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13314Re: [RoboMower] Re: Which brand and type of batteries are used in the current RL1000 models

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  • Christopher Zach
    Aug 1, 2007
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      doodieb2001 wrote:
      > I measured all the voltages, 11.1 and 11.0V when the mower goes back
      > to the dock, 13.1 and 13.2V after the docking station is done
      > charging. I suspect that the charging algorithm is not very good for
      > SLA type batteries (it uses a constant charging current of about 1A
      > and a voltage limit of 28V). In my mind the batteries don't get
      > charged well under these conditions (manufacturers recommend way
      > higher charing currents and constant voltage chargers). In addition
      > the charging is interrupted if the FET is getting hotter than 125°C,
      > which happens frequently. It will restart the charging when the
      > temperature gets below 110°C. Manufacturers recommend to charge the
      > battery w/o interruptions until it is full.

      14.0 volts could be part of the problem. If the batteries are out of
      balance, then it's quite possible for battery 1 to get to 14.4 while
      battery 2 is at 13.6 (still charging) when the Robomower says "done".
      Then the next drive they get more out of sync, the dead battery gets
      dead-er, etc.

      > ALso: The battery pack gets too hot during charge due to sun exposure
      > of the mower and the vibration of the mower are not good for SLA
      > batteries either.

      Probably minor in this case. Batteries usually fail due to three causes:
      Overcharge, Undercharge, and Damage. Overcharge results in the batteries
      drying out. Undercharge results in sulfated batteries that don't accept
      a charge anymore. And damage is when you hit them or short a cell by
      pulling very high currents.

      The Robomower is not pulling very high currents :-)

      A sulfated battery will appear to charge quickly, will zip up to full
      but will fail pretty quickly. A dry battery will charge up, maybe get
      hot during charge, and will have very little voltage drop for awhile
      then fall like a brick. A shorted cell battery will appear normal under
      load with the exception that the voltage is 2 volts less than what you
      think it should be (ie: "Full" at 10, drops to 9 under load, ending at 8
      volts or so).

      If one battery is shot and the other good, it will show up pretty
      clearly in the Robomower due to it's tight voltage standards (22 volt
      cut off could be 11 one each battery or 10 on one and 12 on the other).
      I'm curious as to if the batteries are dry or sulfated.

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