Inverter power from car idle?

[drummerdimitri]

So I need to run two gaming desktop computers on a home network for 5 hours continously with a maximum load of around 400W with an average of about 300W once a week as the electricity may go for a maximum of that much time.

Since I already have a Cheap Chinese inverter rated at 1500W but was tested to be good for about 400W max could it just hook it up to my car as it idles for 5 hours straight? 

How many amps can I draw from it when it is idling with a fully charged battery? The has a 2L turbo engine with a 60AH battery but I do not want the battery to drain and would like to know if the alternator can provide the power needed.

[drummerdimitri]

I did some research and found that the alternator is rated for 140A so does that mean a 1500W inverter is the maximum the car can handle?

[Gyro]

A very curious request.   Running a car at idle for 5 hours sounds as if it's going to be bad for the car (engine cooling system) and amount of fuel burned.

I suspect that the alternator isn't going to be that pleased either, it is going to be running at its minimum speed, meaning that its fan will be pulling minimal ventilation, combined with high under-bonnet temperature. You need to allow for the radiator cooling fan cycling on and off frequently too. Having a 2L turbo engine probably doesn't help either - bigger heat sources. You haven't got your country flag so I can't guess at your ambient temperature.

All in all, I definitely wouldn't reccomend it!

[drummerdimitri]

Thank you for the replies.

It is clear that this is not a good solution mainly due to having a car idling unattended just thought if it could be done.

What is the better alternative? a UPS/inverter connected to large batteries (200AH) or a small portable generator?

[Zbig]

http://us.kipor.com/p0/digital-generator/80.htm

[Gyro]

I would go for the small protable generator (you can get more stable 'inverter' types these days). It's a much better match for the job. You could use batteries, but they would need to deep discharge types to survive the duty and you would be constantly worrying about charge level - and you would have to charge them up again of course.

[drummerdimitri]

A portable generator inverter is definitely more ideal but the problem is finding one where i live (Lebanon).
Would any generator do or does it have to be a "digital" or "inverter" model?

Does it have to be a name brand or is the chinese stuff good enough for my needs? Worst case I can just plug in a dc power supply and use my inverter to get "clean AC" back although it willl not be very efficient due to double conversion.

[Gyro]

An 'ordinary' generator would probably be ok, although the inverter ones aren't much extra these days. Yes you should take the mains output to avoid loosing efficiency.

[KD0CAC John]

One biggy being missed here is , that an alternator only puts out its full rating at a much higher RPM - NOT IDLING  !
Too many variables , but 1,300 - 1,500 RPM average .

[NiHaoMike]

http://priups.com/riddle/answer-1.htm
It has been done before with pretty good results.

[ConKbot]

If you can manage to get a hold of one, my go to suggestion would be a Honda 500W generator. they use an inverter, sip fuel, are quiet, and are the toughest little SOBs I've ever seen. I've even seen 2 come back from having diesel put in, with just pouring the mix out, putting gas in, and manually choking the exhaust with a gloved hand.   

[BrianHG]

I once tried the car generator approach.  Idle speed wasn't enough for the alternator to keep the battery from going too low in voltage, I had to put s stick on the gas pedal to run the engine at 1200rpm.  Also, I was only drawing 500 watts and this was an emergency which only lasted 5 days, I did not attempt this again as it damaged my car battery since the system isn't designed for this non-stop load & drive for 5 days straight.

[Siwastaja]

Typical car alternator, when idling, produces barely enough to supply the car's own needs, lights, etc. Even if you turn off everything you can, you may have as little as 50-100W excess, so it doesn't make much difference whether the engine is idling, or not running at all; you'll be mainly draining the battery, which, by the way, destroys the starter battery not designed to be drained.

You'll probably get your required 400ish watts if you put a brick on the pedal and run high rpm all the time, but you can imagine what the noise, fuel consumption and efficiency will be!

So yeah, a dedicated generator is really needed.

[BrianHG]

Typical car alternator, when idling, produces barely enough to supply the car's own needs, lights, etc. Even if you turn off everything you can, you may have as little as 50-100W excess, so it doesn't make much difference whether the engine is idling, or not running at all; you'll be mainly draining the battery, which, by the way, destroys the starter battery not designed to be drained.

You'll probably get your required 400ish watts if you put a brick on the pedal and run high rpm all the time, but you can imagine what the noise, fuel consumption and efficiency will be!

So yeah, a dedicated generator is really needed.

A Mazda 626 Chronos V6 engine slightly above idle, 1200rpm, with a surplus of 500 watts with everything else in the car off, goes through a tank of gas every 14-16 hours.

[Ian.M]

Build your own battery charging set - a 50cc moped engine is typically good for about 2 to 3HP at redline.  Derated down to 1 HP with an added cooling fan it should be reliable.  A 100A alternator should be OK running at 50A continuous.   Add a large lead acid battery, not maintenance free, which doesn't need to be in great condition, a fuel tank and a BIG silencer for the exhaust, and you'll have something that you can maintain yourself and get parts for locally.

[Nusa]

Idling most cars isn't going to cause any unexpected problems with the car, despite what some people say. It happens all the time in the real world, particularly in very hot or very cold climates where AC or heat becomes very important. Or sometimes just to remain operational -- I've been on more than one ski-trip where letting the car idle all night was way better than attempting to start an extremely cold engine at -20F.

