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Old 01-06-2013, 20:46   #16
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Re: SCUBA tank to drive a batt charger motor

Thanks to all for your info! I wondered about some direct-coupling of an air-driven system to crank the engine - but I still don't know how that could be feasible on my boat (i.e., how to get a motor/crank driven by compressed air - I may look at the truck stuff, but seems remote possibility to me...?). (I have a Universal (Westerbeke) 5424 (Model 30), 3cyl, 24 HP). I can flip a decompression lever to spin engine, then engage and go. In theory. (I believe that I would need to give at least a bit of juice to the glow plugs, and the fuel pump is electric). This is a new (1982 Seidelmann 37) boat to me, so have never done any such "interesting" tests etc.

Not to discount the info that folks have kindly provided, but I still don't really see that it is unfeasible to make something intended for some basic recharge method to get out of a jamb (but perhaps too time-consuming and expensive to build).

If I'm given some real-world-exerienced numbers that say it's not doable, then that may be a good thing - one less project for me to deal with ;-)

Thanks again to all.

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Old 01-06-2013, 20:53   #17
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Re: SCUBA tank to drive a batt charger motor

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Originally Posted by ephyraboater View Post
Thanks for expressing your safety concern. However, the tank doesn't care where the air is delivered - absolutely zero impact on the SCUBA tank (unless it's bled too quickly w/ subsequent temperature-induced condensation on outside - but not gonna happen here). If anything, use of a SCUBA tank to drive pneumatic "tools" is an expensive way to go!
Have to agree on the safety issues. As far as expensive, if you already have the tanks then the expense is negligible. Plus a SCUBA tank is very portable.
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Old 01-06-2013, 20:53   #18
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Re: SCUBA tank to drive a batt charger motor

Yes, using compressed air to turn a generator to charge batteries to turn a starter sounds inefficient. If your concern is being able to start the engine in the event batteries can't to the job, maybe you could consider something more direct, like a spring starter.

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Old 01-06-2013, 20:54   #19
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Re: SCUBA tank to drive a batt charger motor

Run a Blue Sea ACR between the starter and the house batteries. Wire all charging sources (including alternator) to the house, run only the starting motor from the starter battery.

Plenty of juice for everything super simple, no messing with switches.
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Old 01-06-2013, 21:03   #20
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Re: SCUBA tank to drive a batt charger motor

Re my battery system - I have 2 new gels rated at 86 amp-hour and 97 ah, with isolator and selector switch. (New boat for me - will have a solar charging system at some point soon, but, again, I was just wondering about the "emergency" use of a source of stored energy that I will normally have on board......).

I didn't intend to ask/talk about my electrical demands etc - I've got v. efficient systems that I use cautiously. I just know that people (like me) make mistakes, and I like to think in advance of any decent ways to fix my future (hopefully non-existent!) mistakes.
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Old 01-06-2013, 21:09   #21
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Re: SCUBA tank to drive a batt charger motor

I'd buy a solar panel instead of an air tool to turn an alternator.
Every time you change from one form of energy to another you lose a lot. You'd be going from air to rotary to electricity to rotary. Also, air tools that have enough torque to turn an alternator don't turn fast enough. You could gear it up but again, better to spend your time & money on solar that will help you every day rather than a funky air charging system that might help you once.
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Old 01-06-2013, 21:19   #22
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Re: SCUBA tank to drive a batt charger motor

That spring starter is really cool, never seen those before. Makes great sense for not needing a starting battery. Thanks Starbuck!
I'm sure they are quite expensive though.

But kinda the same idea, you could have a hand-crank generator to charge the starter battery, maybe use the tank as an assist. Actually want a supercapacitor not a battery to store and release the juice to the starter.

A spare battery is the "common sense" solution. A small A123 M1 12v pack takes up little space and can put out a lot of power to start an engine, just needs to start and not need a lot of cranking or the battery runs flat quickly.
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Old 01-06-2013, 21:55   #23
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Re: SCUBA tank to drive a batt charger motor

Let me remind the thread-people of my interest: does anyone know of a method to use the energy from a SCUBA tank's compressed air to drive a motor to charge a "dead" battery in the event of electrical load problems/mistakes made by the skipper?

