Wikipedia:Reference desk/Archives/Miscellaneous/2020 January 10

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January 10

using car as generator

I'm looking into ways to occasionally get some electric power while off grid, and am wondering how those relatively high power (like 1kw) 12vdc to 110vac inverters work. You clip them to the car battery terminals, but how do you make sure the car motor is running fast enough that the energy is coming from the alternator rather than the battery? Do you have to crank up the idle speed? Is it ok for the car to leave the motor running for a few hours while running the inverter now and then? This isn't a prepper thing, but I wouldn't want it often enough to justify having a separate generator and dealing with storing it. More like a way to power computers while glamping. Thanks. 173.228.123.190 (talk) 21:18, 10 January 2020 (UTC)

You're right to be skeptical. You need a vehicle with a large engine to support such a device, and would likely need to run it above idle speed, which could cause problems like poor air quality due to all that exhaust. Also, leaving a vehicle running unattended is risking theft of the car or inverter. And when equipment like computers all starts up at once, far more than the average power is required for a few seconds. So, you need to start them up one at a time.

I suggest you use laptops, which have far lower power requirements and can be recharged by a plug connected to the cigar lighter. Some type of battery bank that you bring precharged to the event is another possibility, but make sure you can swap out the battery when it no longer holds much of a charge, rather than replace the entire unit. A dark tent will reduce the brightness needed on screens. NonmalignedNations (talk) 23:34, 10 January 2020 (UTC)

Yes, the idea is to power a laptop from a rechargeable power bank, and to use a car or generator to recharge the power bank when needed. I think you're right that using a car isn't practical, and I don't want to deal with a generator. I'll figure out some other way to do this. Thanks. 173.228.123.190 (talk) 00:14, 11 January 2020 (UTC)

You can drive back to civilization and plug it in some socket. You'd probably have to drive back anyway to charge the car battery, as revving in place overheats the engine. Besides all those car parts you'd be stressing are mighty expensive. Kind of like using the side of a dinner plate to drive a nail into a wall. 89.172.2.98 (talk) 21:44, 11 January 2020 (UTC)

What sort of power bank are you thinking of? Even 50000 mAh power banks normally charge off USB-C or microUSB [1]. To be fair it will take a while, but you're definitely not going to be charging at anything close to 1000 W. If you were charging at 1000 W, it will be very short charge time. Automobile auxiliary power outlet suggests that cigarette lighters use ~10A. This is only for a short time, still I suspect you can probably safely do 60W USB Power Delivery and definitely 30W. It will probably take a few hours to charge your power bank, however you probably don't want to be charging your power bank too fast anyway.

Even fancier devices like the Anker Powerhouse devices [2] [3], which themselves have inverters, do not require AC for charging AFAICT. Frankly it makes little sense since the batteries are surely not AC nor anything close to line voltage. The Powerhouse 200 supports USB PD charging. [4], it also mentions solar charging and AC charging . But the latter is just using an AC-DC adapter AFAICT. The 400 [5], surprisingly there is no mention of USB PD charging. Still it does mention DC Input: 16-17V/6A which is what I'd expect. I mean I'm sure they come with an AC adapter to charge it, but you don't have to use it.

I don't think they're really expecting you to charge either device with a car hence why there's no included adapter etc for charging it from a car. However if you did want to charge either with a car, it doesn't make any sense to use an inverter to me. Sure there are inefficiencies in the DC-DC etc, but it would still be far more efficient to simply use some sort of DC-DC adapter to charge them. The 200 frankly you could probably use a 60W USB PD. Anker don't seem to have a 60W car charger, I think their highest is 30W, however this Satechi may be okay [6] [7]. I assume it can also charge at 30W anyway, just slower. For the 400, you're maybe pushing the limit trying to charge from a cigarette lighter socket at maximum spec. However, it still seems to make much more sense to get something to connect direct to the car battery that does a simply DC-DC and produces the desired voltage and is able to handle the desired current, than putting an inverter in the mix.

As I said, I don't think it's generally expected you will charge these off the car. Either you get one that will last until you have access to electricity or you use a solar charger. But if you did really want to use a car, I'm confused why you'd throw an inverter in the mix. Note that even when charging off solar, it doesn't sound like they're thinking you will use an inverter either. If you're using a real generator to charge the device, then maybe it makes sense to use a lines voltage one with a AC adapter since you're talking about something designed for outputting line voltage. (I did see some with DC output for power tools, but I'm not sure they're a better choice in this case.)

