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

BFL13 wrote:


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


....If you want to save money, don't buy an RV. :)

No one said the "plan" was stupid.

However, unwise, wasteful, not the best use of a huge displacement engine, hard on the engine and drive train, hard on the alternator, yes..

No matter HOW you fudge "the numbers" you WILL be at an economic disadvantage using an automobile engine to fully recharge your "camping experience" battery(ies).

Efficiency is governed by the "Laws of Physics", those laws are unchangeable by you, me or anyone else in this world, they are set in stone. Sure you might have a 300HP capable engine idling for hrs on end and you are only making it work for a measly 1 HP worth of work.. But that engine is using much more than 1 HP worth of fuel per hr and that is reflected in a lot of wasted energy in the form of heat.

Some folks just are not able to visually see and analyze that in their head and MUST "demonstrate" and see it for themselves.. Myself I don't "need" "demonstrations" or "experiments" to realize that while it can be done, it is is very inefficient mechanically, physically and chemically to do so.

Engines REQUIRE very specific min fuel to air ratio just to keep it running, that minimum is a "constant" for ALL gas engines. A smaller engine working at half load will consequently use less fuel than a large displacement engine idling.

Granted with the advent of fuel injection, manufacturers can control the min fuel usage closer, but if you lean the mixture too much, you now run into the possibility of burning valves, damaging pistons and overheating the engine.. There is a fine line between destruction and fuel efficiency when it comes to leaning out an engine.

To the second point I quoted.. YES!Buying a RV is not "cheap living", pretty much the same as buying a boat.. Insert money and flush..

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

You our idea of intermittent is not the same as the auto MFG

Their idea is closer to a few hard minutes right after startup on a hot day with the dash A/C turned on, so that is dump some quick amps into battery while holding in the a/C compressor clutch and running the blower full blast inside for cool down, and the condenser fan, and the electric radiator fans
When the alternator starts getting warm and or the battery voltage starts rising output is reduced

Your idea is run long enough to charge house battery, then do it again next day or day after

Theirs hard high charge rate,for Minutes every time started,
Yours hard high charge rate, for hours every now and then,
I think.. You have already been suggested better ways, DC to dc voltage converter or jumper cables

Converters are more in the low 70s in percent of efficiency from my observations, converted at or near full output are about 73% efficient, converter or charger will use 1000watts to charge at 730watts into battery, now the inverter is also a loss, even at 80% efficiency the inverter will need 1250w input for the battery to be charged at 730w
At 12.5v that's 100amps from the alternator and that will mean your TOW vehicle chassis battery is being drawn down and not being charged, to keep the vehicle battery charged you need 13.8v
13.8v 90.6 amps 1250watts to inverter,
Plus what ever the vehicle engine system needs are possibly another 30amps or more,that's 120amps total for how ever long it takes for the house battery charge rate to start decreasing,
How well is the alternator going to stand up to that kind of treatment, the 800w inverter is not going to carry 1000watts for more than a few seconds trying to power the converter at full 45 amp charge
I suggest doing this before the battery gets any lower than 75%, try to keep the charge rate closer to 30 amps, so as to not damage the alternator
The heavy load amps is going to be from alternator to inverter, that stock alternator it's the item to worry about, ambulances, POlice cars, fire trucks,
All use heavy full duty rated alternators, this is Optional equipment on vehicles sold for consumer use

Since I have a business use for my RV I actually save money by having it. It is a mobile hotel room and I boondock with 15 amp shore power. The hybrid inverter/charger with load support makes that quite easy.

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?
If you want to save money, don't buy an RV. :)

Gdetrailer,

Excellent replies.

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

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..

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.

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

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.