Can I power my 30amp RV from my truck alternator

[MailMacDaddy]

Disclaimer: big time newbie but excited to learn as much as possible.

Here is what I have:
2020 7.3L gas F250 with a 240 amp alternator
Renogy 60 amp DC to DC charger
30 amp Airstream
2,000 watt inverter
270 watts of Zamp Solar
2 - 100ah AGM batteries (Lithium to replace shortly)

Is it possible to run my 30 amp rv using my truck while boon docking? I’m trying to achieve the same as though my RV was hooked up to shore power.

I am open to any suggestions that might help me achieve this.

 

[snoobler]

Welcome to the forum.

30A input requires 120VAC input. That's 3600W.

Your truck only puts out 240A*12V = 2880W. Furthermore, running your alternator at max output at idle will do 1 of two things, 1) the "smart" alternator will see that it's overheating and cut back its output to save itself, or 2) the "not smart" alternator will burn up. The wiring required to get 2880W of 12V to your trailer would likely cost more than a generator.

Your truck would best be used to haul a $400 generator that can run the RV. The generator would use less fuel/Wh of energy since the engine is very small compared to the energy required to keep a massive diesel idling while only producing about 4hp (assuming the alternator doesn't burn up).

 

[MailMacDaddy]

Snoobler,

I totally forgot about multiplying volts x amps. HUGE oversight and thank you for pointing that out.

You bring up a good question about wiring. I’ve seen a lot of videos where people are taking 1 gauge copper coated tin 30 ft jumper cables and cutting off the ends to hook up between the battery and the B2Bcharger then to the battery bank. What size wire would you recommend and would it matter putting the B2B charger near the truck battery or put it in the RV? I know wire is expensive but I am willing to pay the cost for safety. Also, is it overkill to use a 60 amp B2B charger or would you go with a smaller one? I just thought the bigger it is the faster it will replenish the batteries especially lithium.

Here is the jumper cables I’m referring to: https://www.amazon.com/gp/product/B083LV1BNJ/ref=ppx_yo_dt_b_asin_title_o07_s00?ie=UTF8&psc=1


I really appreciate your insight. Not what I was hoping to hear but glad you cleared it up for me.

 

[Pnw_steve]

SnooblerI said it well.

One thing I would suggest is that you consider going with an inverter generator. I dug out my old paintball rifle for this coming camping season especially for the fellow campers who show up with cheap open frame generators.

I got run out of a campsite last time we camped at the river. Dipstick down the way ran his cheap generator all night. I don't know how he slept cause the rest of us didn't......

I ran a Honda for 13+ years. Part of that we were off grid and ran it every day. It finally started to give me a bit of trouble so I bought a Champion as a backup. It is not as quiet or as powerful as the Honda but it is in the ballpark. I do love the remote control.

 

[snoobler]

Snoobler,

I totally forgot about multiplying volts x amps. HUGE oversight and thank you for pointing that out.

You bring up a good question about wiring. I’ve seen a lot of videos where people are taking 1 gauge copper coated tin 30 ft jumper cables and cutting off the ends to hook up between the battery and the B2Bcharger then to the battery bank. What size wire would you recommend and would it matter putting the B2B charger near the truck battery or put it in the RV? I know wire is expensive but I am willing to pay the cost for safety. Also, is it overkill to use a 60 amp B2B charger or would you go with a smaller one? I just thought the bigger it is the faster it will replenish the batteries especially lithium.

Here is the jumper cables I’m referring to: https://www.amazon.com/gp/product/B083LV1BNJ/ref=ppx_yo_dt_b_asin_title_o07_s00?ie=UTF8&psc=1


I really appreciate your insight. Not what I was hoping to hear but glad you cleared it up for me.

Those are copper coated aluminum.

The nice thing about the B2B chargers is they are lower amps. 6awg wire @ 60A and 30' would have a 1.4V voltage drop. That means the alternator would see a higher voltage than what's being delivered to the B2B. It would start tapering its current to maintain the absorption voltage. As current tapers, voltage drop decreases, so it still works, just charges slower as the battery fills.

You can simulate your own scenarios:

Voltage Drop Calculator

This free voltage drop calculator estimates the voltage drop of an electrical circuit based on the wire size, distance, and anticipated load current.

www.calculator.net

That 1awg Aluminum cable will have a 0.7V voltage drop.

 

[John Frum]

Here is a slightly different take on the proposition.
Your 2000 watt inverter can deliver ~16.6A@120VAC full bore.
If you can live with that amount of power then read on.
2000 ac watts / .85 conversion efficiency / 12 volts = ~196 amps
If your truck has a high idle switch you might be able to do this.
Suggest you get a dc clamp meter and a temperature sensor to see how much current your vehicle can deliver at what alternator temperature at high idle.
Remember that your truck needs some alternator power too.
Suggest you put the inverter in the truck as close to the alternator as practical to keep the big thick wire runs as short as possible.
For 120VAC, distance is much less of an issue.

