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Using a high output inverter.

5K views 35 replies 7 participants last post by  smart_guy 
#1 · (Edited)
Hello guys.

I'm on a 88amp battery and a 115amp alternator. Can I use a high output alternator on them connected directly to the battery while the engine is on?

I'm having in mind true wave ~1000watt inverters.

General questions:
1- Is the final amperage on a running engine taken from the battery or the alternator rating?
2- The engine will be idling at 700-750 RPM. How much of the max power is achieved?
3- Are there any effects and risks of this practice?
3- Am I asking valid question on a right scenario here? I'm new to this and all of the above is based on assumptions.

Thank you in advance!
 
#2 ·
Hello guys.

I'm on a 88amp battery and a 115amp alternator. Can I use a high output alternator on them connected directly to the battery while the engine is on?

I'm having in mind true wave 800-1000watt inverters.

General questions:
1- Is the final amperage on a running engine taken from the battery or the alternator rating?
2- The engine will be idling at 700-750 RPM. How much of the max power is achieved?
3- Are there any effects and risks of this practice?
3- Am I asking valid question on a right scenario here? I'm new to this and all of the above is based on assumptions.

Thank you in advance!
How much current are you going to be pulling from the inverter (what are you using it for) powering a laptop or boiling a kettle?? The answer that will help you get better information.

Inverters are pretty inefficient. IIRC, if you are drawing a large current from your inverter, the battery will be discharged quicker than the alternator can top it up. There is a lot of information on the www about this but I'm sure people here, smarter than me, will give you more clarification.
 
#4 ·
Thank you for the reply, Rory.

Err... devices have input and output printed on them. My laptop, for example, has output: 100-240V ~2A and input: 18.5V 6.5A 120W printed on its power brick. Which is the current to take (or multiply with voltage to get the power)?

I wanna use it for a laptop (the above mentioned), a 27" LED power saving monitor, 2.1 small speakers and one or two power saving lights in a small camp. Only the engine would be running.

Any idea how to keep the alternator charging the battery well? Should I worry about the alternator's or the battery's current rating first?

Inverters are inefficient. Let's do the math here to put things in perspective.

1000W / 12V = 83.xx Amps peak needed just for the inverter.

Then add at least the 90Amp alternator your car came with in stock.

That means you would need a alternator with a 175Amp output.

And I am not even going to talk about power needs if you have a decent sound system with stand alone amps.

If you really want to run a high end inverter, start thinking about running 2 alternators just like limos do. Extra wiring, extra hardware, extra powertrain loss, etc, etc, etc.

In a nutshell, I'd deep six the idea of a 1000Watt inventer.
Much obliged, sbaert (dunno how this reads!).

Should I worry about the alternator's or the battery's current rating first? Will the alternator pull from the alternator's 115A or thru the battery's 88A max? And then, would the battery be drained if the total pulled current is more than 88A?

My plan is to use it on a small camp with only the car engine running to power the alternator. How many amps is only the car gonna use up at idle in that case with only the engine on? And how much amps will the alternator give? Looking at the devices now, I don't imagine the total power I'm gonna need from the alternator is gonna reach even 400W. I wanna use it for a laptop (the above mentioned), a 27" LED power saving monitor, 2.1 small speakers and one or two power saving lights in a small camp. That 1000W is a wishful thinking just running my imagination to connect whatever I want.

On a side note, the online user's manual says the stock alternator for my car is 14V/70A (battery 62Ah). Could it be for the early 300E's? I replaced it ages ago so I can't see for myself.
 
#3 ·
Inverters are inefficient. Let's do the math here to put things in perspective.

1000W / 12V = 83.xx Amps peak needed just for the inverter.

Then add at least the 90Amp alternator your car came with in stock.

That means you would need a alternator with a 175Amp output.

And I am not even going to talk about power needs if you have a decent sound system with stand alone amps.

If you really want to run a high end inverter, start thinking about running 2 alternators just like limos do. Extra wiring, extra hardware, extra powertrain loss, etc, etc, etc.

In a nutshell, I'd deep six the idea of a 1000Watt inventer.
 
