Trial by Fire or How to guide my 11yo in building a hovercraft without serious injury.

Discussion in 'Homework Help' started by Pippin, Oct 31, 2015.

  1. Pippin

    Thread Starter New Member

    Oct 30, 2015
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    First off great site and I have read many of the hovercraft threads but am still in need of help.

    My 11yo wants to build a small battery powered remote controlled hovercraft for a school project. I want to keep as hands off as possible but am concerned that he might blow up a battery without some guidance from folks on AAC.

    What we have thus far is an array of batteries and a dozen computer fans of differing amp draw but all 12v DC.
    The batteries are as follows;

    1 x 12-volt 3.0 Amp hour NiMH pack. See link below
    http://www.pitsco.com/TETRIX_12-Volt_Rechargeable_NiMH_Battery_Pack

    2 x 12-volt 1.5 Amp hour lithium-ion. See link below
    https://www.milwaukeetool.com/accessories/batteries-and-chargers/48-11-2401

    4 x 18-volt 1.9 Amp hour Ni-Cd Battery. See link below
    http://www.amazon.com/Ridgid-130254003-18-Volt-NiCad-Battery/dp/B002IKLOZU

    Bunches of Alkaline 9 volt like go into a smoke detector

    Bunches of Alkaline and NiMH AA batteries

    6 12v fans are out of desktop computers with amp draw at 0.7,1.2, 1.4, 1.6, 2.5
    5 12v fans are out of a rack mount server and are about 1/3 the size of the PC fans and draw 1.6 amps

    My main concern is that he could blow up a battery and I would rather he learn about these things vicariously. In the same vain I want him to gain a good understanding of batteries and how to properly plan for there use in devices but here is where I seek your advice.

    What questions should I ask him to answer for himself so he can properly and safely plan the power plant for a hovercraft?

    Thank you kindly -- Pippin
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    Most batteries can explode or cause injury if shorted. Your best option is to teach your Son correct handling procedures and make sure he installs a fuse just after the battery positive terminal.
     
  3. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    He's late. I was blowin' up batteries at 5.
     
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  4. Pippin

    Thread Starter New Member

    Oct 30, 2015
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    0
    Ok, so it would not be likely that any of the batteries listed would blow if under too much discharge, correct?
    My concern was that some of the fans are 2.5A and most of our batteries are rated under that for mAh so I was worried that we could damage the batteries or ourselves.

    @dl324, is there a good rule of thumb or equation for determining the proper fuse to use with a project this size? I found several links online like http://www.optifuse.com/PDFs/FuseSelectionGuide_RevA.pdf but was not able to decipher exactly what we will need for your suggestion.

    Any suggested questions I should have him answer or links you would recommend for an 11yo to get their head around the main design decisions needed for a small hovercraft project?

    Thanks again -- Pippin
     
  5. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    I think you worry too much. In 40+ years of tinkering I've never had a battery explode, wires glowing red hot, but no explosions due to drawing excess current. Take care, follow safe practice when handling batteries and wear eye protection.

    Incidentally, I think you will struggle to make a successful hovercraft using computer fans as they tend to be too heavy. A lightweight ducted fan is your best chance of getting airborne.
     
  6. Pippin

    Thread Starter New Member

    Oct 30, 2015
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    Thanks for both thoughts Blocco! I will check out a ducted fan, but a big part of this project just learning and if he fails with the computer fans I am sure it will provide a great lesson and then he gets to figure out how to jump the next hurdle.
     
  7. dl324

    Distinguished Member

    Mar 30, 2015
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    I would treat any battery with respect. Read the warning labels on the batteries and decide for yourself.
    12V @ 2.5A is nothing to be careless with.

    It depends on the current your fans and controlling circuitry will draw. The fuse is really there to protect against fire. Fuses aren't cheap and are not reusable. Make your Son pay for them so he'll have some additional incentive to be careful.
    The usual precautions. Be mindful of shorting things, particularly the batteries. Fingers and fans rotating at high speed don't get along well. Stalling the fans will increase their current draw significantly. Follow specs; don't try to draw too much current from the batteries, don't try to make them go faster by increasing voltage. Fan motors are inductive, so protect drive circuitry from back EMF.
     
