Re: this is from the horses mouth...
tigerwoods said:
"Thank you for your enquiry, the OptiMate charger is fine for use with batteries fitted to BMW motorcycles.
Karl Dowden"
...which says *nothing* about whether they should be used WHILE THE BATTERY IS CONNECTED UP.
I would like to offer the paraphrased story as I got it from Tim at SPC - whom I asked *specifically* about this issue - and his response went something like this:
1) The 1200GS is weird because of all the single-wire stuff; if you have an optimate (I have a Optimate III) then you can use an Optimate to charge the battery, but MUST NOT DO IT WITH THE BATTERY STILL CONNECTED TO THIE BIKE. TO DO SO RISKS DAMAGE TO THE BIKE ELECTRONICS.
Note that this is in agreement with what Mr Optimate (above) says, if you read it carefully; Mr Optimate is talking about batteries, not bikes.
Tim continued:
2) If you want to charge the battery WHILST THE BATTERY IS STILL CONNECTED to the bike, then you will need the BMW or Wunderlich chargers which are designed for the bike; moreover, there is some special sequence of plugging things in and switching things (eg: ignition) on and off in a specific order, after which (i quote) "The Bike Will Decide Whether The Battery Needs To Be Charged, And Do It If Necessary."
In other words, it's another overly-clever widget, just like the rest of the bike.
Me? I have an Optimate and intend to ride the GS quite a lot, so am not fussed. If by some strange change I flatten the battery, I will remove it from the bike, charge it whilst disconnected from the bike, re-install it, and then reset the clock as-per the BMW manual.
As if anybody bothers reading that - which, incidentally, also tells you that the power outlet socket is not suitable for battery charging purposes, before any cleverclogs suggests *that*.
This is all covered on/around page 147 of the manual, for heaven's sake.