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Giovanni LiCalsi
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    Post  2051476 Thu Nov 03, 2011 12:08 pm

    John I was told to ask you this question.
    I’m building a Robbe U47 my questions is batteries. I have a space issue in the WTC due to using an Engel 825 ml piston tank. I will be using a Robbe Rokraft 120æP Controller, with the motors being brushed Graupner SPEED 400 7, 2 V and have them set up in a Ships N Things Double Fixed ratio: 3to1 frame. With that said it the NiMh vs. Li-Po. What I have read says must use NiMh 7.2V, but at a local R/C hobby shop they said I could use LiPo 7.4V. The Li-Po seams to win the size based on mAh ratio.
    Understand that I must use a Li-Po battery charger, would like comments and opinions
    Thanks,  John
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    Post  Giovanni LiCalsi Fri Nov 04, 2011 1:39 am

    John,
    I use these with great success.
    Hyperion li-po are really good batteries.
    http://www.allerc.com/batteries-c-3.html
    david f
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    Post  david f Fri Nov 04, 2011 3:48 pm

    Hi John,

    (Further to this and your post on Model Boat Mayhem.)

    I would definitely recommend the Lipo route. Just watch out for shorts (as with any battery) and make sure that you fit a low voltage alarm.

    They give you much more flexibility in fitting them in the WTC.

    I have used them on 3 models (Holland, Resurgam, Dzik) over the space of about 4 years. I use 3 cell ones (i.e about 11v) with about 2000mAH capacity.

    Use a standard balancing Lipo charger. I'm not sure about supply and costs in the US but I think that you will not go far wrong using mainstream Lipo types such as Giovannis suggestion.

    I just realise that I have written about Lipos in the past- see the Electronics section.
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    Post  johnrobinson Fri Nov 04, 2011 7:33 pm

    First problem with using lipos in submarine is if cell splits and comes in contact with fresh water there will be a big bang, but if split cell comes into contact with salt water cell will be made harmless because if you wont to do away with a lipo cell you cut the jacket put it in a old bowl with salt water leave for 24hrs and then you can put it in the bin as it has been made harmless.

    Second problem when using lipo cells no matter how good the pack is one cell will always go flat first,and every time the weakest cell is took below 2.75 volts depending how low it has been took below 2.75 volts it will loose so
    much capacity and will never get it back so that cell becomes weaker every time until the cell puffs up mainly because it has been under fast discharge.

    Third problem because you won't to use it in a model submarine because the cell voltage drops off fast on a lipo cell you need to monitor each cell separate to make sure that the weakest cell will give you a warning and blow the tanks before cell voltage goes to low and damages the pack and you have not got enough power to get to the surface. Plus you need to be slowing down speed controller for main drive motors, to be safe you should be having a trip voltage at about 3.4volts to give your self time to get the sub to the bank side

    I have plated about with lipos for many years now i was probably one of the first in the country to use them some of my packs i am pulling 3400watts at 42 volts that is alot of power plane will go vertical cloud base

    If you use lipo cell in submarine tags on end of cells i would cover in silicone rubber as if cells get wet alloy tags on end of cell will rot and drop off Hope info helps SUB JOHN
    Giovanni LiCalsi
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    Post  Giovanni LiCalsi Fri Nov 04, 2011 10:47 pm

    Lipos have come a long way in quality and longevity. Chargers have also gotten much more efficient in balancing the individual cells.
    I also use lithium iron phosphate batteries with great success. The Life brand is good.
    MIT has just tested and proven without a doubt a new lithium battery with super fast conducting anode/cathodes for super fast and low temperature charging and discharging. This type of new generation of lipos will be sold soon.
    Kind regards,
    Giovanni
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    Post  Tom(ADMIN) Tue Nov 08, 2011 8:23 am

    Now here's an interesting little thing worth trying...I was discussing this with Nigel (AMS Chairman) and between us we came up with this idea. May well be worth a try..

    Ok, so we know that if Lipo batteries get wet, there will be a big bang in your wtc, which by it's very nature won't do your boat much good. And if your boat has been down a while, there's every chance that when it's rescued there will be water in the wtc...maybe it's not found it's way to the battery pack as yet, but as soon as it's tilted, the water may run to where the battery pack is...resulting in aforesaid big bang which may well cause damage to the rescuer as well as your boat.

    Now, we also know that dropping a Lipo battery pack into salt water will kill the battery completely and render it harmless.

    Ok, so how about adding salt to your wtc? No, I don't mean getting the salt cellar out of the kitchen and sprinkling everything with salt, but adding salt into water soluble bags and placing a couple at either end of the wtc...see what I mean? You can buy water soluble bags in any fishing tackle shop, or you can empty out a couple of dissolving tablets and fill them with salt.

    Now, if your boat develops a major leak and water enters the wtc, it will start off a reaction which will melt your water soluble container, be it a bag or a tablet case, and the salt will mix with the water, which in theory will kill the battery pack before the big bang happens.

    Please note that this theoretical and as yet hasn't been tested, but it may well be worth a try...it may well save your boat from destruction at the cost of a battery pack. Sounds like a good deal to me.

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    Post  profesorul Tue Nov 08, 2011 12:43 pm

    HI,
    How about the idea of putting in place of salt Silica gel!(in a small bags),considering the weight of course.I was used these with very much succes in my apartment close to the windows to absorbe the humidity!.I have over 100 flower pots in the house!!!, and that"s create a lot of humidity.
    Regards MARIUS.
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    Post  Tom(ADMIN) Tue Nov 08, 2011 12:46 pm

    Does silica gel contain salt? It would absorb the moisture for sure, but if there's no salt content, then you'd still have freshwater inside the wtc, and it's freshwater that causes the damage to the lipo battery.
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    Post  profesorul Tue Nov 08, 2011 6:41 pm

    NO,it doesn't content salt so is NOT good to use.
    You're right about it!.
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    Post  david f Wed Nov 09, 2011 10:27 am

    The salt idea is ingenious (spelling?) but a bit drastic i.e any water gets in and all your electronics gets destroyed!

    My models all have push fit circular O ring seals held together by O rings doubling as rubber bands (Don't laugh) That means that any internal overpressure (or small explosion!) would be released fairly safely.
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    Post  Giovanni LiCalsi Wed Nov 09, 2011 6:58 pm

    The new designed, quality lipo battery packs are sealed very well and have to be soaked in water for a very long time to cause damage. If you are very concerned about wet lipos, you can use the rc car type sealed inside a water-proof case

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