  8. Microwave

    New Member

    Oct 13, 2015
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    Pippin,

    Some thoughts on your questions:

    1. Batteries – They can explode when being discharged rapidly i.e. by short circuiting and when being charged especially if they are over charged, an early warning sign is if they become hot. With care, common sense and good design this is unlikely. Batteries are labelled with their nominal voltage and capacity. The voltages are a function of the number of cells and the capacity is related to their size. A 6 volt battery may consist of 5 Ni cads in series, 4 dry cells or 3 lead acid cells. A 150ma/h cell could be smaller than a match box, a typical AA cell may be 2500 ma/h or a lead acid cell 900x400x350 may be 500 a/h. Ampere/Hour (a/h) indicates how much current can be drawn for one hour and how much current may be applied to recharge the battery in one hour. To extract maximum life from the battery they are usually discharged and charged at something less than 1/5 of their a/h capacity. Careful consideration should be given to the battery specifications drawn from their data sheets.
    2. Fuses blow at double their rating. I.e. 1Amp fuse will blow at 2 Amps, a 10 Amp will blow at 20 Amps etc. There are exceptions to this of course there are slow blow, high rupturing current, time delay and other types for fuses. The fuse should be rated to cover the anticipated load by a small safety margin depending on the design load and charge current. The fuse is best located as near as possible to the battery in the active lead i.e. +ve if the -ve is earthed and -ve if the +ve is earthed. It's function is to protect the battery and the device if there is a fault or malfunction which draws excessive current from the battery from catching fire (if there is an on board charger it will be fused similarly).
    3. Questions: 1) How much voltage does each item need and how much current will each item draw? 2) If there are different voltages how will each item be supplied. 3) Will the battery have +ve or -ve earthed.
     
  9. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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  10. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    @Pippin
    Hover craft are really difficult. Light weight, proper design of chassis and skirt that forms air bellows along the ground is key to success. A good light weight chassis is a disposable Tupperware (hefty brand?). They are really light weight and the bigger the better for wide surface area. Ducted fan is key but you also need compression so you cannot just use a propeller type blade from the model airplane section of the hobby store, you need a screw or wide fan blades (like a computer fan but really light).

    Also, lithium-polymer batteries are super strong, high current and the key to success. These are the batteries used in electric model airplanes. Brushless DC motors are also able to take the super high current load needed (tens of watts). It all gets rather expensive. And, once you get into lithium batteries, it is important that safety glasses are worn and batteries are charged on a concrete surface with no combustibles near by - the Radio-controlled airplane crowd does not use UL-listed batteries, chargers or motor speed controllers. Be careful if you go this direction.

    I think it is great to build something - even if it is an utter failure. Then he can do failure analysis and make better and better iterations of his project. I am a big fan of learning by doing, learning by failure and iterative development. Encourage him, review each iteration with him, and show him you have the cash to support him and his project (if you do). This is a much better goal and much better learning experience than a summer robotics camp - there is plenty of info on the Internet to do most any project. You might have to spend some time on the net to stay a step ahead and guid him a bit.

    Finally, 3D printing can be used to make extremely light weight structures. There are sites where you can download shared files of 3D parts and print your own (Staples, Home Depot (soon)). I am sure someone has posted a hovercraft chassis.

    Anyhow, have fun.

    Finally, be ready to store boxes of junk of partially built projects and failed projects and some successful projects. I currently have (from two sons) a mechanical Xylophone, a nerf dart canon for compresses air, a nerf dart rifle with 6-round clip (very innovative made with PVC), 3 different RC airplanes made from scratch, a series of hovercrafts (made from a shoebox, made from hefty disposable "Tupperware" boxes, made from god-knows-what) - none worked, an electric guitar, a tube amplifier, a weed-wacker turned moped engine on a mountain bike, various robots including a 30-lb battle bot, a line-following bot, a sumo-bot and, this year, a micromouse bot is under construction.

    Good luck.
     
  11. crutschow

    Expert

    Mar 14, 2008
    13,000
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    It's easy to show that you can't do it with computer fans.
    Just lay one on the table face down and power it up. You will see that it will likely stay on the table without moving significantly, certainly not enough to lift itself into the air .
     
  12. ian field

    Distinguished Member

    Oct 27, 2012
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    I still remember when I was a wee lad I got a very painful slap because gran put a battery on the fire, and I got the blame when it burst all over the wallpaper.
     
  13. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    You should have known better than to let her throw that into the fire.
     
  14. ian field

    Distinguished Member

    Oct 27, 2012
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    First I knew about it was the battery bursting and spraying the wallpaper across the room - then the slap!
     
  15. WBahn

    Moderator

    Mar 31, 2012
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    Not a valid test. You could make the same claim for the air exhaust from a canister vacuum cleaner. Yet those are used all the time to make hovercraft that can carry 200 lb adults. The issue is area -- you need to vent the air into a skirted chamber that has a lot of area. The big question is whether you can move enough air to keep up with the air escaping the skirt.
     
  16. WBahn

    Moderator

    Mar 31, 2012
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    Some of the batteries you listed can be quite dangerous, primarily from a fire standpoint, if abused.

    Also, trying to equate 2.5 A with some mAh rating is a fool's errand. It's like trying to draw some conclusion based on whether a car's top speed is greater than its fuel economy -- the numbers fundamentally can't be compared.
     
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