just something that I thought of and was curious about as it would be less to pack than a generator, but I am thinking that the generator would be way less fuel. So I will probably go that route anyway.

[MtnDon]

I thought there was somebody on the forum who had already done just what you are thinking of, but I can't find it. Maybe I'm dreaming or confused about where I saw it. As I think I recall, the person used it mainly while traveling. If the intent was to also use such a system while parked I would only recommend doing that as a last resort and only if a generator and charger was not available. It would be a fuel waste to run a TV engine while parked.

[KTM_Guy]

My question is he said golf cart batteries, those are usually 6 volts. I'm sure they are wired is searies for 12 volts. But what I have been reading is you should charge your batteries at the battery rated voltage. Noco who makes high end chargers even says to charge batteries wired in parallel should be charged individually.

Over at some of the overland sites people are having problems with battery life because their high end AGM batteries are not getting fully charged. A battery getting charged to 80% over and over is just as bad as running them too low. So they do tricks like putting in a diode to fool the charge computer into thinking it needs to put out more voltage. I really doubt car manufactures are thinking of charging a big deep cycle battery when they design their electrical systems. I do not want to play around with my electrical system that may or may not cause a problem when I'm in the middle of no where.

I do not see the problem with running an inverter and battery charger. It might not be efficient but you will get a better charge on the battery. And does efficiency matter when you have an endless supply of power? I just checked my 10 amp Noco and it draws 140w on 120v, out put is 120w at 12v. My Jeep has a 300 watt (I think) inverter built in, no problem running a charge with that. I have no problem running 120v back to the trailer, a few zip tie and all good. Or keep the charger in the TV and run back low voltage wires no deferent than running off the battery.

I would look into a charger made for deep cycle batteries like the Noco's. If you are discharging your batteries where you need a 15 amp charger to get them back up maybe look into a generator. The new inverter generators are nice, small, cheap and quiet. My brothers has one one of the HF ones and loves it. He hasn't used his Honda since.

[tony.latham]

...because their high end AGM batteries are not getting fully charged...

I'm working out the details for my next build and I've got a little man that's sitting on my shoulder whispering, "AGM, AGM, AGM." I just told him to keep his elf lips zipped.

I've had no problems after four (or five?) years on my current lead-acid marine battery --charging while on the move from my rig and from the sun while parked.

Simple is good.

Tony

[MtnDon]

My question is he said golf cart batteries, those are usually 6 volts. I'm sure they are wired is series for 12 volts. But what I have been reading is you should charge your batteries at the battery rated voltage. Noco who makes high end chargers even says to charge batteries wired in parallel should be charged individually.

Lead acid batteries in series connection are no problem to charging. That comes from years of off grid experience and a friend who has been building off grid systems for 25+ years. Parallel connections can have problems due to differences in internal resistance between batteries. Note that I started lead acid batteries. The typical flooded golf cart battery is quite forgiving to slight overcharges but not to being undercharged and never or seldom getting to 100%. AGM and other sealed lead acid batteries can be damaged by both over and frequent undercharging.