Automotive Recharge an RV battery by hand cranking a generator

[grandpa2390]

If you've seen breaking bad you probably already know what I'm asking.
in this episode I'm watching, they are hand cranking the generator to recharge the battery. They are moving nowheres near as fast as the motor would have spun it. My dad had a hand crank device that was, more or less, a toy. so I'm sure they were producing some electricity.

is this artistic license, or is recharging the rv battery enough to start it, by hand cranking the generator, a realistic ambition? How much voltage, wattage, amperage, or whatever could they expect to produce and how long would it take to get that battery charged enough.

just in general. Assuming spherical cows making whatever assumptions you need to make (consistent turning and so forth). How would this be calculated?

 

[berkeman]

With an efficient bicycle crank setup (like an exercise bike), you will be hard pressed to generate 100W for more than a few minutes. Accomplished cyclists can generate 100W-200W for a while, but it takes training to do that.

So how long will it take you to recharge a car battery with a 100W source?

 

[phyzguy]

With an efficient bicycle crank setup (like an exercise bike), you will be hard pressed to generate 100W for more than a few minutes. Accomplished cyclists can generate 100W-200W for a while, but it takes training to do that.
So how long will it take you to recharge a car battery with a 100W source?

Also, your arms are much less powerful than your legs. So I doubt if a hand cranked generator can exceed 25-50 watts.

 

[russ_watters]

Perhaps look up some claims from jump starter battery makers about how many starts you can get per charge, then calculate how many joules that is, then how long it takes to crank.

 

[anorlunda]

I second the guess that 25 watts is a lot for a setup like that. But let's use that number. 25 watts is about 2 amps on a 12 V battery. That's exactly what a trickle charger puts out, 1-2 amps.

If you have a 100 AH start battery that it too low to crank, it must be down by 50% or more. To get it to crank for even one revolution, I'm guessing that it needs 60-70% charge. So you need 10-20 AH from the trickle charger. That's about 5-10 hours at max effort. That's crazy, but not not impossibly crazy considering only energy.

But what about RPM? What RPM can you get by cranking compared to normal running speed of the gasoline engine? If the RPM is too low, you won't get any current at all into the battery.

If it was even remotely possible, I suspect that Mythbusters would have put it on the show. They did other Breaking Bad stunts.

 

[russ_watters]

If you have a 100 AH start battery that it too low to crank, it must be down by 50% or more. To get it to crank for even one revolution, I'm guessing that it needs 60-70% charge. So you need 10-20 AH from the trickle charger.

I think that's an order of magnitude or two too high (cause: you used the same criteria to pick all 3 numbers). If you think in terms of starter draw, that would be 12,000-24,000A for 3 seconds to start a car.

 

[anorlunda]

I think that's an order of magnitude or two too high (cause: you used the same criteria to pick all 3 numbers). If you think in terms of starter draw, that would be 12,000-24,000A for 3 seconds to start a car.

Everything is nonlinear, so an order of magnitude error is certainly possible.

You can crank a car with a little motorcycle battery, but RVs use very large batteries. That's why I said 100 AH battery.

It is certainly true that with a well charged battery, that you can start your engine with 0.5 seconds cranking at 100 amps. That is 0.15 AH. A simplistic analytics says that a 100 AH battery should start you car more than 700 times without any recharging. It won't.

But if your battery is so low that it won't crank at all, it must be down to 10 or 11 volts. That's severe. (The last time I did that, was 50 years ago and the temperature was -40). I'm also not sure how low the voltage has to be before the engine won't turn over. That's an automotive question.

50% of 100 AH is 50 AH.. 50/0.15 is more than 350 times as many AH as should be needed; yet the battery is so low that it won't crank at all.
My point is that the nonlinearities in this case are so severe, that we can't do much by just comparing mechical and electrical energies.

 

[russ_watters]

Everything is nonlinear, so an order of magnitude error is certainly possible.

You can crank a car with a little motorcycle battery, but RVs use very large batteries. That's why I said 100 AH battery.

It is certainly true that with a well charged battery, that you can start your engine with 0.5 seconds cranking at 100 amps. That is 0.15 AH. A simplistic analytics says that a 100 AH battery should start you car more than 700 times without any recharging. It won't.

But if your battery is so low that it won't crank at all, it must be down to 10 or 11 volts. That's severe. (The last time I did that, was 50 years ago and the temperature was -40). I'm also not sure how low the voltage has to be before the engine won't turn over. That's an automotive question.

