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Need Safety tips for large motors/batteries/capacitors

Discussion in 'Electronic Basics' started by [email protected], May 17, 2007.

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  1. Guest

    First of all hey to everyone who reads this group, hope your well.

    Anyways, in my latest project I am hooking up 2 OSMC's (open source
    motor controllers, www.robotpower.com) to 4 wheelchair motors to power
    a small chair around.

    1st set of Questions, regarding the batteries:
    I have 2 24 volt UPS batteries (which are actually 2 12 volt 17 AH
    Lead Acid batteries in series) that I would like to use.

    I have read that with NiMH batteries, they need to be connected to
    bridge rectifiers ala http://www.tlb.org/scooter_battery_schematic.html
    to prevent current flowing into eachother.
    Is this also required for Lead Acid batteries?

    I eventually plan to drop some serious dinero's for Lithium Polymer
    batteries, do those require the rectifier in parrallelling
    applications?

    Finally for batteries, does anyone know of a link for a robust 24 volt
    battery charger schematic? Unbolting the batteries from eachother is
    tiresome, and I would prefer to be able to hook the entire battery
    network up a charger and not have to worry about it.

    2nd set of Questions, regarding the capacitors:
    I recently found 7 25V 30,000uF electrolytic capacitors at a local
    surplus store, which I must admit just scare the bejeezus out of me.

    Now according to the manufacturer of the UPS that the batteries were
    ripped out of, the batteries have a nominal charging voltage of 27.6
    or 28 volts or something in that area. If I connect these fully
    charged batteries to the circuit, will the caps be damaged by the over
    voltage?

    How many of them should I use? the motors are 3 amps no load, and 8
    amps full load, so in absolute worse case scenario there would be 32
    amps drawn from the caps. is 210,000 uF overkill?

    When I put the battery in the circuit, the first thing it does is
    charge the caps. It occurs to me that if I charged up the caps as
    well, it would give my application that much extra longevity between
    charges.

    I have charged little caps before, but since these are like big white
    scary hand grenades I would just like a little brush up so they dont
    burst into flames/explode whatever. So essentially what is the best
    way to charge 7 x 30,000 uF caps up to 24 volts?

    Thats all I can think of for now, if anyone wants to go ahead and
    reply, that would be absolutely great, and much appreciated. And if
    anyone has any resources about building electric vehicles with
    wheelchair motors link would also be appreciated.
     
  2. ehsjr

    ehsjr Guest

    At 32 amps draw, 210,000 uf will do nothing for you. As a point
    of information, when using caps like those you always want to
    use a voltage _lower_ than the cap is rated. Exposing them to
    a voltage higher than their rating is something to avoid.

    Regarding using diodes to isolate the batteries - bridges
    are not required. I would use the diodes, regardless of
    which type of batteries you use. You can use a single diode
    for each series battery setup, like this:


    +--[Batt+]-[Batt+]-[Batt+]-[Batt+]-[Batt+]-[Diode]--+
    | |
    +--[Batt+]-[Batt+]-[Batt+]-[Batt+]-[Batt+]-[Diode]--+---> +
    |
    +-------------------------------------------------------> -

    You need a different charging algorithm depending on battery
    chemistry - you don't want to charge NiMh, Lead Acid and
    Lithium with the same charger, unless it is programmable
    for those types. Otherwise, you need a separate charger for
    each chemistry.

    Ed
     
  3. I just looked at some battery management ICs on websites for TI, Linear
    Technology, and Maxim. They are a little pricey but may be worthwhile for a
    high end application. The big picture seems to be Li-Ion, but they have
    multichemistry chargers and some that are programmable and integrate a
    "fuel gage". They have switching type chargers up to about 4 amps in single
    IC packages. I have decided to try to roll my own using a PIC.

    Energy storage for the capacitors is based on 1/2*C*V^2, so your 210,000 uF
    25 V bank will store 65 J (watt-seconds). If you have 800 watts of motors
    (1 HP), the energy will power the vehicle for less than 0.1 seconds. They
    are most useful for quick energy dumps from dynamic braking, but you will
    need more than that to absorb the kinetic energy of a fast stop.
    Supercapacitors are available, such as Maxwell BPAP1200 – E270, which are
    1200 F at 2.7 V. Ten in series gives you 120 F at 27 volts, and energy
    storage of 43,000 W-Sec, or 50 seconds run time at 800 watts. They cost
    about $46 each.