It's also true that the alternator output curve is based on shaft RPM (not the same as engine RPM, but related). At idle speeds, you may only get 10% of the rated output, if that. (On older cars you may remember a flickering alternator light at low idle was fairly common and not considered a problem in normal use.) So yes, you need a way to set a fast idle for an application like this.

But if this kind of power outage is a regular occurance, as you seem to indicate, you're better advised to pursue a more proper solution.

[BrianHG]

So I need to run two gaming desktop computers on a home network for 5 hours continously with a maximum load of around 400W with an average of about 300W once a week as the electricity may go for a maximum of that much time.

Have you measured this power consumption?
De-rating the clock speed by only 10% can cut power consumption by over 25%, or, get more power efficient PCs.  Doing this and having 3-4 car batteries for your backup inverter and you'll be fine.
Switch to quality laptops and a single car battery would be overkill.

[james_s]

I've done it before in an emergency. Idling a car for several hours is really not a big deal, it's a lot less fuel consumption and wear than driving for 5 hours. A good generator will be more fuel efficient but a cheap 3600 RPM one may not be. Just make sure the car is outside, at least the back end poking out of the garage to prevent exhaust from getting back in. Note that you won't get full power from the alternator at idle but 400W shouldn't be an issue.

[P90]

lots of solar panels and lead acid batteries...

[DBecker]

There is a knowledgeable set of people on this topic: boaters.

Sailboats sometimes run at idle for hours to recharge the house batteries.  The generators are lightly tweaked automotive alternators -- plated or galvanized for corrosion resistance, some hardware replaced with brass or bronze, wire mesh flame arrestors on the cooling vents, and more effectivfe fans.  They put lots of attention on the pulley sizing and regulator tuning to produce a reasonable power output at the lowest RPM, while not blowing up at maximum RPM or overheating at passage-making RPM.

Automobiles alternators are tuned differently.  They barely put out enough power to keep the car running while at idle, and only put out their rated power when at highway RPM with lots of airflow.

Ambulances and other emergency vehicles often use two alternators to provide the power needed for long periods of idling.  Sometimes the second one has a smaller pulley to increase the RPMs at idle, with a clutch to disconnect at higher engine speeds.

[Ian.M]

I would question how knowledgeable the average boater is, unless they are marine engineers and electricians or at least wear those hats on their own vessel.  However the more knowledgeable ones typically have the charging system experiance and are willing to share it.

Most boaters with more sophisticated charging systems run diesel engines.  Its only the smaller sports boats and runabouts + high performance racing powerboats that run petrol engines. 

Many small diesel engine alternators DO NOT have specialist marine adaptations apart from the pulley ratio.  No plating/galvanising, no flame arresting screens (unless required by local regulations).  The base engine is often derived from industrial or construction equipment
('plant') and the alternator may well already be optimised for supplying high current accessories near idle.  Alternatively, if the base engine has automotive origins,  as redline RPM for a marine diesel is very well defined (no engine braking downhill) and typically a lot lower than in an automobile application and most engines have governors that prevent any over-speed (other than catastrophic runaway) its possible to use a higher pulley ratio, leaving less safety margin for the alternator at engine redline. 

Another common adaption is to add an external regulator, either overriding or replacing the internal one, to get more out of the alternator by optimising the charging current and voltage for the battery actually fitted rather than for a generic automotive starter battery.  This can be problematic if the alternator is run too close to its power rating without an added temperature sensor to cut back its output before it overheats, and can be particularly hard on drive belts and bearings.

However marine engine systems are typically at least 20 years behind current automotive practice and there is still a fairly strong resistance to unnecessary electronics including ECUs on the engine that may shut it down or enforce a limp-home mode, (as damaging or even writing off the engine by running it for 5 minutes more after a critical warning, to get the vessel out of immediate danger is an acceptable tradeoff against total loss of the vessel - there is no 'hard shoulder' near exposed coasts or shoals or on fast flowing rivers), so expert knowledge of marine charging systems is unlikely to be much help when dealing with a modern automotive engine with an ECU that monitors the alternator - you know what you want to do , but the ECU and other systems wont let you.

[drummerdimitri]

Thank you for all your suggestions.

I think I will just use my current UPS with two 240 AH batteries or get myself a used honda generator/ inverter depending on what is cheaper.

[nctnico]

The generator will last longer (life span) than the batteries.

[Ian.M]

...... if you use clean dry fuel, use good quality oil of the correct grade and change it at or before the recommended interval of operating hours, don't operate it in excessive ambient temperatures, and change or clean its air filter frequently.

Also if laying it up for longer than a month or two, clean it thoroughly, drain the fuel system and flush with fuel mixed with fuel preservative, change the oil and seal the intake and exhaust to keep dust and bugs out.

Small gensets need quite a bit of TLC to keep them in good working order - you cant simply keep on putting gasoline in at one end to get clean stable electricity out the other month after month, and newer ones with fancier electronic controls are likely to be more expensive and harder for a DIYer to maintain correctly.

[james_s]

I've had the pleasure of borrowing a Honda EU2000i generator a few times and it's a really wonderful machine, can't recommend it enough. It's extremely quiet and sips fuel. I absolutely hate the cheap generators most people have, whenever the power goes out here the neighborhood is blanketed in the roar of dozens of cheap generators, so loud I can't hear myself think. Some pricks even run them all night long.