50% of 100 AH is 50 AH.. 50/0.15 is more than 350 times as many AH as should be needed; yet the battery is so low that it won't crank at all.

I get 0.014 AH from 0.5s at 100A. Regardless, how is a car battery rated to make it possible that it won't crank at all at 50% capacity? That doesn't make sense to me.
This link says that capacity is measured at a steady 25A discharge down to 10.5V: https://batteryuniversity.com/learn/article/how_to_measure_capacity

Is the voltage linear with capacity? If so, and 10.5 is defined as "dead", and 13.6V is full, then 11V would be 16% charge.

In any case, I have a version of this jumper battery (don't buy it, it loses 1V per month so you have to recharge it every month or 2:
https://www.amazon.com/dp/B073VZ9H7Q/?tag=pfamazon01-20

It says with 500A "Peak" and 250A "Starting" it can start a V8. If we assume a flat 250A for 3 seconds at a nominal voltage of 12.5, that's 9,375 J or 0.2 AH. At 20W, it would take about 8 minutes to build up enough charge cranking by hand.

The x-factor would be exactly how dead the battery is and how much you lose with the first failed starting attempt. If it's really really dead, it would take a certain amount of charging just to achieve "dead".

 

[anorlunda]

We need some automotive help on this thread, so I moved it.
'
If your car battery is so dead that it won't crank at all, how long does it take on a trickle charger (1-2 amps) to when it will start your car?

 

[CWatters]

I think cars take 200-300A for a few seconds. Let's say 3 seconds at 250A. At 12V that would be around 9000 Joules.

If the generation and charging process was 100% efficient that would require 25W for 9000/25 = 360 seconds = 6 mins of cranking. Perhaps double or triple that for losses or errors in assumptions?

 

[phyzguy]

I think cars take 200-300A for a few seconds. Let's say 3 seconds at 250A. At 12V that would be around 9000 Joules.

If the generation and charging process was 100% efficient that would require 25W for 9000/25 = 360 seconds = 6 mins of cranking. Perhaps double or triple that for losses or errors in assumptions?

I don't think this answers anorlunda's question. My experience has been, as anorlunda said, that when your battery is too dead to crank, it takes 12-24 hours on a 1-2A trickle charger to get it to the point where it will crank. It's just not as simple as assuming that whatever energy you put into the battery is immediately accessible at whatever voltage and current you want.

 

[anorlunda]

One of the nonlinear complications is that as lead-acid batteries severely discharge, the plates become coated with lead sulphate. The operational significance is that the apparent internal resistance of a sulphated battery increases dramatically. At high current draws, such as starting, that causes large transient drops in terminal voltage.

To recover the battery, the lead sulphate must be dissolved. It is more of a chemical problem than an electrical problem. Watts and joules are not helpful to describe this problem.

 

[Rive]

3 seconds at 250A. At 12V that would be around 9000 Joules.

As I recall at that current it'll be around 9-10V. (That only means that ~ third of that 9kJ energy would be spent on heating up the battery, nothing more, nothing less.)

Regarding the original question: it would require quite an effort to spin up the engine (by hand) for the minimal RPM where the generator would start providing noticeable charge current

Charging is not expected to happen (much) below the idle RPM of the engine.
I did not see the mentioned series/episode but let's say... According what was written here this idea is on par with the average MacGyver stuff

 

[CWatters]

Oh were they hand cranking the whole engine to get the alternator to spin? I never watched the film. If that's the case I think you can write it off immediately as impossible.

 

[berkeman]

Oh were they hand cranking the whole engine to get the alternator to spin? I never watched the film. If that's the case I think you can write it off immediately as impossible.

Let's not be so hasty! I hand cranked my first car every day to drive to school (driving uphill both ways, BTW)...

https://i.ytimg.com/vi/DArvPMlxHeQ/hqdefault.jpg

After I earned some money mowing lawns by hand, I bought my first airplane for commuting...

https://cdn.avweb.com/media/newspics/325/p19hj394ra17k710ullcg13kj135k7.jpg

 

[CWatters]

But hopefully you only held onto the prop for quarter of a turn :-)

 

[gmax137]

leverage

 

[sophiecentaur]

Let's not be so hasty! I hand cranked my first car every day to drive to school (driving uphill both ways, BTW)...

That point has been ignored up until now.
It interests me that you used to live in Esher Land ; uphill both ways.

 

[CWatters]

When I was a university student the Pub was always down hill bothways from our accomodation. At least it always felt like it was down hill as we cycled back afterwards.

