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 > [SOLVED] Can an inverter on a car charge the house battery?

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SlowBro

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Posted: 01/04/21 09:08am Link  |  Quote  |  Print  |  Notify Moderator

Gdetrailer wrote:

At least you do now realize the gen you already have will use far less fuel..


No it uses far more, because it's not an inverter genny. It's a cheap inefficient Chinese genny. Uses at least 50% more gas per hour, maybe as much as 300% more. My car uses about 0.25gph idling but the rating on the genny at half load is 0.47gph, and if that's overly optimistic it would be even worse. The RV would only need half the genny load so maybe it is only 0.47gph.

However, the cost difference for such a small amount of usage is what convinced me. Not worth saving eight bucks or so.

Alrighty thanks everyone, I have the answers I need and have marked this thread solved, so I am unsubscribing from this thread. Have a great day!

* This post was last edited 01/04/21 12:47pm by SlowBro *   View edit history


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pianotuna

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Posted: 01/04/21 09:53am Link  |  Quote  |  Print  |  Notify Moderator

There is no magic in inverter generators. Under the same load they use about the same amount of fuel as a non inverter generator. The only time they 'save' fuel is when the inverter generator is idling and producing little power.

In my case, that is almost never. I have remote electric start. So the generator only idles until my inverter charger "flips" to generator power.


Regards, Don
My ride is a 28 foot Class C, 256 watts solar, 556 amp hours of AGM in two battery banks 12 volt batteries, 3000 watt Magnum hybrid inverter, Sola Basic Autoformer, Microair Easy Start.

valhalla360

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Posted: 01/04/21 10:30am Link  |  Quote  |  Print  |  Notify Moderator

SlowBro wrote:

those cheap and inefficient Chinese generators and my van uses less gas per hour idling, despite having a larger engine.


Gdetrailer wrote:

Most car alternators are designed for INTERMITTENT heavy charging


Oh then that's perfect. I have in mind intermittent use, otherwise I would be buying solar.


By INTERMITTENT heavy charging, that means for a few minutes at a time. Basically when you start the motor, it's pulling hundreds of amps to turn the starter for a few seconds...so a few minutes charging at 80-100amps will replace the amp-hr that were pulled. After that it just puts out a few amps to run the lights, radio and other light loads. Nothing close to 150amps.

You can buy specialty after market alternators designed to put out high amperage for a longer period of time. Most are geared toward full size pickups with big V8 engines that won't notice the power needed to run them. They are also frequently optimized to put out significant amperage at lower engine RPM.

As you indicated it's the stock alternator, I would be very surprised if it can withstand anything close to 150amps output for more than a few minutes. If you are lucky, there will be a cut out that stops the output. Otherwise, very good chance of burning out the alternator. (This is made even worse by the idea of doing it with the vehicle parked and no airflow flushing the heat out of the engine bay)

From the battery to the inverter, you will need some pretty heavy cables as you are going to be pulling upwards of 80-100amps off the truck battery.

You will need to look at the continuous output of the inverter. Usually, they give a peak load rating as the main number. That peak load is usually only good for seconds or minutes. There is a continuous rating (often hard to find) that is much lower.

If you upgrade everything from the alternator thru the inverter to handle the loads, it's pretty straight forward and at 120v, the cabling to go back to the trailer converter (charger) doesn't have to be excessively large due to the higher voltage (typical extension cords is good for this size load) and as previously mentioned once you get 120v power of sufficient amperage back to the trailer, the converter won't know the difference between your system and shore power.

If you have the alternator that can provide the amps, it's actually a good solution to charging while towing (not so much while stationary). The problem with charging directly off the 12v system is the wires going to the hitch are tiny and often the connections are of poor quality with low voltage (12v) you get a lot of power lost. This results in only a few amps making it back to the trailer battery.

Probably not worth replacing the generator if it's in good running condition but watch and you can find a good quality inverter generator for $4-500 (helped a friend pick out a 3100w dual fuel Champion last summer for $500). And yes, they do save on fuel consumption compared to the old cheap fixed speed generators. Only under high loads will it be comparable fuel consumption. So an 800w load on a 3000w generator will be throttled way back with an inverter generator and do significantly better (and be much quieter).


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2112

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Posted: 01/04/21 11:12am Link  |  Quote  |  Print  |  Notify Moderator

SlowBro wrote:

2112 wrote:

Can you get your vehicle close enough to the house battery to use jumper cables? That's how I do it.


Per this battery manufacturer, alternators direct connected (using battery cables or jumper cables) are not designed to charge batteries, only maintain them. Also, I recall an Interstate battery tech telling me the same.

But inverters are common, provided I get the sizing right, engine RPMs, all that
So let me see if I got this correct: You are concerned about using your charging system going straight from the source (alternator) to your depleted group 24 battery, but you're ok with going from the source, through an inefficient inverter, back through an inefficient converter to recreate the DC you just had? How could that possibly be better?

I bulk my group 29 battery for about 20 minutes with jumper cables when needed. That gets me back to about 75%. You don't need to completely recharge your battery every time. It's inefficient doing so with a generator. Give it a good full charge when you get home.


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QCMan

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Posted: 01/04/21 11:30am Link  |  Quote  |  Print  |  Notify Moderator

If it takes six hours of charging then you probably do not have batteries but do have acid filled paperweights.


