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12v battery advice... — Parallax Forums

12v battery advice...

RontopiaRontopia Posts: 139
edited 2007-10-17 00:21 in Robotics
well guys its been a while sence I last posted.. all my robot studies had to take back seat to work for a while. well the last 5 months really. anyway im getting close to the end of my projects and my time is freeing up..

I have a bot i have been trying to work on for the last 6 months. I have a 4 wheel rover with 12v motors. I have a propeller chip that I have been learning how to program and have have wazoos of sensors I will add over time. whil the bot is much bigger than a boebot.. its not hudg.. its a lynxmotion 4w. so space for 12v battereys has been on my mind for a while now. lead acid has loads of power but also loads of weight.. anyway I was looking around on ebay to see what I might find and found this..

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=190154058334&ssPageName=STRK:MEWA:IT&ih=009

I wondered what yall think of this? its cheap and 1800mA which I believe will do. the 12v motors at peak are about 400 mA and all the other stuff I doubt will draw more than 400mA .. even if it did current draw would only be high for micro seconds at a time. .. maybe I would use 2 of them?

please post thoughts and comments.

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IC layout designer
Austin Texas

Comments

  • sam_sam_samsam_sam_sam Posts: 2,286
    edited 2007-10-16 16:49
    Rontopia

    [*]Brand New
    [*]Capacity of Battery: 1800mA
    What they are not telling you is at what rate of discharge this is at
    Duration of Power up to 8 hours........at what rate are they talking about
    Compact DC 12V
    Input voltage: 12.6V
    Output voltage: 12.6V - 10.8V
    Size: 98 x 60 x 23mm



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    ··Thanks for any·idea.gif·that you may have and all of your time finding them

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    Sam
  • RontopiaRontopia Posts: 139
    edited 2007-10-16 17:09
    i assume that 1800 mA would be there no matter what time I needed it?

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    IC layout designer
    Austin Texas
  • BeanBean Posts: 8,129
    edited 2007-10-16 17:41
    You could use two of these with 10 "AA" 2000mAh NiMH batteries.

    http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=300159420059

    I'm the person selling these by the way...

    Bean.

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    Post Edited (Bean (Hitt Consulting)) : 10/16/2007 6:41:13 PM GMT
  • allanlane5allanlane5 Posts: 3,815
    edited 2007-10-16 18:19
    Battery capacity is rated in "milli-Amp Hours". This means an 1800 mA-H battery can put out 1800 mA for an hour, or 900 mA for two hours, or 450 mA for 4 hours, or... you get the idea.

    Actually it turns out there's a curve involved. So at LOW current use rates -- 450 mA -- you may get 4 hours. But at HIGH current use rates -- 1800 mA -- you may only get 30 minutes. The difference is "internal resistance" of the battery, which at HIGH discharge rates will eat up a lot of your capacity. ESPECIALLY for a Lithium battery -- Nickel-Cadmium (NiCad) and Nickel-Metal Hydride (NiMH) batteries have very low "internal resistance" and can discharge very fast with lots of current if you put a very low resistance load on them. Like a paper clip -- don't 'short out' a fully charged NiCad with a paper clip, the paper clip will probably melt.

    Bottom line -- Lithium batteries are GREAT for 100 mA use in a camera or computer. For driving motors, they're not so good. For $25 or so, you could try it and find out. Just don't think you're going to get 1.8 AMPS out of the poor thing for very long.
  • allanlane5allanlane5 Posts: 3,815
    edited 2007-10-16 18:21
    Oh, and their "8 hour" life would be at 225 mA (1800 mA-H / 8 hours). So that may be the 'higest' discharge rate these things will support.
  • RontopiaRontopia Posts: 139
    edited 2007-10-16 21:35
    I C.. that all makes since to me now.

    so what I would need if I really wanted 1800 mH would be 8 of them.. which is not what I want at all. thanks for setting me stright.

    so 2 of these will do the trick?
    http://www.lynxmotion.com/Product.aspx?productID=331&CategoryID=48

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    IC layout designer
    Austin Texas
  • allanlane5allanlane5 Posts: 3,815
    edited 2007-10-16 22:16
    That looks really nice. And yes, you can drive servo's off of Vin using that. At 400 mA you'll get 4 hours -- if that's good enough, then that's good enough.

    Oh, but to get your 12 volt DC drive, you'll need two of them in series.· That shouldn't be a problem, right?
  • RontopiaRontopia Posts: 139
    edited 2007-10-16 22:47
    nope no problem at all.

    i just reread the specs on the motor.. its rated for 233mH at peak load and 12v.. so I dont knwo where I was getting the 400mH number. anyway there are 4 of them wired up in a tank tread config.. oh oh.. now I know where I got the 400 number.. it was for both sides of the bot.. see sometimes I have the memory of a gold fish.

    ok.. so 466 mh x 2 at peak power = 932mH max, plus sensors and propeller.. really I should be able to get away with

    http://www.lynxmotion.com/Product.aspx?productID=64&CategoryID=48

    this should run the bot for 45 min's to an hour? or should i just get the bigger ones and deal with the weight?

    man I sure can waffle huh.

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    IC layout designer
    Austin Texas
  • allanlane5allanlane5 Posts: 3,815
    edited 2007-10-16 22:57
    Well, I don't know what a 'mH' is -- I assume you meant 'mA', for "milli-Amp'.

    And that 12-volt pack is nice. I wonder what the charger costs?

    In any event, yes, you should be able to use that for 45 minutes to an hour of runtime.
  • RontopiaRontopia Posts: 139
    edited 2007-10-17 00:21
    sorry yes I ment mA.. you know im still not all the way put back together yet.. thanks again.

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    IC layout designer
    Austin Texas
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