 

[grandpa2390]

With an efficient bicycle crank setup (like an exercise bike), you will be hard pressed to generate 100W for more than a few minutes. Accomplished cyclists can generate 100W-200W for a while, but it takes training to do that.

So how long will it take you to recharge a car battery with a 100W source?

I found one site that said the average car requires 480 watts to start. so 5 hours for the average car?

 

[berkeman]

I found one site that said the average car requires 480 watts to start. so 5 hours for the average car?

Units, units, units...

 

[sophiecentaur]

I found one site that said the average car requires 480 watts to start. so 5 hours for the average car?

That's only 40A and your average starter motor takes a few hundred Amps when the engine is cold and hasn't been running for some while. I could believe 40A to re-start a stalled engine at traffic lights, though.
But it's not just the Power needed. It's the Energy (Watt Hours) or, alternatively, the Charge (Amp Hours) that counts. Having coaxed many a tired old engine into life with a tired old battery, I know that the engine can be turned over once with no result but two or three times can kick it into life. Getting enough Energy to start once is only half of the job because there are other demands on the battery - if only the leakage in an ageing one. You'd need to be talking in terms of several tens of AHr for a useful charge.
A great advantage of hand cranking is that there are no losses due to charging and discharging a battery and your input is 'direct'. A peak mechanical input of several hundred Watts would be possible for a second or two, using the body mass intelligently, could be expected. But not for an old gimmer like me!

 

[russ_watters]

That's only 40A and your average starter motor takes a few hundred Amps when the engine is cold and hasn't been running for some while.

I was thinking it might have been an average; starting at a few hundred, then dropping as the torque decreases and it gets up to speed.

 

[sophiecentaur]

I was thinking it might have been an average; starting at a few hundred, then dropping as the torque decreases and it gets up to speed.

That effect does tend to happen. Thing is, topping up the battery manually could perhaps be significant as long as the battery is in good fettle because the total Energy for starting is not all that high - something like a few kJ. I have managed to start an old diesel marine engine by hand but that was only in desperation and involved releasing the compression and getting the flywheel spinning first. The engine was a tiddler (1300cc), compared with an RV and used to start well enough when the battery could turn it over fast, a couple of times.
I reckon the OP would be better advised to look into PV systems. Cheap and off the shelf and supplying an Amp or so (effortlessly), all day. Someone else has done the design work there, which gets my vote.

 

[anorlunda]

The OP refers to a TV episode. Here's a still picture of the scenario from that episode.

To answer the OP, at least three related topics must be considered.

1. How much energy at what rate (power) does it take to start a RV gasoline engine?
2. How much electric energy at what rate can you expect to produce pulling on the start cord of a portable generator?
3. What does it take to recover a depleted lead-acid battery enough to meet the requirement of #1?

The consensus on #2 seems to be zero. That makes #1 and #3 moot.

 

[russ_watters]

2. How much electric energy at what rate can you expect to produce pulling on the start cord of a portable generator?

The consensus on #2 seems to be zero. That makes #1 and #3 moot.

Consensus from where? I don't remember seeing discussion of that.

Googling the scenario though, I'm seeing it discussed that a certain rpm is required to generate the needed voltage, so no charging would happen. I'm not an EE, but if that's the case it would be a dealbreaker.

 

[anorlunda]

Consensus from where? I don't remember seeing discussion of that.

Googling the scenario though, I'm seeing it discussed that a certain rpm is required to generate the needed voltage, so no charging would happen. I'm not an EE, but if that's the case it would be a dealbreaker.

Yup, that was mentioned in #5, #13. Nobody that I see offered a counter-opinion.

 

[russ_watters]

Yup, that was mentioned in #5, #13. Nobody that I see offered a counter-opinion.

Oh, I was reading "pulling on the start cord" literally. You meant cranking on the generator? To be honest, since the statements didn't contain an explanation, I wasn't sure what they meant. It sounds like maybe the same thing as what I just said...

...though literally "pulling on a start cord" instead of cranking might overcome the minimum rpm issue.

 

[Rive]

Here's a still picture of the scenario from that episode.

Very thanks!

According to the picture the correct term to look up would be something like 'petrol battery charger'. As I know these are optimized for stable RPM around the maximal efficiency point of the engine. And that's usually somewhere between 2000-4000RPM. It happens that for a generator that's a quite convenient RPM, so they are usually in sync with the engine - they cannot be disconnected.
I don't know how the regulator works, but I doubt anything would come out below 1000RPM.

What RPM would be possible by hand (let' say there is no compression)?

