I think it's wise to fit a battery disconnect to all infrequently used machines.

The only downside on a M3W is that any updated settings in the ECU adaptive maps are lost when the power is disconnected. Not a big deal if the machine is stock. But if you have intake/exhaust/cam mods it will take a number of miles for the engine to get back to it's happy place.

A replacement unlocked ECU with merged maps solves this issue, but isn't an absolute requirement.

Older photos, as this was the very first mod I did on my car. Disconnect is on the negative side of the battery. Have since fitted covers to the exposed terminals.

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Steve
Late 2012 M3W