Your "stepper" may change the voltage but is unlikely to be able to change the AC frequency the charger's transformer is expecting, so a flash and a puff of smoke is a possibility. If you've searched these forums you'll have noted a general lack of enthusiasm for the BMW charger, and a good amount of praise for Optimate/Accumate/Acumen/Oxford. In your part of the world you may find it easier to get hold of either of the Yuasa Smartshot chargers- one is 900 milliamp, the other 1500 milliamp. Both are excellent (even maybe better than the chargers I mentioned earlier); they come with tails you can connect easily to your battery leaving the charger connector poking out the underside of your seat. As far for the story about your 2nd battery getting ruined because it wasn't hooked up to an AGM-specific charger, unless that was a fast charger designed for car batteries etc. I call hokey, b/s, and profiteering. I'd expect an AGM battery to survive several boosts from a regular lead-acid charger, though you wouldn't want to make a habit of it.