#6 ·
I concur on the Honda EUxxxxI series of DC inverter generators, or the equivalent Yamahas. not the other lesser names, they tend to be junk.

if you DO go with a DC inverter, go for one thats between 1.5 and 2X the max load you expect to pull. 1000 watts is WAY too muchl Your laptop is 120 watts MAX but likely draws half that most of the time, a LED backlit 27" 'FHD' (1080p) LCD is likely well 40 or 50 watts, your speakers are way likely less than that unless you're really rocking out (hey, my kid has a sound system that actually uses 7000 watts[1]), LED lamps are like 7-10 watts each, 200 watts is probably your max actual draw, with 100 watt typical, so get a 300W inverter. modern inverters are 85% efficient when used in the 50-80% of their capacity range (pulling 200 watts from a 1000W inverter will be less efficient than pulling 200 from a 300W).

100 watts actual draw, add 15% for (in)efficiency loss, lets round that to 120 watts, that's 10 amps off your alternator, all the sudden way more reasonable.




[1] note the EU7000iS on the front bumper, wooden cabinet has a pair of 3000 watt Crown amplifiers, speakers on the upper deck are a pair of dual-18" subwoofers, and a pair of dual-15"+horn mains...the truck is a 5 ton, not a deuce-and-a-half.
 
#8 ·
That DC inverter generator idea is fancy. I guess the price is what counts then. Good solution for lengthy and demanding camping. My plans are to stay for mere 6-7 hours max and use just the mentioned devices.

Are my settings (i.e. the 115A alternator and 88A battery) prepared to run a 300-350W (that 50 is for efficiency preparations) inverter to run the mentioned devices for let's say 4-5 hours without flattening the battery?

[1] note the EU7000iS on the front bumper, wooden cabinet has a pair of 3000 watt Crown amplifiers, speakers on the upper deck are a pair of dual-18" subwoofers, and a pair of dual-15"+horn mains...the truck is a 5 ton, not a deuce-and-a-half.
That... should be good enough to run my laptop :)
 
#7 ·
We once lost power for just over a week. I used a Harbor Freight 1500/3000 watt inverter off my 85 300SD to power the fridge, some lights, and a few other essentials (phone, cable, TV, cell chargers).




I kept a voltage meter attached to make sure I was getting positive voltage, i.e. charging rather than draining. This was key.



If the load sucked too much juice, I squeezed a clamp on the cruise control linkage to up the idle until I was running positive again - usually between about 1200-1700 RPM.



I ran this set-up straight through for days, only shutting it down late at night. The OM617 purred along without a blip or hiccup the entire time. I think I went through two tanks of diesel.
 
#9 ·
We once lost power for just over a week. I used a Harbor Freight 1500/3000 watt inverter off my 85 300SD to power the fridge, some lights, and a few other essentials (phone, cable, TV, cell chargers).

I kept a voltage meter attached to make sure I was getting positive voltage, i.e. charging rather than draining. This was key.

If the load sucked too much juice, I squeezed a clamp on the cruise control linkage to up the idle until I was running positive again - usually between about 1200-1700 RPM.

I ran this set-up straight through for days, only shutting it down late at night. The OM617 purred along without a blip or hiccup the entire time. I think I went through two tanks of diesel.
Is it just me, or is the "like" option (love, more like it) missing for your post?

Thank you for sharing such an experience.

Any idea what was the amps rating of the alternator and the battery? What should the voltage reading be to have things run the way they should? Any idea what was the max wattage the fridge pulled?
 
#10 · (Edited)
Again, let's do the math. Assume we're looking at 360W (it's just a easier number to work with)

360 Watt / 12 Volt = 30A (per hour)

30 Amps * 5 hours = 150A expected loss

Take your cold cranking amps (worst case scenario) and subtract the expected loss in capacity.

Example: 800CCA (for a young healthy group 49/DIN100 battery) - 150 Amps consumed = 650 Amps

Substitute CCA number depending on your battery size and also deduct 20% for each year the battery has been in use. A fresh M103 MB battery (stock group 48 size) has about a 600CCA rating. So to do your calculation on a 2 year old group 48 battery is 600CCA * 60% = 360CCA before substracting the expected loss from the inverter.

Then you need to determine if that leaves enough juice to fire her up.

Another thing to consider is that lead acid batteries do not like to get discharged and recharged repeatedly. For such purposes, you should use a deep cycle battery such as used in marine applications.

Personally, my vote goes to the portable Honda generator.
 
#11 ·
I hope those Honda generators aren't that expensive then :)
 
#12 ·
#19 ·
Now those are some prices alright! Those inverters in the links are really ugly too.

