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Battery Trouble — Parallax Forums

Battery Trouble

ElectrolordElectrolord Posts: 12
edited 2010-04-20 18:49 in BASIC Stamp
Hi!

I purchased two of the serial servo controllers b4 they were discontinued (first link).· I'm tryin to build a sort of robotic spider.· 8 servos for leg back and forth and 8 for leg up and down.·I finaly have some time on my hands, and was gona sit down today and plug in some servos for some fun...· Then I realized that i need a power supply that provides a large current.·· so my question is this.· If I were to stick on 16 servos, (140 +/- 50 mA)(second link), (2.24A-3.0A).·· I would hopefully only run about·8 to·10·at a time,· what kind of supply would i be looking at?· would it be simaler to the boe-bots 4 AA supply?

I've been reading forums the last hour about mA hours and understand that they are a measure of how long a battery will last depending on the current drawn.· But I dont understand if a AA can pump out a significant·current for a short period of time say like 10-15 minutes or if that would blow it up.· Any help would be appreciated.·· (first time posting in forum hope this was the right one... lol)


Electro



http://www.parallax.com/Store/DiscontinuedProducts/tabid/795/CategoryID/69/List/0/Level/a/ProductID/376/Default.aspx?SortField=ProductName%2cProductName


http://www.parallax.com/Portals/0/Downloads/docs/prod/motors/900-00008-CRServo-v2.0.pdf

Comments

  • ercoerco Posts: 20,256
    edited 2010-04-10 16:00
    Alkaline batteries will die pretty quick. Suggest you use rechargeables for a variety of reasons; life, overall cost, convenience, performance, consistency, etc. LiPo is always your best power to weight, but expensive and cheap ones have exploded. Next choice is Ni-MH batteries. Best here is an assembled hobby-grade NiMH battery pack for an RC car or plane. Sub-C packs will obviously last longer than AAs. Cheapest rechargeable option is individual AA batteries (some rechargeable C batteries are AAs inside a fat sleeve!). They are better than alkalines, some are rated nearly 3000 mAH. Note that if you put them in a plastic battery holder, 3 amps will get hot and may melt your battery holder. It HAS happened.

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    ·"If you build it, they will come."
  • Mike GreenMike Green Posts: 23,101
    edited 2010-04-10 16:04
    Servos can draw on the order of 1A each at peak load (near stall). When moving, they draw maybe 250mA. Idle is much less.
  • ercoerco Posts: 20,256
    edited 2010-04-10 16:23
    Excellent point, Mike. And walkers (per original post) are notoriously inefficient; they gobble power in big intermittent current spikes as the weight shifts from one leg to the next.

    Different servos have different internal gearing. Depending on the walker's weight, leg geometry, and servo gearing, the servos will consume a certain amount of power even if the walker isn't walking. Just standing up without collapsing will drain the battery.

    Walkers are very cool and engaging to watch, but they will quickly make you appreciate the simplicity and efficiency of wheels!

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    ·"If you build it, they will come."
  • W9GFOW9GFO Posts: 4,010
    edited 2010-04-10 19:09
    I think you would be best off with a 5 cell sub-c NIMH pack. But then you will need to get a charger for it and you will either need to build your own pack or modify an existing six cell pack which is easy to find. AA cells are not meant to supply large currents, the sub-c RC car packs are.

    Rich H

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    The Simple Servo Tester, a kit from Gadget Gangster.
  • ElectrolordElectrolord Posts: 12
    edited 2010-04-11 14:39
    hah ive made my profile pic the image of what i made! hurah!

    mabye ill have to go back to a roller design. I'm attending a college electronics program right now so ill see if they have any fancy battery packs i could borrow/buy. Ill look into teh rc battery packs. Thanks guys!
    If i get it moving ill post a link to a movie!
    electro
  • ElectrolordElectrolord Posts: 12
    edited 2010-04-11 14:57
    Is this the pack you were thinking I may have to modify if i got somthing for a 6-cell sub-c that would need to be modified?
    http://www.onlybatteries.com/showitem.asp?ItemID=14501.55&cat1=12&uid=1010

    Is this sort of what you guys meant for rc nickel metal pack?

    http://www.thesource.ca/estore/Product.aspx?language=en-CA&catalog=Online&category=Battery_Toys&product=2300321


