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The update process does take some time (30 to 45 minutes), so it is important to have a battery charger (10A to 12A recommended, or a charger with a "flash" setting will work) on the car's battery during this process. It would also be a good idea to keep a charger on the PIWIS II computer as well.
If you have a PIWIS III tool, the workflow procedure may slightly differ.
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