If the battery is really shot (i.e. won't hold charge at all) an mc21 will struggle to run (splutter at low revs, won't idle) and won't run at all with it's lights on, it'll just splutter and die.

An mc18(PGM-II) will lose a couple of bhp in the same situation.

And as mentioned you'll be lucky to even get a '28 started at all with a dead battery, if the steering lock is on you're shot.
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