 

[JeepDaddy]

Good info so far.

I would add that alternator output is typically rated at around 6,000 rpm of the alternator shaft. Engine RPM will be lower than that based on the ratio of the serpentine pulley diameters.

You won't get near rated capacity at idle.

 

[John Frum]

Good info so far.

I would add that alternator output is typically rated at around 6,000 rpm of the alternator shaft. Engine RPM will be lower than that based on the ratio of the serpentine pulley diameters.

You won't get near rated capacity at idle.

I recall that some manufacturers publish an current by rpm graph.

 

[MailMacDaddy]

I really appreciate all the info you guys are providing. I only wish I fully understood. Still trying to learn this whole new language!

I’ve read that you should expect half the amp output at idle. And for about $500 I could install a 370 amp alternator. I do have a high idle switch on my truck. And I have no issue with upgrading my inverter to a 3,000 watt output. I’m also planning to install and soft start on my 15k a/c unit.

Back to the wiring. So basically the fatter the wire assuming it’s all copper the better, right? Is it possible and detrimental to have too fat of a cable?

At the end of the day I’m trying to be able to provide as much power as possible to enable off grid camping. Generators are loud and some camp sites won’t allow them to run late at night. But I could run my truck at idle for an hour or 2 in order to recharge for the night.

 

[FilterGuy]

Is it possible and detrimental to have too fat of a cable?

Define detrimental....?? Electrically, more copper is better. I can't think of a situation where a fatter cable is worse electrically. However, even with the best marine grade thin-strand cable, when you get to 2/0 it is a pain in the ass to route and may not even fit where you want it to go. Going bigger just makes it harder to deal with physically. Furthermore, when a heavy stiff cable is connected to something, if there is movement or vibration, the stiffness can put stress on whatever you have it connected to. At the extreme, if you put a heavy copper bar between two devices and those devices move in relation to each other..... something has to give.

Finally....more copper will be detrimental to your wallet.

Engineering is all about understanding trade-offs.

 

[Factory400]

A couple of things I did with a similar size travel trailer......

I got a soft-starter for the AC. This is a device that reduced the starting current needed to get you AC compressor started up. The BIG benefit is that
it can now run with no problem on a EU2200i Honda inverter genny.

EasyStart™ 364 Soft Starter

EasyStart™ single phase soft starter lets you start your A/C without buying a second or bigger generator or inverter & run 2 air conditioners on a 30-Amp cord.

www.microair.net

I also have a 2nd companion EU2200 that can run in parallel if I want to run the AC, microwave, and everything else at the same time. That arrangement essentially gives you a backup generator since you can easily just use one.

Powering from the truck has so many problems compared to 1 or two mini-gennies like the Honda.

I eventually got a large battery system and solar, but that is a whole new level of diving into the deep end.

 

[John Frum]

Back to the wiring. So basically the fatter the wire assuming it’s all copper the better, right? Is it possible and detrimental to have too fat of a cable?

The big big wires are required for the 12VDC side.

2000 ac watts / .85 conversion factor / 13.5 vdc = ~174 dc amps
~174 dc amps / .8 fuse headroom = 217 fuse amps

3000 ac watts / .85 conversion factor / 13.5 vdc = ~261 dc amps
~261 dc amps / .8 fuse headroom = 326 fuse amps

Have a look at the ampacity table here https://baymarinesupply.com//bosns_corner_wire_sizes

You want the column that says inside engine room 2-3 wires.

Then you need should look at the voltage drop calculator at the same link.

 

[Factory400]

I’ve read that you should expect half the amp output at idle. And for about $500 I could install a 370 amp alternator. I do have a high idle switch on my truck. And I have no issue with upgrading my inverter to a 3,000 watt output. I’m also planning to install and soft start on my 15k a/c unit.

Running 12V 300+AMPS any distance in a vehicle is:

1. Dangerous
2. Difficult
3. To manage voltage drop - massive cable will be needed.
4. The inverter needs to be as close to power source as practical - which is a pain in the butt.

Personally, I would never, ever even come close to considering the possibility of doing it this way.

 

[Hedges]

Your AGM batteries have 200 Ah x 12V = 2400 Wh of storage. If you drain to 75% DoD (doable, limited cycle life), that's 1800 Wh.
Can you get by on that much power per day?

When you drive, battery gets recharged. Need more than 150Ah to replenish.
If alternator can keep up with 60A DC-DC charger, 2.5 hours driving.
60A is 0.3C of battery, possibly more than it wants, possibly OK.
270W of PV - add more, it's cheap. Might produce 250W aimed at the sun, 1250 Wh/day.
Two panels could replenish in one day the 1800 Wh used.
Several panels could let you draw more during the day.

 

[MailMacDaddy]

You guys are killing me with all this good info. And I am so glad I joined this forum!

