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364Re: [force_ev] Power and lack thereof

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  • David Roden (Akron OH USA)
    Oct 27, 2001
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      On 27 Oct 2001, at 11:59, umarc@... wrote:

      > I was driving home from work the other day when on starting up a hill I
      > found myself without enough power to sustain 30 mph (I was down about 10
      > amp-hours from full charge at this point, according to the meter).

      It could be a controller problem, or perhaps it was cutting back because
      the motor was overheating. Did you check the motor temperature?

      The most likely cause of this would be a failing battery. One cell with
      unusually high internal resistance will cause a large voltage drop. The
      more current that's used, the greater the voltage drop. One cell can
      actually cause far more than its own 2 volts of reduction because it acts
      like a resistor.

      Under normal circumstances, the controller tries to be kind to the
      batteries by limiting current to keep voltage above 1.75 volts per cell
      (126 volts for a 144v pack, or 136.5 volts for a 156v pack). Thus a low
      charge will cause a drop in top speed and acceleration rate. The same
      thing will happen in case of a failing battery which drags down the pack
      voltage under load.

      Ideally you should test the pack under operating load by putting a 50 amp
      (or more) dummy load across the pack and measuring each battery. This
      would quickly tell you which one is acting up. I use a dummy load I made
      from heating elements from derelict heat pumps, adding a fan and a spare
      contactor. This eats 100-110 amps at 144 volts (and as a cool weather
      bonus also warms up my garage nicely while I'm testing).

      If you don't have such a load, you may be able to determine the
      troublesome one by turning on the heater and all the accessories in your
      garage, and measuring the voltage of each battery in turn. Look for a
      battery that's about 2 volts lower than the others.

      Too bad Solectria didn't fit all cars with voltmeters. This was an
      option which should have been standard, as it alerts you to these kinds
      of problems.



      David Roden - Akron, Ohio, USA
      1991 Solectria Force 144vac
      1991 Ford Escort Green/EV 128vdc
      1979 General Engines ElectroPed 24vdc
      1970 GE Elec-trak E15 36vdc
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