    Do you guys think a six volt lantern type battery like this may be able to pump out enough current?

    http://www.canadiantire.ca/AST/browse/7/Electronics/HouseholdBatteries/Batteries6Volt/PRD~0651046P/Energizer%2B6%2BVolt%2BSquare%2BLantern.jsp?locale=en
  • ercoerco Posts: 20,256
    edited 2010-04-11 20:37
    Walkers can be built with just 2 motors, instead of over a dozen servos. Check out the simple but elegant robot dicussed in this thread:
    http://forums.parallax.com/showthread.php?p=874324· and now the builder Jamie is building a big one to ride in... His·many Youtube videos·are amazing.

    And of course, there is an add-on to make a six-legged walker out of a 2-wheel BoeBot.

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    ·"If you build it, they will come."
  • icepuckicepuck Posts: 466
    edited 2010-04-12 02:59
    Considering what you are building rechargeable batteries are the only way to go.
    With that said, it opens up a can of worms when it comes to battery chemistry.
    Here's a link that should help get you started.
    www.powerstream.com/BatteryFAQ.html
    -dan

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    Arguing with myself--sometimes me, myself, and I don't always agree.
    (Former) Caterpillar product support technician
  • W9GFOW9GFO Posts: 4,010
    edited 2010-04-12 07:07
    Electrolord said...
    Is this the pack you were thinking I may have to modify if i got somthing for a 6-cell sub-c that would need to be modified?
    http://www.onlybatteries.com/showitem.asp?ItemID=14501.55&cat1=12&uid=1010

    Something like that; this is a bit cheaper -

    You'll need a charger too, and that's kinda the downside to using these types of batteries. The good news is that a charger like this will last you many years and will be able to charge just about anything you need.

    This one is a bit more expensive because you can plug it into a wall outlet as well as hook it up to a car battery. Most chargers like this need a 12 volt supply.

    Rich H

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    The Simple Servo Tester, a kit from Gadget Gangster.
  • ZuddieZuddie Posts: 5
    edited 2010-04-12 08:46
    The best type of batteries you could use for your project are the NiMH rechargeable. Don't even bother trying a 6V lantern battery. You only need 4 AA batteries. To save current, only enable one servo at a time. I'm not familiar with the servos you're using, but most will allow you to disable them so they don't draw any current.
  • ElectrolordElectrolord Posts: 12
    edited 2010-04-12 22:02
    Okay I think i have a plan!tongue.gif·

    The d-cells are on sale at an electronics outlet (The source).·· I figure ill rig 5 or 4 D cells in series (1.2V*4 or 5=·4.8-6V).· The D-cells are half off so it should be perfect!· They sell 1 and two d cell holders, unfortuneatly plastic.··

    I realized that I have a second servo controller so I can put 8 servos on each reducing the current draw(means 2 power sources thou).·

    do you guys think this will work?· Thanks for all the feedback!!



    http://www.thesource.ca/estore/Product.aspx?language=en-CA&catalog=Online&category=Rechargable+Batteries&product=2300523

    http://www.thesource.ca/estore/Product.aspx?language=en-CA&catalog=Online&category=BatteryAccessories&product=2700386
  • ElectrolordElectrolord Posts: 12
    edited 2010-04-12 22:10
    hmmmmm i just had another look at the site energizers are on sale for 2 days.· but cost about 7 bucks more.· which ones do you think would be better?



    energizer



    http://www.thesource.ca/estore/Product.aspx?language=en-CA&catalog=Online&category=Rechargable+Batteries&product=2311464


    other

    http://www.thesource.ca/estore/Product.aspx?language=en-CA&catalog=Online&category=Rechargable+Batteries&product=2300523
  • mikedivmikediv Posts: 825
    edited 2010-04-12 23:26
    Electrolord, this may be a little off topic but I just converted my serial servo to use an RS-232 and run the Parallax PSCI program , it works really well. Just in case you want to check it out
    bmp
    bmp
    rs232schematic.bmp
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    2272 x 1704 - 990K
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    2272 x 1704 - 1005K
  • ElectrolordElectrolord Posts: 12
    edited 2010-04-13 02:20
    pulled out my multimeter and measured my servos.·(probably should have done this b4 building the frame and mounting them lol)


    One type draws about 100 ma no load and 0.46 A loaded and stopped.