 

[anorlunda]

The OP said:

in this episode I'm watching, they are hand cranking the generator to recharge the battery.

To me, that means the same as pulling on the start cord.

Portable generators have the same recoil start mechanism as a lawn mower. If you repeatedly pull the start cord on your lawn mower with the kill switch on (so it doesn't start), what average RPM do you think you could maintain?

@Rive had the best engineering comment; coupling it to the idle speed of the engine.

Charging is not expected to happen (much) below the idle RPM of the engine.

If we applied load at idle speed, the engine would likely stall. The portable generator designers would have to make sure it doesn't stall. They could do that electrically, or mechanically with a centrifugal clutch between engine and generator, set to engage as some RPM higher than idle speed. It is the same in a manual transmission car; you have to run it slightly faster than idle speed before letting out the clutch.

 

[dlgoff]

After I earned some money mowing lawns by hand, ...

I think I've seen your mower somewhere?

 

[sophiecentaur]

I think I've seen your mower somewhere?

View attachment 233071

One of those cut me up in heavy traffic the other day.

 

[russ_watters]

What RPM would be possible by hand (let' say there is no compression)?

No more than about 120rpm before torque drops off rapidly. Optimal is around 90-110.

 

[russ_watters]

To me, that means the same as pulling on the start cord.

Portable generators have the same recoil start mechanism as a lawn mower. If you repeatedly pull the start cord on your lawn mower with the kill switch on (so it doesn't start), what average RPM do you think you could maintain?

If there's a threshold rpm above which you are generating enough voltage to charge and below which you aren't, using the average rpm would give the wrong answer. For example you might calculate the average never exceeds the threshold when in reality you exceed it half the time and are therefore be charging half the time. I don't want to dwell on this though, since it isn't the OP's scenario and would be horribly inefficient physiologically: most people couldn't do it for more than a minute.

To sum up though:
1. Physiologically, a human is capable of putting out enough energy in a reasonable time to start an engine.

but:

2. The particular scenario doesn't work because they aren't spinning the alternator at its required rpm to generate enough voltage to charge the battery.

Incidentally, there are commercial products for exactly this purpose:
https://www.k-tor.com/pedal-powered-generator/

That's rated for 3A at 14V. Thinking about this I remembered that in some yachting races they take the human and wind power thing to the extreme where all their sophisticated electronics are human powered; when they aren't trimming sails, crewmen are charging batteries.

 

[sophiecentaur]

There is a vast difference between starting an engine manually and charging the vehicle battery. The picture that's way up the thread of an aeroplane being started by hand is an example of what you can do with a single compression. Aero engines of that sort have fancy magnetos that produce a really meaty spark (Modern electronic ignition will do the same for you) and the engine is very high compression for work at altitude. There are stories of aero engines starting when someone leans on the prop, when it's been primed and at the right point in the cycle. (They chop off arms apparently- or so a pilot was telling me when we were standing near the prop and chatting.) Wartime engines were started using a blank rifle cartridge - a one off impulse.
But an RV engine is not an aero engine and the OP was basically about a bit of poetic license in a film.

 

[Rive]

Wartime engines were started using a blank rifle cartridge

I've just come across this video today...

If there's a threshold rpm above which you are generating enough voltage to charge and below which you aren't

Well, as it seems there is...

 

[russ_watters]

Well, as it seems there is...

What confused me about the comments previously on this is that they (and the graphs in that link) are focused on current, when it appears to me that what matters most for this discussion is voltage.

E.G., all of those graphs/ratings are at 12V. So if the graph says at XXX rpm the charging current is zero, that is only true if your battery is at 12V. Earlier in the thread it was speculated you could start a car with 11 Volts; a level where the battery is considered "dead" and the charger is in fact providing substantial charging current.

For the purpose of answering the question by the OP the threshold rpm is so far above what a hand crank can generate it doesn't change the yes/no answer, but the explanations were not making sense to me for that reason.

 

[Rive]

If we are free to patch the plot holes, then we have ways around the RPM threshold (for example with something based on these), but we actually have a whole system what got abused by that hand cranker

 

[jim hardy]

This was a fun thread to read . One can watch the question come into focus.

At first i thought they'd taken off the fanbelt and were spinning the generator by hand which is laughable.

The picture was worth a thousand words.

Starting that generator's engine is one thing
cranking it even to idle speed is quite another.

Is there a thread on physics blunders in movies ?
Like in :The Martian" where duct tape and plastic sheet over a five foot hatch holds against an atmosphere of pressure ?

