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224Re: [force_ev] Pb-acid Battery Charging

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  • David Roden (Akron OH USA)
    Apr 3, 2001
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      On 3 Apr 2001, at 9:05, umarc@... wrote:

      > I wish there were an easy way for me to tell, when I get out of my car and
      > it says I'm down 15 amp-hours, whether I'm at 20% or 30% or 50% depth of
      > discharge.

      It's standard to define 100% discharge as the point at which your battery
      voltage drops to 1.75 volts per cell while under load. If you have a
      144v nominal pack, that would be 126 volts; if it's a 156v pack, 136.5
      volts.

      Connect a good quality accurate digital voltmeter to your pack (careful,
      insulate well!) and drive until the voltage on the flat at about 35 mph
      is at that level. Read your amp-hour counter. This is your real-world
      battery capacity right now (not the manufacturer's rated capacity or
      something theoretical).

      > The manual also says, by the way, that I should charge until the
      > amp-hour meter reads -4, but that is damned near impossible ...

      Assuming your amp-hour counter is accurate, it would be normal for it to
      run past zero when charging, because batteries aren't 100% efficient. An
      overcharge of 8-10% wouldn't be too far out of line. If it doesn't do
      that, there are a couple of possibilities.

      One is that the counter is not accurate. I have one here which reads
      actual amp-hours in and out differently!

      Another is that your charger really is undercharging the pack. This is
      probably happening if you have flooded batteries and the charger is set
      up for gel batteries. Long-term undercharging is as bad as long-term
      overcharging, so you should investigate this.

      The best way is to use a known-accurate amp-hour counter. There are some
      issues of concern with the Solectria/Brusa amp-hour counters; some people
      have suggested that they may sometimes miss a fraction of an amp-hour
      when the current peaks during acceleration. You might consider getting
      the newer design Brusa counter, or a Cruising Equipment E-meter.

      Another way would be to measure the actual battery state of charge. This
      isn't 100% accurate, but it's close enough to give you a good idea.

      After charging, let the car sit for 24 hours.

      If you have flooded batteries, read the specific gravity with a good
      quality temperature compenstated hydrometer (available from large battery
      vendors). A reading of 1.275 to 1.285 would indicate a fully charged
      cell. Take readings in all the cells and average them.

      Note that if your pack is old and sulfated it will not come up to full
      specific gravity. Also, if you find unusually low cells it indicates a
      need for equalization and/or possible weakness in those cells.

      If you have gel cells, with no load at all on the battery, read the
      voltage with your accurate digital voltmeter.

      12.85 - 12.95 volts => fully charged
      12.65 volts => 25% discharged
      12.35 volts => 50% discharged
      12.00 volts => 75% discharged
      11.80 volts => 100% discharged

      These values are approximate and again will be lower for old batteries.

      Again take note of any batteries that are appreciably lower than the
      others in the pack, and consider them either in need of equalization, or
      possibly they are sulfated weak links.

      If the SG or voltage in most batteries seems low, perform a long, slow
      equalization charge. I don't think your Zivan can do this, so you will
      need to either find a manual (adjustable) high-voltage charger, or charge
      the batteries individually with one or more 12-volt chargers. Charge at
      around 1 amp until the on-charge voltage stops rising. It may take days.
      Be careful to keep the current at or below 1 amp! Then repeat the above
      test. If you can't get the open-circuit voltage up to 12.6 volts or so,
      or specific gravity up past 1.250, it's probably getting on toward time
      for replacement.

      Solectria recommends a simple test which may be useful. Drive the car
      until it won't exceed 35 mph on level ground. Park and turn on the
      heater and some other stuff to keep the load going and immediately
      measure the voltage of the batteries (yeah, I know, it's a pain to open
      up the box under the hood in this situation; I recommend adding some
      external plugs to measure them). Any battery that reads under 10.5 volts
      should be considered suspect.

      There are people who have reported good experiences with the pulse
      "desulfators," but I believe that what they really do is essentially
      identical to the above-described regimen -- a long, slow equalization. I
      don't think they're worth the money, but as I say, some people like them.

      Whew, I wrote more than I thought I would. I hope this isn't more
      confusing than helpful. And I'm sorry if some of this seems vague.
      Unfortunately there's almost as much art as science in lead battery care!


      David Roden - Akron, Ohio, USA
      1991 Solectria Force 144vac
      1991 Ford Escort Green/EV 128vdc
      1979 General Engines ElectroPed 24vdc
      1974 Honda Civic EV 96vdc
      1970 GE Elec-trak E15 36vdc
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