Nil Einne (talk) 12:14, 13 January 2020 (UTC)

So is an idling car engine necessarily (or currently in practice) significantly less efficient in generating electricity than a small emergency diesel generator? By how much is it less efficient? A few tens of percents might be tolerable, several hundred percent might not be. 85.76.77.44 (talk) 00:23, 11 January 2020 (UTC)

An inverter delivering 1kw will draw more than 1000/12 = 83 amps from a 12V battery. This can be obtained with clip connections to the battery terminals (it is too much for a cigarette lighter socket) but a car alternator is current limited and usually unable to replenish such high current continuously, see Alternator (automotive)#Output current. Low price DC-to-AC inverters cannot deliver a low-distortion sine wave and their harmonic distortion can upset sensitive electronics, especially radio receivers and audio amplifiers. Hybrid electric vehicles such as Toyota Prius can provide a very efficiently self-charged supply of over 200VDC (see Toyota Prius#Batteries) that can serve appliances that accept DC (lighting, heating, Universal motors and regulated power supplies for electronic units). The recommendation above to use laptop(s) for computing away from home is good and you can charge devices such as phones from the laptop USB ports. DroneB (talk) 00:57, 11 January 2020 (UTC)

For emergency generators, I'd consider fuel efficiency to be almost a non-issue, but car motors are BIG compared to portable generators and it must take a lot of power just to keep them turning. Diesel motors are terrible for emergency generators by the way: starting up is hard on them and they want to run for long periods. The issues with using a car as I saw it were 1) regulating the motor speed to keep up with power demand, 2) capacity of the alternator to produce that much power for long periods, 3) thermal and other considerations running the car motor for hours with the car not moving (NonmalignedNations brought up emissions as another issue). For an emergency or occasional-use generator I'd consider a propane powered unit as an alternative to a gasoline one, but I haven't looked into it much. I've been in a few prolonged power outages and actually found them relaxing. A few flashlights and a battery powered radio were enough. Generators and computers were not needed. 173.228.123.190 (talk) 04:20, 11 January 2020 (UTC)

Starting up stresses diesel motors, but the motor itself starts more reliably than a gasoline motor. And with the typical use patterns of an emergency generator either it won't see enough activations to wear down, or it'll run often and for long enough periods that it'll wear down less than a gasoline motor. They aren't made for diesel for no reason. 89.172.2.98 (talk) 21:44, 11 January 2020 (UTC)

Well I don't know much about it, and was going partly by this (warning: NSFW language and off-color humor, but I find the guy's videos informative and I like watching them). He got a surplus diesel generator super cheap because it had gotten messed up by too many cold starts through normal period testing. Not mentioned is that transfer switches also regularly get messed by periodic tests, since they are often designed to be operated just a few times under load in their entire lifetime. 173.228.123.190 (talk) 22:04, 11 January 2020 (UTC)

By the way some trucks and tractors have power take-off (PTO) shafts that can be used to run generators, but my question was about using an ordinary passenger car. 173.228.123.190 (talk) 04:24, 11 January 2020 (UTC)

• Vehicle-to-grid is a recent way to use plug-in hybrid vehicles as a battery reserve on the grid. Relies on smart grids and automated control.

Otherwise cars are generally a terrible way to provide a generator. Generators are diesels, not petrol, and engines designed for that sort of use cycle. Cars aren't really set up to suit this, and their generators are tiny too. In particular, petrol engines are inefficient on part loads, and car generators are tiny compared to the engine power. Andy Dingley (talk)

I've never heard of a 1kw diesel generator. Generators that small are usually portable, powered by petrol (gasoline) or in some cases propane. Inverter-type generators are supposed to be more efficient at partial loads. There are some cheap small inverter generators out there[8] or even cheaper but I guess non-inverter-type [9], presumably crap, but maybe ok for very occasional use. But beyond the upfront purchase price, I don't have a reasonable place to store the thing, don't want to deal with smells and explosion/fire hazards from residual petrol in the tank, etc. Propane might be worth considering[10] if I was going to make a regular thing out of this, but it doesn't seem likely. A solar panel might be more practical than a generator if it can be set up away from tree cover. I'll look into that approach. Vehicle to grid sounds great with an electric car but for now I only have a traditional petrol-powered car. 173.228.123.190 (talk) 01:35, 12 January 2020 (UTC)