My SCUBA tank is always there (assuming it's always filled!), I can add a second if needed (for me or other divers), and it just seems logical to use this energy source in a pinch.

I just wonder if there is something practical that can be driven by ~2 cfm or so air supply. I have no idea, thus the specific question to you experts!

And, if there are any specific sources for how to use compressed air to drive a system to crank a boat diesel (re. above posts), that would be interesting.

I appreciate the info on good practices/setups for electrical systems - but that's not what I'm really interested in for this specific thread.
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Old 01-06-2013, 22:58   #24
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Re: SCUBA tank to drive a batt charger motor

Folks, he did say "emergency". This doesn't have to be efficient, it just has to work. He is already carrying the filled scuba tank.

A reasonable question is if there are alternative emergency charging (or starting) methods that are simpler, smaller, lighter, more practical, etc. Obviously the first step is to have a well thought-out battery system, but I assume the OP already has that taken care of.
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Old 02-06-2013, 01:19   #25
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Re: SCUBA tank to drive a batt charger motor

a quick google How much energy is there in a filled scuba tank

shows a full tank has 1242000 watt/seconds of energy, which is more than I'd expect.
If we believe their number, convert that to 12v at 100amps to run a starter, and it will operate it for 17 minutes at 100% efficiency. If 10% efficiency, engine crank for 1.7 minutes. So given this first step looks like enough energy to bother to extract it,
now find a way to convert the pressure to charge a battery.

The simplest thing would probably be to remove your engine belt to the alternator and attach a pulley to an air grinder and use it to spin the alternator, and it will charge the battery without anything special. Or maybe skip the belt if you can use a direct coupler, which I guess if you want to build something custom that might do.

Again, I'm not sure why you would want to carry around a custom air grinder, when a small A123 emergency battery is smaller and lighter and could be used for other things.

Maybe you can just blow on the alternator pulley, tape on some fins like a water wheel, might work, but maybe now just 1% efficiency.

So perhaps best to buy a wind generator, then nothing special to carry, you just blow air from the tank as wind into the generator.
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Old 02-06-2013, 06:09   #26
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Just a FYI..some very large backup generators start with compressed air. Doesn't get old...
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Old 02-06-2013, 07:03   #27
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Here's a wild crazy idea: Sail the boat to somewhere you can get the problem fixed ???

The second best idea use a portable generator or solar panels.

Running your start battery down is *amatuer*. Shouldn't happen.

So if the SCUBA tank is empty can you fill it with a superduper bicycle pump??

Crazy.
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Old 02-06-2013, 07:16   #28
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Re: SCUBA tank to drive a batt charger motor

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Originally Posted by nimblemotors View Post

So perhaps best to buy a wind generator, then nothing special to carry, you just blow air from the tank as wind into the generator.
Jack beat me to this answer....

Honestly man... You are trying to reinvent the wheel with only a sea sponge at your disposal....

Possible??? Sure.... You may power a communications device... and a light or two... You will never juice up a dead battery enough to crank an engine... Just buy a jump box and be done with it....

If you are consumed with using your air tank for something else useful... Invent the pneumatic martini stirrer....
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Old 02-06-2013, 07:21   #29
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Re: SCUBA tank to drive a batt charger motor

I have a lot of air tools.... ALL pull more than 2cfm! Also the torque required by a 100amp alternator would likely stall any small air tool (not to mention increase the required CFM).

If you wanted to set something up just for the heck of it, I would think a heavy flywheel with fins on the outside "ring" somehow coupled to a clutch. Use full pressure (unregulated) air through a jet to spin up the flywheel, then engage the clutch!

I think the SCUBA tank would be the cheap part; the mechanism to capture the energy and transfer it to the engine would get pricey. Personally, I'd still use the cash to buy a Honda eu2000; not only would it charge your batteries, it will do something even more important..... run your Mr Coffee in the morning!
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Old 02-06-2013, 08:06   #30
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Re: SCUBA tank to drive a batt charger motor

HappyMdRSailor nailed the basic reason I thought of doing this: was getting consumed with using my air tank to do something else useful. Just seemed like an interesting application...

However, from what I've read here, even if it were technically feasible to make a device, I'm not interested/able to take the time/expense of fabricating something. Which probably wouldn't really work after all of the (fun) effort. Thanks.
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