I'm keeping this inverter generator idea in mind and it's a one step ultimate answer (I'm collecting information too), but let's learn more of car inverters.



No intentions are made so far, I'm just putting expected plans for my coming vacation and I'm new to this. So dunno of any places really.

Also keep in mind that your 115a alternator only outputs about 65a at idle.
That's one answer I've been looking for all along now. Cool!

Does that mean it's granted to have continues 780W from that 65A to run, let's say, a 350W inverter for a long time? If that 350W inverter works full power, then it takes ~29A leaving ~36A for the car running the engine alone and recharging the battery. Will that keep the battery from dying for let's say 4-5 hours? Is it okay to rev up the engine to like 1000RPM for that long to get better charging?

The numbers above are of worst case scenario. I don't think I'll need more than 300W as sbaert mentioned before.
 
#17 ·
We had a power outage a few days ago due to storms, I lit up the whole house (led bulbs) off my 4 year old group 48 battery through an 800w inverter. When the power came back on after 5hours of draining the car's battery, it fired right up. The rated size of the inverter is irrelevant, it's the load you put on it that matters. If it went much longer I would've had to plug in the fridge and fire up the engine.

Also keep in mind that your 115a alternator only outputs about 65a at idle.
 
#23 ·
leaving an engine idling or at an elevated idle for a long time can be hard on the cooling system, but as long as thats all working well, you'll be fine. I think you'll find your actual power usage will be way under 200 watts I've got a beefy big tower desktop here, 2 big LCD monitors, speakers, subwoofer, and a staick of network hubs and stuff, and I'm at about 150 watt actual going into my UPS.
 
#27 ·
Yeah, that's the impression I'm having now thanks to your calculation earlier. I did check the power need for the devices and found they are 120W for the laptop, 60W for the monitor, 14W for the speakers and let's say 40W for the lights. That's a pessimistic over worst case scenario which means it's almost impossible to eave reach that. I'm saying almost because I'm crazy, I crank the volume with the bass, use all monitor image improving features and use the max performance power plan on the laptop. Lights could increase to 3 or 4 too. That's just a pessimistic calculation, if that expression makes sense.
 
#26 ·
I would suggest getting a voltage monitor[1] and keeping an eye on it, as long as the voltage at the battery stays above 13.2V or so, the battery won't discharge.

also keep an eye on the engine temperature...

another note, the 12V DC wiring from the engine to the inverter is carrying a lot more current than the 120V AC wiring from the inverter to your loads, so keep the inverter close to the car, and use 120VAC extension cords to bring the AC to your computer and whatever. wire the meter to the battery, NOT to the inverter. since there's nearly zero current going through the meter, the wires from the battery to the meter can be as long as you like, and can be as thin wire as is practical, like door bell wire, so you can run the voltage meter to near where you're sitting...


[1] https://www.amazon.com/dp/B000EVWDU0
or https://www.amazon.com/dp/B005UWD2J4

I recommend these over using a multimeter because most multimeters will time out and shut off after a few minutes.
 
#31 ·
Yeah, I've been thinking about those too. I always open the hood fully and if there is wind, even little, I'd keep it against it. I also have one of these:

Does it do the trick? That cig. lighter one is attractive tho. I can even use it while driving! Side question: how many amps does W124's cig. lighter give max?
 
#32 ·
#33 ·
Yeah, that battery tester is huge. I was wondering if the reading of these devices is reliable, just in case.

Got it. The user's manual says it's 16A. Looks like MB decided to use 8-16A instead of 5-10A for the lower fuses.

This means I can get up to 192W out of the cig lighter? That's enough for the laptop and the monitor. Seems too high to me. Are cig lighters that efficient? Unless maybe there is something else to the cig lighter's electrical wiring system. My 300W modified sine wave inverter (the thread is about pure/true sine wave ones, but power consumption calculation here is still relevant) ran the laptop alone very well (with battery disconnected) and I thought it was using the cig lighter close to the limit. I was wrong, I guess.
 
#35 ·
Good point! That's actually the intended use. Good observation.

Practically I only used a laptop and it pulls at worst a max of 10A, which I don't think I reached even at surges, if any happened at all.
 
#36 ·
Just for the reference, here is the modified sine wave inverter I have:





It comes with a cig lighter and a battery clamp connections:



I was wrong, it's 400W, not 300W as I mentioned before. Here's the slip that came with it:
 
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