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Ideas for High current AA cell holders

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by MrAl, Aug 7, 2010.

  1. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello there,

    The AA cell holders you get at RS and other places even on the web are made for maybe 100ma up to 1 amp or so. I would like to construct at least two AA cell holders to work up to a full 10 amps, so i am asking for some ideas on ways to do this.
    The contacts would have to support 10 amps as well as any leads running from the contacts, and this means very low contact resistance. If the contact resistance is too high the area where the contact is made will heat up too much.

    Also, if anyone has any data on maximum cell contact pressure for a typical cell or a specific cell i'd like to look at that too. I would not want to push too hard on either cell contact on the AA cells.


    Thanks for any ideas or suggestions.
     
    Last edited: Aug 7, 2010
  2. vne147

    vne147 Member

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    I don't really have an idea that can help you sorry but I do have a question. Is it even possible/safe to pull 10A from a AA? I'd imagine they would get extremely hot if they could.
     
  3. k7elp60

    k7elp60 Active Member

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    Mr. Al,
    I may want to look a Keystone electronics battery holders and contacts. I have attached a PDF of some of their holders. I have 4ea of the 139(single cell holders that have pc terminals) there are excess to my needs I would be glad to send you if you think they would be suitable. Also you might check their website as they do send samples.
    Ned
     

    Attached Files:

  4. dave

    Dave New Member

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  5. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello again,


    Thanks for the ideas and suggestions.


    vne:
    In a word, 'yes'. Li-ion cells of that size can push 10 amps, but really i need to do some tests on some NiMH cells of the AA size. There are a few commercial chargers that push 10 amps through the NiMH AA cells, and they are called "15 minute chargers" made for NiMH of course. You can check them out on the web if you like.
    I cant say how good it is for the cells, but they do work.

    k7elp:
    Well that's a very generous offer i might have to take you up on, but i would have to compensate you for at least the shipping.
    Those look like very capable cell holders so thanks for the link.
     
  6. vne147

    vne147 Member

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    Didn't know that. Thanks for the info.
     
  7. MRCecil

    MRCecil Member

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    Hi MrAl,

    Regarding contact resistance, pressure and materials I would suggest the UL and/or CSA standards as a starting point. From this link below, you can contact them for assistance:

    Need Help with UL Certifications?
     
  8. Sceadwian

    Sceadwian Banned

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    10 amps? This would have to be a massively parallel connection to get that kind of power out of a AA pack. In a large parallel pack like that you'd only need to have each connection (in the case of 10 cells) support one amp, the output bus would be the only portion that had to support a full 10 amps.

    You'll find very few to no specifications for contact pressure on a given conductor for a given ampacity because of the extreme variability of the surface oxide layer or general configuration.
    This is why batteries of this power delivery to the cells power that is almpost always via a soldered connector. There is not pure science applicable to physical connectors, too many variables over a given usage spectrum.
     
  9. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,

    Thanks for the suggested link i'll check that out.

    Scead:
    When i asked about contact pressure i was talking about the cell itself. I want to know how hard i can press on the end of the cell before it begins to collapse in on the cell body. For example, if i put it into a holder with a fine pitch screw at one end and tightened the screw to get really good contact, how hard can the end of the screw (assuming wide enough area of contact) press into the cell end cap, either end.
     
  10. Sceadwian

    Sceadwian Banned

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    You're not going to find specs like that either. I'd just use common sense, torque it down enough that you feel comfortable with the electrical contact but obviously before the metal starts to distend. I would use a grinder + polishing wheel to make sure the end of the screw conforms to the shape of the cell, you want as much contact area as possible and modest contact pressure, massively cranking down on the cell will gain you nothing past a certain point, and that certain point is going to be well before the cell deforms, making sure the contacts are clean and have a good surface area contacting each other is more important than the actual pressure.
     
  11. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello again,


    I've seen the contacts get so hot they heat up the contact area so much it would burn your finger if you touched it (i found out the hard way :) Squeezing the sides of the holder helps, which puts more pressure on the contact area. I cant hold the thing for as long as 30 minutes though so i was after something better. I dont really want to modify the existing holder either, would rather go with a new more sturdy design.
     
    Last edited: Aug 8, 2010
  12. Sceadwian

    Sceadwian Banned

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    10amps is a lot of current, normal battery contacts aren't rated for it. What is this for?
     
  13. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi again,

    Charging NiMH, but more for testing while charging at various current levels. I might end up restricting my charge current to 5 amps, but even with that current the holder i have now gets hot so i want to move to a better made holder.

    The Energizer 15 minute charger charges at around 10 amps. It wont accept the cell though if the resistance is too high, which could mean the contacts are dirty. Interestingly, i modified it to charge at 1/2 the normal charge level and accept a slightly higher resistance (it's ok with 1/2 the current as normal) and sent the design to Energizer and they wrote back that they couldnt use the design because they *might* have already started to implement something like that already (ha ha).
     
    Last edited: Aug 8, 2010
  14. Noggin

    Noggin Member

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    Dielectric grease on the contact points may help as well to reduce corrosion.
     
  15. Sceadwian

    Sceadwian Banned

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    Dielectric grease? It should be avoided at all costs on the portions that actually contact each other, it could be brushed on afterwards to protect, but what would be the point? The entire setup would have to be cleaned every time the batteries were changed.
     
  16. Noggin

    Noggin Member

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    Why? The purpose of it is to stop oxygen/moisture from getting to the contact points and in some cases reduce/eliminate fretting corrosion. If he is going to go so far as to use a screw to increase pressure/contact area, then why not protect it with grease? I suppose that in some circumstances it might reduce the contact area, but if he's going to screw something down onto it, then I wouldn't think that is the case.
     
  17. Sceadwian

    Sceadwian Banned

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    Noggin.... It's a dielectic, an insulator, the protection it offers from corrosion is also protection from physical contact unless the contacts are perfectly smooth and spring loaded to displace the silicon grease. For good conduction in high amperage conectors you want one thing and one thing only, direct clean non-oxidized metal on metal contact, in this case Gold is actually probably the best solution for connector material, but good luck finding gold plated batteries =)
     
  18. Noggin

    Noggin Member

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    yeah, but he did mention tightening it down with screws. I'd think that any contact that is going to be made with the screw is going to be enough to displace the grease. In any case, I don't think the benefits would be all that large unless he's expecting moisture. Just tightening it down will go a long way to reduce the effects of corrosion (I think)
     

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