    The other kind of servo that i have draws about 0.25 A loaded and stopped

    (measurements taken at 5V through the basic stamp board for the boe-bot with a 9V battery attached)



    I have 8 of each type so that means that...

    8*0.46 =· 3.7A
    8*0.25 = 2A

    total = 5.7 A
    total divided by 2 = 2.85 A




    Will 5 D cells be capable of providing 2.85 amps without a boom?
    W9GFO said...
    I think you would be best off with a 5 cell sub-c NIMH pack. But then you will need to get a charger for it and you will either need to build your own pack or modify an existing six cell pack which is easy to find. AA cells are not meant to supply large currents, the sub-c RC car packs are.

    Rich H
    reason im figuring on d cells is becasue W9GFO mentions a 5 cell sub c pack.· my understanding is this is esentialy would be 5 c cells in series making 6ish volts.· Is this right?· thanks again guys*!

    *and gals!


    electro
    ·
  • ElectrolordElectrolord Posts: 12
    edited 2010-04-13 02:21
    or alternativly would 5 d cells be able to provide 5.7 A no boom.· (only one source!)(half da weight)!
  • W9GFOW9GFO Posts: 4,010
    edited 2010-04-13 03:09
    You will probably be okay with AA NIMH cells. Having two separate packs should more than double your runtime. The slower you pull the energy out of a pack, the more efficient it is. I would use five cells packs for snappier performance from the servos - if they are rated for 6 volts. Six cells would be snappier yet but may burn out the servos.

    The current drawn from or going into a battery can be expressed as "C". If the battery capacity is 2,000mah then a 1C discharge is 2 amps (2,000 mah / 1C = 2,000ma aka 2 amps) and a 10C discharge would be 20amps.

    It is common for RC packs to be rated for 10C - which means you could drain the pack in 6 minutes! Some LiPos can handle up to 60C for short periods.

    So, 5.7 amps for those 2,500mah "D" sized cells would be 2.3C. However, you probably would only be drawing half that at any one time so that brings it down to just over 1C. Those "D" cells don't offer any additional capacity over AA cells so the AAs would also be about 1C. I think it is reasonable to assume that nearly any AA NIMH can handle a 1C charge current (1 hour to charge) and therefore can also withstand the same discharge current.

    The reason the sub-c cells are more desirable is that they are made for high discharges, whereas AAs are designed for lighter duty (those "D" cells may be just AAs in a larger package). If you draw too much current from a AA it will overheat, possibly with a runaway thermal reaction. I do not know how much is too much for a AA but I would not feel comfortable with more than 1C.

    Rich H

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    The Simple Servo Tester, a kit from Gadget Gangster.
  • Spiral_72Spiral_72 Posts: 791
    edited 2010-04-13 13:43
    If it were mine, I'd be running a 4-cell, 2/3A 1500NiMh pack.... I'd be tempted to purchase a ~2500mah pack if I were buying one anyways from: www.cheapbatterypacks.com

    I've run the poo out of my pack from cheapbatteries, 5x1500mah 2/3A Elite cells.... Average 5A discharge rate on a brushed motor system....... The motor is toasty, the pack is about 110degrees and I've never noticed a change in the pack.

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    "puff"...... Smile, there went another one.
  • ElectrolordElectrolord Posts: 12
    edited 2010-04-14 14:12
    ahhhhhhhhhh so for rc packs thats y they can take 5 hours to charge but only 20 minutes to discharge. what is a sub-c cell versus a c cell? what is the difference?
  • Mike GreenMike Green Posts: 23,101
    edited 2010-04-14 14:37
    A sub-C cell is smaller than a C cell, usually a 1/2 or 1/3-C size with a corresponding capacity.
  • W9GFOW9GFO Posts: 4,010
    edited 2010-04-14 18:37
    Electrolord said...
    ahhhhhhhhhh so for rc packs thats y they can take 5 hours to charge but only 20 minutes to discharge. what is a sub-c cell versus a c cell? what is the difference?
    It all depends on the charge rate. Trickle charge is 1/10C, that's an overnight type charger. A "real" charger will be able to charge at much higher rates. A fast charge for a typical RC pack can be as little as 30 minutes - maybe less. If I am not in a hurry I generally charge at 1 amp or 1/2C, whichever is greater.

