Usually you are OK just disconnecting the battery but, and it's a big but, "usually" doesn't guarantee every eventuality and won't cover the cost of a new ECU should something go wrong!

Remember 'The Long Way Round', when they welded up Claudio's subframe? The mechanic disconnected the battery, but still succeeded in frying the ABS control unit!

I've got away MIG and stick welding by just removing the battery in the past, but I've only ever done it on classic bikes with no electronic trickery, just an alternator/dynamo and ignition/magneto to worry about.

To be on the safe side, I'd remove the battery and disconnect the ECU. Also make sure the earth point you use is as close as possible to the area your welding - make sure the current takes the shortest route possible to earth. Make sure your earth is a good one too. Also try and avoid any welding current flowing through bearings; it doesn't do them any good.

You can get big capacitors (surge/spike protectors) to clamp across the battery terminals to protect onboard electrics so welding can be done safely with everything in situ.

TBH though, if you're only going to tack the frames to the footrest mounts and then remove them to do the welding proper, you shouldn't have much issue.

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