Recharge an RV battery by hand cranking a generator

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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?
 

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  • #2
berkeman
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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?
 
  • #3
phyzguy
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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.
 
  • #4
russ_watters
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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.
 
  • #5
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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.
 
  • #6
russ_watters
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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.
 
  • #7
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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.
 
  • #8
russ_watters
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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".
 
  • #9
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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?
 
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  • #10
CWatters
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I think cars take 200-300A for a few seconds. Lets 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?
 
  • #11
phyzguy
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I think cars take 200-300A for a few seconds. Lets 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.
 
  • #12
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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.
 
  • #13
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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:woot:

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:smile:
 
  • #14
CWatters
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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.
 
  • #15
berkeman
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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

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

p19hj394ra17k710ullcg13kj135k7.jpg
 

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  • #16
CWatters
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But hopefully you only held onto the prop for quarter of a turn :-)
 
  • #17
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leverage

cafe-racer-kick.jpg
 

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  • #18
sophiecentaur
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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. :wink:
 
  • #19
CWatters
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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.
 
  • #20
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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?
 
  • #21
berkeman
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I found one site that said the average car requires 480 watts to start. so 5 hours for the average car?
Units, units, units...
 
  • #22
sophiecentaur
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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!!
 
  • #23
russ_watters
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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.
 
  • #24
sophiecentaur
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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. :smile:
 
  • #25
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The OP refers to a TV episode. Here's a still picture of the scenario from that episode.
breaking-bad-season-2-episode-91.jpg


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.
 

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