 

[Spinnor]

Some WWII era aircraft were started with inertia starters, one or two guys would input energy for a minute or so via a hand crank to spin up a rotating mass and then the pilot would engage the starter. Since the conversion of rotational energy to electrical energy is fairly efficient I am guessing an inertial energy storage system could be coupled to a specially designed generator that could put out large currents for short periods but long enough to start the engine?

 

[jim hardy]

an inertial energy storage system could be coupled to a specially designed generator

That'd be an interesting idea to tinker with.

Flywheel stores kinetic energy ½Iω² ,
where I = moment of inertia and ω is radians per second.

good refresher for units. (I still use slugs , feet and RPM)

old jim

 

[anorlunda]

Volvo used to make diesel engines with a wind-up starter. It was like a clock mainspring.

But to help everyone stay on topic to the OP, I will rephrase the the scenario the OP refers to in #1

Two men are in the desert. They have an RV with a dead battery. They have a portable generator that won't start. Their solution is to wire the DC output of the generator to the RV battery, then pull on the portable generator recoil starter cord to spin it. They used that rig to recharge the RV battery enough to start the RV and then drove home. Is that realistic? How long would it take?

 

[jim hardy]

Is that realistic?

I vote no.

 

[Rive]

No way. That RPM difference (apparently any engine-paired alternator would require 1000+RPM*) is just too much.

* at least I could find nothing what would work below this threshold. I mean nothing, what could be present in a common RV... Any wind based battery charger would be cheating I think.

 

[arydberg]

There are two parts to this question. One is there enough energy available given vigorous arm motion for perhaps 15 minutes to start the engine. ( the answer is yes) Two can someone efficiently couple this energy to the engine. A generator and battery is one way to do this but the generator must be rotated at a high RPM. I saw the show but forgot how the generator was turned.

On a side note inerta starters were used to start large aircraft engines. I saw one where as a person cranked the flywheel spun faster and faster . At some point it was coupled to a reduction gear to start the engine.

 

[CWatters]

There are two parts to this question. One is there enough energy available given vigorous arm motion for perhaps 15 minutes to start the engine.

I'm old enough to remember when vigorous arm motion for about half a turn was enough to start the engine on a warm day at least. I guess the remaining 14 mins 59 seconds of cranking are lost in the generator, charging process and the starter motor :-)

 

[Tom.G]

I'm old enough to remember when vigorous arm motion for about half a turn was enough to start the engine on a warm day at least. I guess the remaining 14 mins 59 seconds of cranking are lost in the generator, charging process and the starter motor :-)

Fantastic efficiency: 0.1% or less

 

[ZMacZ]

I think a spring engine much like in a watch would work better..
If the engine is in a well kept state it would need to rotate but a few times..
So storing the manual labour into a spring, which then gets used to apply all labour performed in a much shorter while,
will get the engine started..
I'd say about 200 rotations for one engine rotation, possibly 300 or 400..
So, you'd crank the engine by hand, but the actual force applied, would be compressed into a single rotation..
Did you know that the old 2CV of the Citroen brand did have a hand crank for it's engine ?
(One could also use a two stage engine, using a small 1 cilinder engine to crank the RV engine..)
(now you would only need to have a 25 CC diesel engine, that starts with a handcrank,
or kickstarter..diesel works so nice and simple, air+fuel+compression = running..)

But hey, whatever works and gets the engine going..

 

[Rive]

So storing the manual labour into a spring, which then gets used to apply all labour performed in a much shorter while,
will get the engine started..

I have a feeling that it is not exactly a new idea... I think these guys here are doing exactly just that

Ps.: Well, not exactly. As it seems for those engines it was an inertial starter - a flywheel...

 

[Rx7man]

The way I see the problem is that you need MUCH more torque to bring the first cylinder up through the compression stroke, after that it'll help you start to regain the energy as it decompresses.. so you go from needing something like 500-800 amps to 200-400 amps, depending on engine size of course.

Also most alternators require a certain amount of power going into the exciter coil.. I've found most of them need about 5 amps at 12V... = 60W... That's already more than you can produce for any extended time.

Most generator units are the same, with a small permanent magnet to get it going.. key word: SMALL.. Our 5kw generator needs to rev to nearly full speed (3600 RPM) until it comes online, Once it's alive, it can rev down to 1400 RPM or so and continue making power.

Based on those two facts alone, their method of generating the power isn't going to work AT ALL. If they had a small permanent magnet DC motor they might have a chance of getting some useful power out of it.

Here's a Tiger tank getting started with a flywheel