    The Sub-C cells commonly used in RC packs are just slightly smaller than a standard C. They measure about 22mm x 44mm vs. 26mm x 50mm for a standard C.

    Rich H

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    The Simple Servo Tester, a kit from Gadget Gangster.
  • ElectrolordElectrolord Posts: 12
    edited 2010-04-16 13:54
    ok after much deliberation and consideration, and searching, I have scraped the d cell plan and have found a canadian company(im from canada and hate shipping fees!) that provides rc packs with C ratings!· Now my final question is "is there any point to getting a balancer?"· is it worth the extra cash? im gona get a 7.4 volt pack and regulate it down to 6 or 5 volts.·

    Here is the balancer http://www.rctoys.com/rc-toys-and-parts/TP-205V/RC-PARTS-CHARGERS-BALANCERS-POWER.html
    here is the battery i plan on getting http://www.rctoys.com/rc-toys-and-parts/TP-2600-2SPL2/RC-PARTS-THUNDER-POWER-LITHIUM-BATTERIES.html.·
    here is the power supply http://www.rctoys.com/rc-toys-and-parts/K-TP-535-1205P/RC-PARTS-COMBOS-LITHIUM-CHARGER.html

    thanks!

    ·
  • Spiral_72Spiral_72 Posts: 791
    edited 2010-04-16 14:21
    First, I would recommend AGAINST using a Lipo for THIS project. There is a lot to worry about and the instant one cell in your two cell pack drops below about 2.9V, it's typically ruined. If you short the battery by accident, it's usually toast. Lipo batteries can supply a huge amount of power in a very short time.

    Aside from that, a balancer will monitor each cell's voltage while charging and prevent overcharging one cell in that two cell pack. The more you use a pack, the less balanced the cells will typically be..... and die long before it's time. Been there, because I didn't want to spend the $30 on a balancer.

    I'd say forget the Lipo, and if you still buy it.... the balancer is NOT optional. Never ever leave it unattended in charge or discharge. We lost $20,000 in building and equipment because of a charging Lipo was left unattended.

    I use em' all the time in RC. Just saying, they require respect and attention.



    The Thunder Power Lipos are usually VERY good batteries. That's not a bad price either.

    Good luck
    cool.gif

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    "puff"...... Smile, there went another one.

    Post Edited (Spiral_72) : 4/16/2010 2:35:40 PM GMT
  • W9GFOW9GFO Posts: 4,010
    edited 2010-04-16 15:39
    I agree, it would be better to use NiMH for the reasons stated. Standard 6 cell NiMH RC packs are cheaper and a lot easier to modify too.

    For just a bit more you can get this charger - www.rctoys.com/rc-toys-and-parts/TP-610C/RC-PARTS-CHARGERS-BALANCERS-POWER.html - which can charge all types of batteries, and it has a built in balancer. You'd still need to get the power supply though.

    Rich H

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    The Simple Servo Tester, a kit from Gadget Gangster.
  • hover1hover1 Posts: 1,929
    edited 2010-04-16 19:34
    +1 Lipos are unforgiving. As said before, discharging below 2.9..trash. Balancer..a must. Slightest physical damage will result in a ruined battery. Sometimes causes fires when charging.

    I use Thunder Power batteries in all my RC planes and helicopters, but I would probably not use it in a robot. I'm more a fan of RC car NiMH or SLA (Sealed Lead Acid) for a robot platform.
    The batteries are cheaper, the chargers are cheaper and you can get them at Radio Shack, Wallmart, Target.

    My 2 bits
    Jim
  • ElectrolordElectrolord Posts: 12
    edited 2010-04-16 23:26
    okay· judging from that response I wil kick out lipo plan!· I like my house fresh and not crispy!
    I have foudn a website that has a wide varity of nimh batteries for 6V.· This one has 10 amps max and many other 6V batteries!


    http://www.onlybatterypacks.com/showitem.asp?ItemID=10413.7

    This may be a stupid question but does anyone know about or ordered from this company?· credit card fraud and all worries me...lol

    thanks!
    ·
  • ElectrolordElectrolord Posts: 12
    edited 2010-04-20 18:49
    Thanks to everyone that posted I have ordered some ni-mh batteries!·· I couldnt have figured it out without your comments.· Thanks again!!
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