It may help if you better articulate what you're trying to do. As I mentioned above, it's not clear to me why you'd ever want to use an inverter to charge a power bank from a car. Nil Einne (talk) 12:17, 13 January 2020 (UTC)

That would be for a power bank recharged from AC. Alternatively it could possibly be recharged directly from 12VDC but that still poses the same question about load on the alternator. The idea was to have perhaps 1kwh of power bank batteries ([11] or maybe some cordless tool packs recharged with something like this) and charge them at a fairly fast rate to not have the car running for too long. But I'm liking the solar panel idea better now. The purpose isn't camping per se, but to get some distraction-free programming done by going off grid for a while. That might not be a great idea in the first place, but considering it at all means figuring out what to do about power. 2601:648:8202:96B0:0:0:0:DF95 (talk) 16:52, 15 January 2020 (UTC) Added: question is to some extent just a general inquiry about how people are supposed to use those big inverters, because of the need to have the car motor running. 2601:648:8202:96B0:0:0:0:DF95 (talk) 20:38, 15 January 2020 (UTC)

They can be useful during home power failures. For example, I've powered the fan for forced air gas heat in winter and the refrigerator and freezer in summer. I was able to do this every few hours, and sit in the car and listen to the radio while it was on. This kept the pipes from freezing and the food from being ruined. (I also used flashlights and a battery-powered radio and was able to make warm food by pouring hot tap water over canned beans until they warmed up.) A whole-house generator would be more useful, but they cost thousands of dollars, can be loud and toxic, and are a prime target for theft. So, the inverter was the most cost effective way to make it through a power failures without suffering major losses. NonmalignedNations (talk) 22:38, 15 January 2020 (UTC)

Right but now we're back to the original questions: was it hard on the car? How did you make sure the motor was running fast enough to power the inverter without draining the car battery? Also for refrigerators, there is usually a huge surge in current at the moment the motor starts, which means you end up getting an oversized generator to deal with that. I wonder if they could get around it somehow, without taking an efficiency hit or else resorting to 3 phase power. 2601:648:8202:96B0:0:0:0:DF95 (talk) 04:32, 16 January 2020 (UTC)

The refrigerator startup surge was an issue, we had to be sure that nothing else was plugged in when it first started up. The car's voltmeter and the meters on the inverter told us how close we were to the limit. The sound of the engine "pulling down" also let us know. And the inverter had a cut-off to keep it from running if we pulled down too much power. It's best to have a large margin-of-error over the average load to deal with the variable loading. One thing we didn't have enough power for was the air conditioning during the summer blackout. So yes, with an inverter like this you have to nurse it along, but it did prevent disaster, so it was worth the effort. Now if you have daily outages for hours at a time, then a whole house generator might be justified. But, for us, we have a major outage maybe once a year, and this is enough to get us through it. NonmalignedNations (talk) 05:49, 16 January 2020 (UTC)

Even something like this [12] is simply a bunch of ordinary cells. I don't know the default connection details but if you're going to go for something bespoke like that you should be adapting it to your use rather than being forced into a nonsensical solution. So I still strongly question the wisdom of using an inverter for AC charging as I explained in detail above. You're basically adding a far more complicated conversion step than you need to. While it may not make a big difference to the demands on the car, you need to get the basics of your plan right otherwise you're just wasting time.

Also if you haven't actually tried this to see if it works for you, have you considered starting off small? I'm not suggesting you get something that will be a waste but if you're just programming unless you're programming games that you need to debug or something your computing demands shouldn't be that high. Even something like the Anker Powerhouse 400 should provide enough power for a few days in that case. If your getting a 1000 Wh power bank, you can probably get a week of power. That will probably be enough time to get a feel of the idea if you do it a few times. If it does work, then you can make it last longer. Maybe you will still want to have some plan in minds for that before you buy anything since it could affect what you choose for your power bank, but it seems to be it's often worth starting slow with untested ideas like this.

Nil Einne (talk) 09:20, 16 January 2020 (UTC)