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Thread: MicroVAX TOY battery backup replacement project

  1. #1

    Default MicroVAX TOY battery backup replacement project


    Hi All,

    This little project solved the annoying re-setting of the time and date and console language preference every time I powered up my MicroVAX II’s and III's.

    The old 180mAH NiCad battery pack is hard to come by, slow to charge, and memory effect, ugly leaks and old age made it useless on all my MicroVAX computers.

    I have designed a PCB that plugs piggyback into an the existing SLU (serial line unit). It uses a 14500-type Lithium Ion 3.7V AA-size rechargeable battery, in place of the original NiCad battery pack. The charging is managed by a MCP73883 charger IC.

    The charge current is about 300mA, so you can have a battery fully charged in a couple of hours, ready for many months of battery backup.

    The measured current consumption of the TOY clock circuitry (based on the now extinct MC14681 is between 50uA and 120uA. Thus, if you believe the battery manufacturers’ 1200 mAh rating, you can get anywhere from 415 to 1000 days of battery backup on a fully charged battery. The original battery, at best, could only give 2 or maybe 3 months on a full charge. I have never seen one last more than a couple of weeks though.

    Detailed description, bill of materials, production files, installation instructions and many photos are available here:


  2. #2
    Join Date
    Aug 2009
    Oslo, Norway
    Blog Entries


    That's nice. Well done!

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