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Gdetrailer

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Posted: 01/04/21 11:53am Link  |  Quote  |  Print  |  Notify Moderator

SlowBro wrote:

Gdetrailer wrote:

At least you do now realize the gen you already have will use far less fuel..


No it uses far more, because it's not an inverter genny. It's a cheap inefficient Chinese genny. Uses at least 50% more gas per hour, maybe as much as 300% more.

However, the cost difference for such a small amount of usage is what convinced me. Not worth saving eight bucks or so.


Fuel usage on portable gens are typically are for HALF LOAD or FULL LOAD, need to read pretty closely. They don't typically give no load figures since for the most part you WILL be putting some sort of load on the gen..

Found some info on fuel typical useage at idle for vehicles..

HERE

2 liter engines .16 Gal per hr
4.6 liter engines .39 gal per hr
5-7 liter engines .84 gal per hr

Although, those numbers look suspiciously low, but I will humor these folks by using the the numbers above for comparison..

It takes a certain amount of fuel to air ratio to keep any ICE engine just running, period. Even with the use of fuel injection, idle cannot be sustained if you do not supply enough fuel for the air in each cylinder.

The more "displacement", the MORE fuel you will use just to keep the engine running at idle with no load.

A typical non inverter 4Kw portable gen uses a 6.5HP 212 CC engine like THIS one. While this gen doesn't "idle" and it runs at 3600RPM all the time it is better on fuel than you think.

The gen in the link is rated at 16hrs of run time at 50% load, the fuel tank is only 4 gallons which comes out to only .25 gal (ONE QUARTER OF A GALLON)of fuel per hr at half load!

a Lighter load on that gen than half will most likely result in even a slightly lower amount of fuel used per hr.

Another example is a smaller 2Kw gen I used to have yrs ago, a "Buffalo Tools" Gen154, 2.4 HP engine, had 1.32 gallon tank and was rated at 9hrs run time at half load..

That is only .15 Gallon per hr at half load..

You can find some info on that one HERE

So, for a car to use less fuel idling than what that a 4Kw gen uses you would HAVE to have a displacement much smaller than a 2.0 liter engine and to find a car that uses less fuel than the old school 2Kw gen you would have to have a car with nothing more than a 2.0 HP lawn mower engine!

ANYONE that says else wise is only kidding themselves and totally ignoring physics.

Gdetrailer

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Posted: 01/04/21 12:08pm Link  |  Quote  |  Print  |  Notify Moderator

valhalla360 wrote:

SlowBro wrote:

those cheap and inefficient Chinese generators and my van uses less gas per hour idling, despite having a larger engine.


Gdetrailer wrote:

Most car alternators are designed for INTERMITTENT heavy charging


Oh then that's perfect. I have in mind intermittent use, otherwise I would be buying solar.


By INTERMITTENT heavy charging, that means for a few minutes at a time. Basically when you start the motor, it's pulling hundreds of amps to turn the starter for a few seconds...so a few minutes charging at 80-100amps will replace the amp-hr that were pulled. After that it just puts out a few amps to run the lights, radio and other light loads. Nothing close to 150amps.



Typically in the seconds to a minute time frame a alternator might putout more than HALF the rating after startup, batteries can be discharged heavily while starting an engine and the regulator will attempt to put that back into the battery as quick as possible but it may take 15 minutes to an hr to FULLY recharge the battery since the battery voltage goes up as it is charged and the alternator voltage will typically max out around 14.4V so the amperage going back into the battery will taper down very quickly often within a couple of seconds..

It wasn't until you got into the 1990s when autos started to get alternators larger than 35A-60A and the reason was because of additional high power accessories like power windows becoming standard on vehicles.. Most accessories are ALSO intermittent loads, used for SECONDS at a time..

I have repaired alternators, trust me, there is very little space in there to dissipate heat from the regulator, windings and the diode packs..

noteven

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Posted: 01/04/21 04:31pm Link  |  Quote  |  Print  |  Notify Moderator

Who here prefers the sound emissions of an idling fuel injected V8 to the “pleasant hum” of a 3500rpm generator?

pianotuna

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Posted: 01/04/21 05:13pm Link  |  Quote  |  Print  |  Notify Moderator

Gdetrailer,

Excellent replies.

BFL13

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Posted: 01/04/21 05:55pm Link  |  Quote  |  Print  |  Notify Moderator

I tried the OP's idea way back as seen in that photo, because I wanted a back- up plan if the gen would not work.

In that photo posted earlier I was actually trying for a faster recharge with more amps, by adding the amps from the charger on the inverter- truck combo with the other chargers' amps being run from the Honda 3000 in the bed of the truck at the same time.

That worked until the amps tapered to where the Honda would run the total amps being accepted, and the amps from the extra charger on the inverter were not needed anymore. ( I did not have solar then or it would have added even more amps)

The OP's plan is not stupid. He just needs to get his spec numbers correct to make it work.

In my test, I ran the truck in idle for an hour, no problem. (I did not set fire to the wet grass with the cat cov either!) Adding some gas to get more revs did nothing at all to help make it work better.

ISTR trying it with the lights on too, but that sure didn't help--I read that somewhere, but think it must have been an idea from the old days before the newer style alternators and their voltage regulation.

As for efficiency--that means nothing! You are off-grid and just want to get by ok. So what if it costs something to get gas the next time you do a "fill-up" earlier than it would have been time for a fill-up?
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