    You might do best with two independent systems so you don't need to
    parallel the batteries. You should be able to connect the controllers so
    they synchronize motor speed, and you probably want to have a speed
    differential for turning.

    For any high power circuitry, it is a good idea to have it enclosed in a
    rugged electrical box, and wear eye protection when testing. Judicious use
    of fuses and other current limiting devices is also advised.

    Good luck,

    Paul
     
  4. Guest

    Hope my well what? :)

    Seriously, have a look at this
    http://www.buchmann.ca/toc.asp
    and Yahoo groups for relevant interests. The power assist group is
    knowledgeable about batteries controllers and motors for bicycles.
    Then there are scooter parts houses for cheap Chinese motors,
    controllers and batteries.
    There is one in Australia called Oatley Electronics at
    http://www.oatleyelectronics.com/

    jack
     
  5. Coleston

    Coleston Guest

    Thats awesome, thanks for the advice. Ok, so when I get around to
    buying li-poly batteries, I was really planning on buying a nice
    charger for it, cause they have expensive and I dont want to screw
    them up, so the multi chemistry charger is not really an issue, thanks
    for the advice tho.

    For the lead acid batteries they are fairly simple to charge from what
    ive seen, so I want to whip up a charger for them.
    I see the wisdom of seperating the circuits to keep the batteries
    seperate, Ground from the 2 circuits should be tied together tho,
    correct?

    Now the motors I have a geared down to 260 or so RPM's, with a worm
    gear reduction. This does not allow for free wheeling, so energy
    storage from braking is not such an issue, as the mechanical segment
    has to soak up any excess energy during braking.

    The super caps are interesting, especially since I have been reading
    about these fantastical claims from eestor in texas, do you have any
    resources on charging them? I was just under the impression that all
    motor driver circuits require big caps in parallel with the motors,
    for acceleration from a dead start, should I not put them in the
    circuit? What would be the optimal capacitor arrangement for
    gearmotors?

    As for fuses and current limiting devices, what sort of thing would be
    best? I am most concerned with shorts, so a fuse for about 40 amps
    would probably do the trick. Gotta check for DC breakers, hmmm.

    Do you happen to know the code for 15 amp diodes? Are those still of
    the 1N4*** series? Also, I think in the end I am going to put all teh
    motor circuitry in an enclosed oil filled box for extra robustness and
    heat dissipitation, so I dont need to massively over design all the
    parameters, when i was in school I saw a 1N4002 take 8 amps for over
    20 minutes by being enclosed in a barrel of oil.

    Sorry for all the annoying questions, just with the last motorized
    vehicle I attempted to build I blew all sorts of hip4080's and the
    busbar for all my caps melted and a variety of other silly mishaps
    that I would like to avoid this time.

    Thanks a ton
    -Coleston
     
  6. Coleston

    Coleston Guest

    Thanks alot man, that first link is extremely helpful. Learned a
    ton. :)
     
  7. Probably a good idea, and should be bonded to chassis.
    You need to look at the surge current requirements for the motors and the
    capacity of the batteries. The capacitors can help reduce high current
    surges through the batteries, as well as a lower ESR to keep voltage
    steady.
    Many AC breakers are also rated for DC use, but inductive loads can be a
    real problem if snubbers or freewheeling diodes are not installed (or are
    defective). Sometimes two or three poles are hooked in series to get higher
    DC rating. Fuses are ultimately more fail-safe, but expensive and
    cumbersome to replace. Fast blow and even high speed semiconductor fuses
    are probably best for DC. It's probably a good idea to put a fuse on the
    positive terminal of each large capacitor if they are in parallel. Ordinary
    250 volt or 600 volt fuses at 40 amps should not be too expensive. I bought
    a lot of DC rated high current breakers surplus on eBay for a couple bucks
    each. If you need a few I'll get more details on them if it's worthwhile to
    ship some (from 21030). My company designs and builds breaker test sets, so
    I can check them out.

    I think the oil filled box would be more trouble than it's worth, and
    possibly dangerous. Just a strong steel NEMA type enclosure should be fine
    for blast containment. If anything, you might consider filling it with very
    clean sand.

    Sounds like my kind of fun!

    Paul
     
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