So what I’m hearing is my idea is a terrible idea. I’m ok with that and appreciate your input. So let me ask this question. Generally speaking do all of you think its a good idea to even use the truck alternator to charge the rv batteries while driving or just buy a generator?

 

[Factory400]

You guys are killing me with all this good info. And I am so glad I joined this forum!

So what I’m hearing is my idea is a terrible idea. I’m ok with that and appreciate your input. So let me ask this question. Generally speaking do all of you think its a good idea to even use the truck alternator to charge the rv batteries while driving or just buy a generator?

I run my generator while I am driving.

My entire power system is in the bed of the truck - I run the genny on eco mode and it limit the charging to about 700w so it is nice and quiet. Pretty simple. If my batteries, inverter, etc have any issues - I still have a genny (and a backup genny).

I love simplicity.

 

[Hedges]

You guys are killing me with all this good info. And I am so glad I joined this forum!

So what I’m hearing is my idea is a terrible idea. I’m ok with that and appreciate your input. So let me ask this question. Generally speaking do all of you think its a good idea to even use the truck alternator to charge the rv batteries while driving or just buy a generator?

What's important is that you don't burn up your alternator.
So long as that can be avoided (e.g. a DC-DC converter that doesn't draw too much), seems very convenient.

The closest I've come is a dual-battery isolator I put in a Saab. I soldered three flying wires off the positive diodes in my alternator and rectified with 3 more diodes to charge a battery for my trailer.

 

[Dzl]

You guys are killing me with all this good info. And I am so glad I joined this forum!

So what I’m hearing is my idea is a terrible idea. I’m ok with that and appreciate your input. So let me ask this question. Generally speaking do all of you think its a good idea to even use the truck alternator to charge the rv batteries while driving or just buy a generator?

To a large degree I think the answer to this heavily depends on context, how much will you drive? how much will you sit? how much power do you need to generate and what sort of usage pattern. The way lots of RV'ers live/travel, they will need a generator one way or another (or be tethered to shorepower most nights), in which case maybe its not worth spending extra money on alternator charging. Personally, I lived in a van for 2 years with alternator charging as the only charge source, but power needs were super modest (cell phone, LED lights, and a 12V fridge) and on average we drove 2-6 hours a day. Side note (I was very ignorant of battery best practices at the time and beat the living crap out of that battery, it was surprisingly resilient).

If your usage pattern is to drive to a campsite, camp for a week or a weekend, and drive home, alternator charging is probably not very useful. If you move everyday, maybe alternator charging would make it so you don't have to use the generator at all outside of emergencies or long stays (giving you and your neighbors more peace/quiet) If you drive everyday but have electric cooling (A/C) or heating (Heat pump or water heater) you might need the genny regardless.

Long story short, think through your expected (or actual) usage pattern, that should direct your design

 

[John Frum]

2x 100ah agm batteries are not going to run your 2000 watt inverter at or near capacity for more than a few minutes
That means no air conditioning.
No microwave.
Nothing even approaching a shore power experience.

 

[airmon]

I'm new to this forum, but have been a full time RVer for the last 6 years. We spend most of our time boondocking - 80% of our nights or more, we've gone for several months without being plugged in and often run the generator for exercise, not energy, so I know a bit about about this.
We're on our second RV with Lithium batteries, this one has 400Ah@12v and 780W of solar on the roof.

IMHO, living exactly like you're on shore power isn't really practical, even with a generator. Most of your daily activities, short of running the air conditioning, can be done happily off a moderate battery bank and some solar.
Higher demand items, like heating or cooling with electric, or running your propane/electric fridge is possible for a small amount of time, but even with a generator long term operation of those items becomes problematic since running a generator or idling a truck for hours on end has it's own set of issues ( noise, maintenance, fuel requirements, etc. ).

My suggestion would be to upgrade the batteries ( move them inside if you can ) and the solar. Zamp is pretty low end and 270W isn't a lot. I'd also add a battery meter with a shunt, once you have a good feel for how much energy you need in a day ( we use about 200Ah@12v daily, but a mostly sunny day is enough to put all of that back )
Then, on consecutive cloudy days, use the DC to DC charger to top off the batteries from the truck.

FWIW, we live a comfortable life, but don't live exactly like we can on shore power. On shore power, heat often comes from electric space heaters, while out in the boonies we use a catalytic propane heater and the built in RV furnace. The fan needs power, but we've got plenty for any reasonable night down into the teens.
We heat water in a pot and use a pour over coffee pot ( no electric coffee maker ) but frequently use an electric toaster oven for toasting or light baking needs. We use our microwave about as much as we want. My wife and I typically work at our laptops much of the day since we're not retired. At night, LED lights hardly draw any power at all and we've got a 34" TV with a sound bar that we waste too much time watching.
Hot weather means fans, move somewhere cooler or find somewhere with power. As I write this, the sun is just going below the point where it's generating any useful power, we're at 95% even after I used about 15% to run the water heater for dishes.

Good luck and safe travels!