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EV Digest, Vol 18, Issue 73

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    Message 1 of 1 , Jan 31, 2009
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      Today's Topics:

      1. Re: Re fill level for flooded batteries? (Roger Stockton)
      2. Re: Good idea or not ... motor safety contactor (Lee Hart)


      Message: 1
      Date: Sat, 31 Jan 2009 10:49:56 -0800
      From: Roger Stockton <rstockton@...>
      Subject: Re: [EVDL] Re fill level for flooded batteries?
      To: "'Electric Vehicle Discussion List'" <ev@...>
      Content-Type: text/plain; charset="us-ascii"

      Rob Trahms wrote:

      > Simple question...
      > I wanted to know what folks use as the level guideline for
      > refilling floodies.
      > I read on the USBattery site that topping off with distilled
      > water, leaving 1/4" space below the cap. Sound right to everyone?


      The electrolyte level should be about 1/8-1/4" below the bottom of the "fill well". The "fill well" is the little bit of pipe/tube that extends down from the opening that you pour the distilled water in through. Usually, there is a slot in one or both sides of the tube.

      If you filled the battery to 1/4" below the cap you would dilute the electrolyte tremendously and would lose a bunch of it out the fill caps on charge.

      One other note: check the water levels before charging and top up only as required to make sure all of the plates are covered; after charging, recheck the levels and top up fully (to 1/4" below the bottom of the fill well). The reason you don't top them up fully before charging is that the electrolyte level rises a bit during charge.




      Message: 2
      Date: Sat, 31 Jan 2009 13:29:20 -0600
      From: Lee Hart <leeahart@...>
      Subject: Re: [EVDL] Good idea or not ... motor safety contactor
      To: Electric Vehicle Discussion List <ev@...>
      Message-ID: <4984A690.3010101@...>
      Content-Type: text/plain; charset=ISO-8859-1; format=flowed

      Ryan Bohm wrote:
      > I've been thinking a lot about the fail-on nature of PWM controlled DC
      > series wound configurations. If the controller fails, you're left with your
      > main contactors to protect you.
      > Basically, it's just a contactor across the motor. Under normal
      > circumstances, it would never engage. The only time it would engage is if
      > the main contactor was commanded off, but stayed on...

      It's called a "crowbar" circuit when you short something to stop it by
      forcing a fuse or circuit breaker to blow. The colorful name is inspired
      by the idea of throwing a crowbar into a piece of machinery to stop it
      in an emergency. :-)

      In general, you do not want to crowbar motors or circuits with big
      capacitors. The peak currents can be exceedingly high, which almost
      guarantees damage. The capacitors in the controller can easily supply
      thousands of amps. Shorting a spinning motor can make it become a
      generator -- besides the huge current it could deliver into a short, it
      could generate so much braking force that it skids the tires, which on a
      front wheel drive vehicle could cause a serious accident.

      That said... There are ways to make this work. A typical crowbar circuit
      has a fuse, the crowbar switch (which is often an SCR), and enough
      resistance to limit the peak current to a reasonable value (like 2x to
      4x the fuse's current rating). This is enough current to guarantee the
      fuse will open in a reasonable time, yet not so much current that you
      risk destroying batteries, wiring, or other parts before the fuse blows.

      It is usually easier though, to have a contactor, switch, or circuit
      breaker that you can guarantee will turn *off* to stop the fun if things
      go wrong. Fuses and circuit breakers are specifically designed and
      tested to do this (which is in part why they are more expensive than a
      random piece of wire or an ordinary switch).

      Contactors can also do it if properly sized. But because they have
      "failed shorted" modes, it is common to use more than one, or a
      "proving" circuit that tests the contactor to be sure it works before
      relying on it.
      Ring the bells that still can ring
      Forget the perfect offering
      There is a crack in everything
      That's how the light gets in -- Leonard Cohen
      Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net


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      End of EV Digest, Vol 18, Issue 73
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